Degradable quantum channels are an important class of completely positive trace-preserving maps. Among other properties, they offer a single-letter formula for the quantum and the private classical capacity and are characterized by the fact that a complementary channel can be obtained from the channel by applying a degrading channel. In this paper, we introduce the concept of approximate degradable channels, which satisfy this condition up to some finite epsilon {\textgreater}= 0. That is, there exists a degrading channel which upon composition with the channel is e-close in the diamond norm to the complementary channel. We show that for any fixed channel the smallest such e can be efficiently determined via a semidefinite program. Moreover, these approximate degradable channels also approximately inherit all other properties of degradable channels. As an application, we derive improved upper bounds to the quantum and private classical capacity for certain channels of interest in quantum communication.

%B IEEE TRANSACTIONS ON INFORMATION THEORY %V 63 %P 7832–7844 %8 dec %R 10.1109/TIT.2017.2754268 %0 Journal Article %J Physical Review X %D 2017 %T Autonomous {Quantum} {Clocks}: {Does} {Thermodynamics} {Limit} {Our} {Ability} to {Measure} {Time}? %A Erker, Paul %A Mitchison, Mark T. %A Silva, Ralph %A Woods, Mischa P. %A Brunner, Nicolas %A Huber, Marcus %X Time remains one of the least well-understood concepts in physics, most notably in quantum mechanics. A central goal is to find the fundamental limits of measuring time. One of the main obstacles is the fact that time is not an observable and thus has to be measured indirectly. Here, we explore these questions by introducing a model of time measurements that is complete and autonomous. Specifically, our autonomous quantum clock consists of a system out of thermal equilibrium—a prerequisite for any system to function as a clock—powered by minimal resources, namely, two thermal baths at different temperatures. Through a detailed analysis of this specific clock model, we find that the laws of thermodynamics dictate a trade-off between the amount of dissipated heat and the clock’s performance in terms of its accuracy and resolution. Our results furthermore imply that a fundamental entropy production is associated with the operation of any autonomous quantum clock, assuming that quantum machines cannot achieve perfect efficiency at finite power. More generally, autonomous clocks provide a natural framework for the exploration of fundamental questions about time in quantum theory and beyond. %B Physical Review X %V 7 %P 031022 %U https://link.aps.org/doi/10.1103/PhysRevX.7.031022 %R 10.1103/PhysRevX.7.031022 %0 Journal Article %J Quantum %D 2017 %T Decoupling with random diagonal unitaries %A Nakata, Yoshifumi %A Hirche, Christoph %A Morgan, Ciara %A Winter, Andreas %X Yoshifumi Nakata, Christoph Hirche, Ciara Morgan, and Andreas Winter, Quantum 1, 18 (2017). https://doi.org/10.22331/q-2017-07-21-18 We investigate decoupling, one of the most important primitives in quantum Shannon theory, by replacing the uniformly distributed random unitaries commonly used to achieve the protocol, with… %B Quantum %V 1 %P 18 %8 jul %U https://quantum-journal.org/papers/q-2017-07-21-18/ %R 10.22331/q-2017-07-21-18 %0 Journal Article %J Physical Review Letters %D 2017 %T Discrimination Power of a Quantum Detector %A Hirche, Christoph %A Hayashi, Masahito %A Bagan, Emilio %A Calsamiglia, John %B Physical Review Letters %V 118 %8 Jan-04-2017 %U http://link.aps.org/doi/10.1103/PhysRevLett.118.160502http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevLett.118.160502/fulltexthttp://link.aps.org/article/10.1103/PhysRevLett.118.160502 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.118.160502 %0 Journal Article %J PHYSICAL REVIEW X %D 2017 %T Efficient Quantum Pseudorandomness with Nearly Time-Independent Hamiltonian Dynamics %A Nakata, Yoshifumi %A Hirche, Christoph %A Koashi, Masato %A Winter, Andreas %XQuantum randomness is an essential key to understanding the dynamics of complex many-body systems and also a powerful tool for quantum engineering. However, exact realizations of quantum randomness take an extremely long time and are infeasible in many-body systems, leading to the notion of quantum pseudorandomness, also known as unitary designs. Here, to explore microscopic dynamics of generating quantum pseudorandomness in many-body systems, we provide new efficient constructions of unitary designs and propose a design Hamiltonian, a random Hamiltonian of which dynamics always forms a unitary design after a threshold time. The new constructions are based on the alternate applications of random potentials in the generalized position and momentum spaces, and we provide explicit quantum circuits generating quantum pseudorandomness significantly more efficient than previous ones. We then provide a design Hamiltonian in disordered systems with periodically changing spin-glass-type interactions. The design Hamiltonian generates quantum pseudorandomness in a constant time even in the system composed of a large number of spins. We also point out the close relationship between the design Hamiltonian and quantum chaos.

%B PHYSICAL REVIEW X %V 7 %8 apr %R 10.1103/PhysRevX.7.021006 %0 Journal Article %J PHYSICAL REVIEW A %D 2017 %T Enhancement of low-temperature thermometry by strong coupling %A Correa, Luis A. %A Perarnau-Llobet, Marti %A Hovhannisyan, Karen V. %A Hernandez-Santana, Senaida %A Mehboudi, Mohammad %A Sanpera, Anna %X We consider the problem of estimating the temperature T of a very cold equilibrium sample. The temperature estimates are drawn from measurements performed on a quantum Brownian probe strongly coupled to it. We model this scenario by resorting to the canonical Caldeira-Leggett Hamiltonian and find analytically the exact stationary state of the probe for arbitrary coupling strength. In general, the probe does not reach thermal equilibrium with the sample, due to their nonperturbative interaction. We argue that this is advantageous for low-temperature thermometry, as we show in our model that (i) the thermometric precision at low T can be significantly enhanced by strengthening the probe-sampling coupling, (ii) the variance of a suitable quadrature of our Brownian thermometer can yield temperature estimates with nearly minimal statistical uncertainty, and (iii) the spectral density of the probe-sample coupling may be engineered to further improve thermometric performance. These observations may find applications in practical nanoscale thermometry at low temperatures-a regime which is particularly relevant to quantum technologies. %B PHYSICAL REVIEW A %V 96 %R 10.1103/PhysRevA.96.062103 %0 Journal Article %J Physical Review A %D 2017 %T Entanglement and nonlocality in diagonal symmetric states of %A Quesada, Ruben %A Rana, Swapan %A Sanpera, Anna %B Physical Review A %V 95 %8 Jan-04-2017 %U http://link.aps.org/doi/10.1103/PhysRevA.95.042128http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevA.95.042128/fulltexthttp://link.aps.org/article/10.1103/PhysRevA.95.042128 %! Phys. Rev. A %R 10.1103/PhysRevA.95.042128 %0 Journal Article %J PHYSICAL REVIEW X %D 2017 %T Equilibration Time Scales of Physically Relevant Observables %A Garcia-Pintos, Luis Pedro %A Linden, Noah %A Malabarba, Artur S. L. %A Short, Anthony J. %A Winter, Andreas %XWe address the problem of understanding, from first principles, the conditions under which a quantum system equilibrates rapidly with respect to a concrete observable. On the one hand, previously known general upper bounds on the time scales of equilibration were unrealistically long, with times scaling linearly with the dimension of the Hilbert space. These bounds proved to be tight since particular constructions of observables scaling in this way were found. On the other hand, the computed equilibration time scales for certain classes of typical measurements, or under the evolution of typical Hamiltonians, are unrealistically short. However, most physically relevant situations fall outside these two classes. In this paper, we provide a new upper bound on the equilibration time scales which, under some physically reasonable conditions, give much more realistic results than previously known. In particular, we apply this result to the paradigmatic case of a system interacting with a thermal bath, where we obtain an upper bound for the equilibration time scale independent of the size of the bath. In this way, we find general conditions that single out observables with realistic equilibration times within a physically relevant setup.

%B PHYSICAL REVIEW X %V 7 %R 10.1103/PhysRevX.7.031027 %0 Journal Article %J Physical Review A %D 2017 %T Erratum: Resource theory of coherence: Beyond states [Phys. Rev. AThe detection of change points is a pivotal task in statistical analysis. In the quantum realm, it is a new primitive where one aims at identifying the point where a source that supposedly prepares a sequence of particles in identical quantum states starts preparing a mutated one. We obtain the optimal procedure to identify the change point with certainty-naturally at the price of having a certain probability of getting an inconclusive answer. We obtain the analytical form of the optimal probability of successful identification for any length of the particle sequence. We show that the conditional success probabilities of identifying each possible change point show an unexpected oscillatory behavior. We also discuss local (online) protocols and compare them with the optimal procedure.

%B PHYSICAL REVIEW LETTERS %V 119 %R 10.1103/PhysRevLett.119.140506 %0 Journal Article %J New Journal of Physics %D 2017 %T Flexible resources for quantum metrology %A Friis, Nicolai %A Orsucci, Davide %A Skotiniotis, Michalis %A Sekatski, Pavel %A Dunjko, Vedran %A Briegel, Hans J. %A Dür, Wolfgang %X Quantum metrology offers a quadratic advantage over classical approaches to parameter estimation problems by utilising entanglement and nonclassicality. However, the hurdle of actually implementing the necessary quantum probe states and measurements, which vary drastically for different metrological scenarios, is usually not taken into account. We show that for a wide range of tasks in metrology, 2D cluster states (a particular family of states useful for measurement-based quantum computation) can serve as flexible resources that allow one to efficiently prepare any required state for sensing, and perform appropriate (entangled) measurements using only single qubit operations. Crucially, the overhead in the number of qubits is less than quadratic, thus preserving the quantum scaling advantage. This is ensured by using a compression to a logarithmically sized space that contains all relevant information for sensing. We specifically demonstrate how our method can be used to obtain optimal scaling for phase and frequency estimation in local estimation problems, as well as for the Bayesian equivalents with Gaussian priors of varying widths. Furthermore, we show that in the paradigmatic case of local phase estimation 1D cluster states are sufficient for optimal state preparation and measurement. %B New Journal of Physics %V 19 %P 063044 %U http://stacks.iop.org/1367-2630/19/i=6/a=063044 %R 10.1088/1367-2630/aa7144 %0 Journal Article %J IEEE Transactions on Information Theory %D 2017 %T From Log-Determinant Inequalities to Gaussian Entanglement via Recoverability Theory %A Lami, Ludovico %A Hirche, Christoph %A Adesso, Gerardo %A Winter, Andreas %B IEEE Transactions on Information Theory %V 63 %P 7553 - 7568 %8 Jan-11-2017 %U http://ieeexplore.ieee.org/document/8004445/http://xplorestaging.ieee.org/ielx7/18/8071168/08004445.pdf?arnumber=8004445 %! IEEE Trans. Inform. Theory %R 10.1109/TIT.2017.2737546 %0 Journal Article %J JOURNAL OF MATHEMATICAL PHYSICS %D 2017 %T Genuine-multipartite entanglement criteria based on positive maps %A Clivaz, Fabien %A Huber, Marcus %A Lami, Ludovico %A Murta, Glaucia %X Positive maps applied to a subsystem of a bipartite quantum state constitute a central tool in characterising entanglement. In the multipartite case, however, the direct application of a positive but not completely positive map cannot distinguish if a state is genuinely multipartite entangled or just entangled across some bipartition. We thus generalise this bipartite concept to the multipartite setting by introducing non-positive maps that are positive on the subset of biseparable states but can map to a non-positive element if applied to a genuine multipartite entangled state. We explicitly construct examples of multipartite non-positive maps, obtained from positive maps via a lifting procedure, that in this fashion can reveal genuine multipartite entanglement in a robust way. Published by AIP Publishing. %B JOURNAL OF MATHEMATICAL PHYSICS %V 58 %8 aug %R 10.1063/1.4998433 %0 Journal Article %J NEW JOURNAL OF PHYSICS %D 2017 %T Global and local thermometry schemes in coupled quantum systems %A Campbell, Steve %A Mehboudi, Mohammad %A De Chiara, Gabriele %A Paternostro, Mauro %X We study the ultimate bounds on the estimation of temperature for an interacting quantum system. We consider two coupled bosonic modes that are assumed to be thermal and using quantum estimation theory establish the role the Hamiltonian parameters play in thermometry. We show that in the case of a conserved particle number the interaction between the modes leads to a decrease in the overall sensitivity to temperature, while interestingly, if particle exchange is allowed with the thermal bath the converse is true. We explain this dichotomy by examining the energy spectra. Finally, we devise experimentally implementable thermometry schemes that rely only on locally accessible information from the total system, showing that almost Heisenberg limited precision can still be achieved, and we address the (im)possibility for multiparameter estimation in the system. %B NEW JOURNAL OF PHYSICS %V 19 %8 oct %R 10.1088/1367-2630/aa7fac %0 Journal Article %J PHYSICAL REVIEW E %D 2017 %T Heating without heat: Thermodynamics of passive energy filters between finite systems %A Muñoz-Tapia, R. %A Brito, R. %A Parrondo, J. M. R. %XPassive filters allowing the exchange of particles in a narrow band of energy are currently used in microrefrigerators and energy transducers. In this Rapid Communication, we analyze their thermal properties using linear irreversible thermodynamics and kinetic theory, and discuss a striking phenomenon: the possibility of simultaneously increasing or decreasing the temperatures of two systems without any supply of energy. This occurs when the filter induces a flow of particles whose energy is between the average energies of the two systems. Here we show that this selective transfer of particles does not need the action of any sort of Maxwell demon and can be carried out by passive filters without compromising the second law of thermodynamics. This phenomenon allows us to design cycles between two reservoirs at temperatures T-1 {\textless} T-2 that are able to reach temperatures below T-1 or above T-2.

%B PHYSICAL REVIEW E %V 96 %R 10.1103/PhysRevE.96.030103 %0 Journal Article %J PHYSICAL REVIEW LETTERS %D 2017 %T Improved Sensing with a Single Qubit %A Sekatski, P. %A Skotiniotis, M. %A Duer, W. %XWe consider quantum metrology with arbitrary prior knowledge of the parameter. We demonstrate that a single sensing two-level system can act as a virtual multilevel system that offers increased sensitivity in a Bayesian single-shot metrology scenario, and that allows one to estimate (arbitrary) large parameter values by avoiding phase wraps. This is achieved by making use of additional degrees of freedom or auxiliary systems not participating in the sensing process. The joint system is manipulated by intermediate control operations in such a way that an effective Hamiltonian, with an arbitrary spectrum, is generated that mimics the spectrum of a multisystem interacting with the field. We show how to use additional internal degrees of freedom of a single trapped ion to achieve a high-sensitivity magnetic field sensor for fields with arbitrary prior knowledge.

%B PHYSICAL REVIEW LETTERS %V 118 %R 10.1103/PhysRevLett.118.170801 %0 Conference Paper %B 2017 {INTERNATIONAL} {SYMPOSIUM} {ON} {WIRELESS} {COMMUNICATION} {SYSTEMS} ({ISWCS}) %D 2017 %T Information Theoretic Principles of Universal Discrete Denoising %A Noetzel, Janis %A Winter, Andreas %XSocial media platforms make tremendous amounts of data available. Often times, the same information is behind multiple different available data sets. This lends growing importance to latent variable models that try to learn the hidden information from the available imperfect versions. For example, social media platforms can contain an abundance of pictures of the same person, yet all of which are taken from different perspectives. In a simplified scenario, one may consider pictures taken from the same perspective, which are distorted by noise. This latter application allows for a rigorous mathematical treatment, which is the content of this contribution. We apply a recently developed method of dependent component analysis to image denoising when multiple distorted copies of one and the same image are available, each being corrupted by a different and unknown noise process. In a simplified scenario, one may assume such a distorted image to be corrupted by noise that acts independently on each pixel. We answer completely the question of how to perform optimal denoising, when at least three distorted copies are available: First we define optimality of an algorithm in the presented scenario, and then we describe an aymptotically optimal universal discrete denoising algorithm (UDDA).

%B 2017 {INTERNATIONAL} {SYMPOSIUM} {ON} {WIRELESS} {COMMUNICATION} {SYSTEMS} ({ISWCS}) %S International {Symposium} on {Wireless} {Communication} {Systems} %P 205–210 %@ 978-1-5386-2913-0 %0 Journal Article %J PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES %D 2017 %T Interferometric visibility and coherence %A Biswas, Tanmoy %A Garcia Diaz, Maria %A Winter, Andreas %X Recently, the basic concept of quantum coherence (or superposition) has gained a lot of renewed attention, after Baumgratz et al. (Phys. Rev. Lett. 113, 140401. (doi: 10.1103/PhysRevLett.113.140401)), following Aberg (http://arxiv.org/abs/quant-ph/0612146), have proposed a resource theoretic approach to quantify it. This has resulted in a large number of papers and preprints exploring various coherence monotones, and debating possible forms for the resource theory. Here, we take the view that the operational foundation of coherence in a state, be it quantum or otherwise wave mechanical, lies in the observation of interference effects. Our approach here is to consider an idealized multi-path interferometer, with a suitable detector, in such a way that the visibility of the interference pattern provides a quantitative expression of the amount of coherence in a given probe state. We present a general framework of deriving coherence measures from visibility, and demonstrate it by analysing several concrete visibility parameters, recovering some known coherence measures and obtaining some new ones. %B PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES %V 473 %R 10.1098/rspa.2017.0170 %0 Journal Article %J PHYSICAL REVIEW A %D 2017 %T Logarithmic coherence: Operational interpretation of l(1)-norm coherence %A Rana, Swapan %A Parashar, Preeti %A Winter, Andreas %A Lewenstein, Maciej %XWe show that the distillable coherence-which is equal to the relative entropy of coherence-is, up to a constant factor, always bounded by the l(1)-norm measure of coherence (defined as the sum of absolute values of off diagonals). Thus the latter plays a similar role as logarithmic negativity plays in entanglement theory and this is the best operational interpretation from a resource-theoretic viewpoint. Consequently the two measures are intimately connected to another operational measure, the robustness of coherence. We find also relationships between these measures, which are tight for general states, and the tightest possible for pure and qubit states. For a given robustness, we construct a state having minimum distillable coherence.

%B PHYSICAL REVIEW A %V 96 %R 10.1103/PhysRevA.96.052336 %0 Journal Article %J Quantum %D 2017 %T Macroscopic superpositions require tremendous measurement devices %A Skotiniotis, Michalis %A Dür, Wolfgang %A Sekatski, Pavel %X Michalis Skotiniotis, Wolfgang Dür, and Pavel Sekatski, Quantum 1, 34 (2017). https://doi.org/10.22331/q-2017-11-21-34 We consider fundamental limits on the detectable size of macroscopic quantum superpositions. We argue that a full quantum mechanical treatment of system plus measurement device is required,… %B Quantum %V 1 %P 34 %8 nov %U https://quantum-journal.org/papers/q-2017-11-21-34/ %R 10.22331/q-2017-11-21-34 %0 Journal Article %J PHYSICAL REVIEW B %D 2017 %T Magnetic phases of spin-1 lattice gases with random interactions %A McAlpine, Kenneth D. %A Paganelli, Simone %A Ciuchi, Sergio %A Sanpera, Anna %A De Chiara, Gabriele %X A spin-1 atomic gas in an optical lattice, in the unit-filling Mott insulator (MI) phase and in the presence of disordered spin-dependent interaction, is considered. In this regime, at zero temperature, the system is well described by a disordered rotationally invariant spin-1 bilinear-biquadratic model. We study, via the density matrix renormalization group algorithm, a bounded disorder model such that the spin interactions can be locally either ferromagnetic or antiferromagnetic. Random interactions induce the appearance of a disordered ferromagnetic phase characterized by a nonvanishing value of the spin glass order parameter across the boundary between a ferromagnetic phase and a dimer phase exhibiting random singlet order. We also study the distribution of the block entanglement entropy in the different regions. %B PHYSICAL REVIEW B %V 95 %8 jun %R 10.1103/PhysRevB.95.235128 %0 Journal Article %J PHYSICAL REVIEW LETTERS %D 2017 %T No-Go Theorem for the Characterization of Work Fluctuations in Coherent Quantum Systems %A Perarnau-Llobet, Marti %A Baumer, Elisa %A Hovhannisyan, Karen V. %A Huber, Marcus %A Acín, Antonio %XAn open question of fundamental importance in thermodynamics is howto describe the fluctuations of work for quantum coherent processes. In the standard approach, based on a projective energy measurement both at the beginning and at the end of the process, the first measurement destroys any initial coherence in the energy basis. Here we seek extensions of this approach which can possibly account for initially coherent states. We consider all measurement schemes to estimate work and require that (i) the difference of average energy corresponds to average work for closed quantum systems and that (ii) the work statistics agree with the standard two-measurement scheme for states with no coherence in the energy basis. We first show that such a scheme cannot exist. Next, we consider the possibility of performing collective measurements on several copies of the state and prove that it is still impossible to simultaneously satisfy requirements (i) and (ii). Nevertheless, improvements do appear, and in particular, we develop a measurement scheme that acts simultaneously on two copies of the state and allows us to describe a whole class of coherent transformations.

%B PHYSICAL REVIEW LETTERS %V 118 %R 10.1103/PhysRevLett.118.070601 %0 Journal Article %J Physical Review A %D 2017 %T Non-{Gaussian} operations on bosonic modes of light: {Photon}-added {Gaussian} channels %A Sabapathy, Krishna Kumar %A Winter, Andreas %X We present a framework for studying bosonic non-Gaussian channels of continuous-variable systems. Our emphasis is on a class of channels that we call photon-added Gaussian channels, which are experimentally viable with current quantum-optical technologies. A strong motivation for considering these channels is the fact that it is compulsory to go beyond the Gaussian domain for numerous tasks in continuous-variable quantum information processing such as entanglement distillation from Gaussian states and universal quantum computation. The single-mode photon-added channels we consider are obtained by using two-mode beam splitters and squeezing operators with photon addition applied to the ancilla ports giving rise to families of non-Gaussian channels. For each such channel, we derive its operator-sum representation, indispensable in the present context. We observe that these channels are Fock preserving (coherence nongenerating). We then report two examples of activation using our scheme of photon addition, that of quantum-optical nonclassicality at outputs of channels that would otherwise output only classical states and of both the quantum and private communication capacities, hinting at far-reaching applications for quantum-optical communication. Further, we see that noisy Gaussian channels can be expressed as a convex mixture of these non-Gaussian channels. We also present other physical and information-theoretic properties of these channels. %B Physical Review A %V 95 %P 062309 %U https://link.aps.org/doi/10.1103/PhysRevA.95.062309 %R 10.1103/PhysRevA.95.062309 %0 Journal Article %J Quantum %D 2017 %T Quantifying high dimensional entanglement with two mutually unbiased bases %A Erker, Paul %A Krenn, Mario %A Huber, Marcus %X Paul Erker, Mario Krenn, and Marcus Huber, Quantum 1, 22 (2017). https://doi.org/10.22331/q-2017-07-28-22 We derive a framework for quantifying entanglement in multipartite and high dimensional systems using only correlations in two unbiased bases. We furthermore develop such bounds in cases whe… %B Quantum %V 1 %P 22 %U https://quantum-journal.org/papers/q-2017-07-28-22/ %R 10.22331/q-2017-07-28-22 %0 Journal Article %J Quantum %D 2017 %T Quantum metrology with full and fast quantum control %A Sekatski, Pavel %A Skotiniotis, Michalis %A Kołodyński, Janek %A Dür, Wolfgang %X Pavel Sekatski, Michalis Skotiniotis, Janek Kołodyński, and Wolfgang Dür, Quantum 1, 27 (2017). https://doi.org/10.22331/q-2017-09-06-27 We establish general limits on how precise a parameter, e.g. frequency or the strength of a magnetic field, can be estimated with the aid of full and fast quantum control. We consider uncorr… %B Quantum %V 1 %P 27 %8 sep %U https://quantum-journal.org/papers/q-2017-09-06-27/ %R 10.22331/q-2017-09-06-27 %0 Journal Article %J Physica Scripta %D 2017 %T Quantum optics and frontiers of physics: the third quantum revolution %A Celi, Alessio %A Sanpera, Anna %A Ahufinger, Veronica %A Lewenstein, Maciej %B Physica Scripta %V 92 %P 013003 %8 Jan-01-2017 %U http://stacks.iop.org/1402-4896/92/i=1/a=013003?key=crossref.1d2d860672d166faed2db4e6d4b217abhttp://stacks.iop.org/1402-4896/92/i=1/a=013003/pdfhttp://stacks.iop.org/1402-4896/92/i=1/a=013003?key=crossref.1d2d860672d166faed2db4e6d4b217ab %! Phys. Scr. %R 10.1088/1402-4896/92/1/013003 %0 Journal Article %J PHYSICAL REVIEW A %D 2017 %T Resource theory of coherence: Beyon} states %A Ben Dana, Khaled %A Garcia Diaz, Maria %A Mejatty, Mohamed %A Winter, Andreas %XWe generalize the recently proposed resource theory of coherence (or superposition) [T. Baumgratz et al., Phys. Rev. Lett. 113, 140401 (2014); A. Winter and D. Yang, Phys. Rev. Lett. 116, 120404 ( 2016)] to the setting where not just the free (”incoherent”) resources, but also the manipulated objects, are quantum operations rather than states. In particular, we discuss an information theoretic notion of the coherence capacity of a quantum channel and prove a single-letter formula for it in the case of unitaries. Then we move to the coherence cost of simulating a channel and prove achievability results for unitaries and general channels acting on a d-dimensional system; we show that a maximally coherent state of rank d is always sufficient as a resource if incoherent operations are allowed, and one of rank d(2) for “strictly incoherent” operations. We also show lower bounds on the simulation cost of channels that allow us to conclude that there exists bound coherence in operations, i.e., maps with nonzero cost of implementing them but zero coherence capacity; this is in contrast to states, which do not exhibit bound coherence.

%B PHYSICAL REVIEW A %V 95 %R 10.1103/PhysRevA.95.062327 %0 Journal Article %J PHYSICAL REVIEW A %D 2017 %T Scaling maps of s-ordered quasiprobabilities are either nonpositive or completely positive %A Ivan, J. Solomon %A Sabapathy, Krishna Kumar %A Simon, R. %X Continuous-variable systems in quantum theory can be fully described through any one of the s-ordered family of quasiprobabilities Lambda(s) (a), s is an element of [-1,1]. We ask forwhat values of (s,a) is the scalingmap Lambda(s) (alpha) -{\textgreater} a(-2) Lambda(s) (a(-1)alpha) a positive map? Our analysis based on a duality we establish settles this issue: (i) the scaling map generically fails to be positive, showing that there is no useful entanglement witness of the scaling type beyond the transpose map, and (ii) in the two particular cases (s = 1,{\textbar}a{\textbar} {\textless}= 1) and (s = -1,{\textbar}a{\textbar} {\textgreater}= 1), and only in these two nontrivial cases, the map is not only positive but also completely positive as seen through the noiseless attenuator and amplifier channels. We also present a “phase diagram” for the behavior of the scaling maps in the s-a parameter space with regard to its positivity, obtained from the viewpoint of symmetric-ordered characteristic functions. This also sheds light on similar diagrams for the practically relevant attenuation and amplification maps with respect to the noise parameter, especially in the range below the complete-positivity (or quantum-limited) threshold. %B PHYSICAL REVIEW A %V 96 %8 aug %R 10.1103/PhysRevA.96.022114 %0 Journal Article %J JOURNAL OF MATHEMATICAL PHYSICS %D 2017 %T Unitary 2-designs from random X- and Z-diagonal unitaries %A Nakata, Yoshifumi %A Hirche, Christoph %A Morgan, Ciara %A Winter, Andreas %XUnitary 2-designs are random unitaries simulating up to the second order statistical moments of the uniformly distributed random unitaries, often referred to as Haar random unitaries. They are used in a wide variety of theoretical and practical quantum information protocols and also have been used to model the dynamics in complex quantum many-body systems. Here, we show that unitary 2-designs can be approximately implemented by alternately repeating random unitaries diagonal in the Pauli-Z basis and Pauli-X basis. We also provide a converse about the number of repetitions needed to achieve unitary 2-designs. These results imply that the process after l repetitions achieves a Theta(d(-l))-approximate unitary 2-design. Based on the construction, we further provide quantum circuits that efficiently implement approximate unitary 2-designs. Although a more efficient implementation of unitary 2-designs is known, our quantum circuit has its own merit that it is divided into a constant number of commuting parts, which enables us to apply all commuting gates simultaneously and leads to a possible reduction of an actual execution time. We finally interpret the result in terms of the dynamics generated by time-dependent Hamiltonians and provide for the first time a random disordered time-dependent Hamiltonian that generates a unitary 2-design after switching interactions only a few times. Published by AIP Publishing.

%B JOURNAL OF MATHEMATICAL PHYSICS %V 58 %R 10.1063/1.4983266 %0 Journal Article %J Physical Review A %D 2017 %T Vortex stability in Bose-Einstein condensates in the presence of disorder %A Pons, Marisa %A Sanpera, Anna %B Physical Review A %V 95 %8 Jan-03-2017 %U https://link.aps.org/doi/10.1103/PhysRevA.95.033626http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevA.95.033626/fulltexthttp://link.aps.org/article/10.1103/PhysRevA.95.033626 %! Phys. Rev. A %R 10.1103/PhysRevA.95.033626 %0 Journal Article %J Physical Review A %D 2016 %T Achieving sub-shot-noise sensing at finite temperatures %A Mehboudi, Mohammad %A Correa, Luis A. %A Sanpera, Anna %B Physical Review A %V 94 %8 10/2016 %N 4 %! Phys. Rev. A %R 10.1103/PhysRevA.94.042121 %0 Journal Article %J Physical Review B %D 2016 %T Autonomous {Quantum} {Refrigerator} in a {Circuit}-{QED} {Architecture} {Based} on a {Josephson} {Junction} %A Hofer, Patrick P. %A Perarnau-Llobet, Marti %A Brask, Jonatan Bohr %A Silva, Ralph %A Huber, Marcus %A Brunner, Nicolas %K Condensed Matter - Mesoscale and Nanoscale Physics %K Quantum Physics %X An implementation of a small quantum absorption refrigerator in a circuit QED architecture is proposed. The setup consists of three harmonic oscillators coupled to a Josephson unction. The refrigerator is autonomous in the sense that it does not require any external control for cooling, but only thermal contact between the oscillators and heat baths at different temperatures. In addition, the setup features a built-in switch, which allows the cooling to be turned on and off. If timing control is available, this enables the possibility for coherence-enhanced cooling. Finally, we show that significant cooling can be achieved with experimentally realistic parameters and that our setup should be within reach of current technology. %B Physical Review B %V 94 %U http://arxiv.org/abs/1607.05218 %R 10.1103/PhysRevB.94.235420 %0 Journal Article %J Journal of Mathematical Physics %D 2016 %T Bipartite depolarizing maps %A Lami, Ludovico %A Huber, Marcus %X We introduce a 3-parameter class of maps (1) acting on a bipartite system which are a natural generalisation of the depolarizing channel (and include it as a special case). Then, we find the exact regions of the parameter space that alternatively determine a positive, completely positive, entanglement-breaking, or entanglement-annihilating map. This model displays a much richer behaviour than the one shown by a simple depolarizing channel, yet it stays exactly solvable. As an example of this richness, positive partial transposition but not entanglement-breaking maps is found in Theorem 2. A simple example of a positive yet indecomposable map is provided (see the Remark at the end of Section IV). The study of the entanglement-annihilating property is fully addressed by Theorem 7. Finally, we apply our results to solve the problem of the entanglement annihilation caused in a bipartite system by a tensor product of local depolarizing channels. In this context, a conjecture posed in the work of Filippov [J. Russ. Laser Res. 35, 484 (2014)] is affirmatively answered, and the gaps that the imperfect bounds of Filippov and Ziman [Phys. Rev. A 88, 032316 (2013)] left open are closed. To arrive at this result, we furthermore show how the Hadamard product between quantum states can be implemented via local operations. %B Journal of Mathematical Physics %V 57 %P 092201 %U http://aip.scitation.org/doi/10.1063/1.4962339 %R 10.1063/1.4962339 %0 Journal Article %J Problems of Information Transmission %D 2016 %T Classical capacities of quantum channels with environment assistance %A Karumanchi, S. %A Mancini, S. %A Winter, A. %A Yang, D. %X A quantum channel physically is a unitary interaction between an information carrying system and an environment, which is initialized in a pure state before the interaction. Conventionally, this state, as also the parameters of the interaction, is assumed to be fixed and known to the sender and receiver. Here, following the model introduced by us earlier [1], we consider a benevolent third party, i.e., a helper, controlling the environment state, and show how the helper’s presence changes the communication game. In particular, we define and study the classical capacity of a unitary interaction with helper, in two variants: one where the helper can only prepare separable states across many channel uses, and one without this restriction. Furthermore, two even more powerful scenarios of pre-shared entanglement between helper and receiver, and of classical communication between sender and helper (making them conferencing encoders) are considered. %B Problems of Information Transmission %V 52 %P 214–238 %8 jul %U https://link.springer.com/article/10.1134/S0032946016030029 %R 10.1134/S0032946016030029 %0 Journal Article %J Contemporary Physics %D 2016 %T Conditional and unconditional Gaussian quantum dynamics %A Genoni, Marco G. %A Lami, Ludovico %A Serafini, Alessio %B Contemporary Physics %P 1 - 19 %8 01/2016 %! Contemporary Physics %R 10.1080/00107514.2015.1125624 %0 Journal Article %J Physical Review Letters %D 2016 %T Entanglement and Coherence in Quantum State Merging %A Streltsov, A. %A Chitambar, E. %A Rana, S. %A Bera, N. %A Winter, A. %A Lewenstein, M. %B Physical Review Letters %V 116 %8 Jan-06-2016 %U http://link.aps.org/doi/10.1103/PhysRevLett.116.240405http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevLett.116.240405/fulltexthttp://link.aps.org/accepted/10.1103/PhysRevLett.116.240405http://link.aps.org/article/10.1103/PhysRevLett.116.240405 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.116.240405 %0 Journal Article %J Journal of Mathematical Physics %D 2016 %T Entanglement-saving channels %A Lami, L. %A Giovannetti, V. %B Journal of Mathematical Physics %V 57 %P 032201 %8 03/2016 %N 3 %! J. Math. Phys. %R 10.1063/1.4942495 %0 Journal Article %J Journal of Functional Analysis %D 2016 %T Estimating quantum chromatic numbers %A Paulsen, Vern I. %A Severini, Simone %A Stahlke, Daniel %A Todorov, Ivan G. %A Winter, Andreas %B Journal of Functional Analysis %V 270 %P 2188-2222 %8 03/2016 %N 6 %! Journal of Functional Analysis %R 10.1016/j.jfa.2016.01.010 %0 Journal Article %J Physical Review A %D 2016 %T Heisenberg-{Weyl} {Observables}: {Bloch} vectors in phase space %A Asadian, Ali %A Erker, Paul %A Huber, Marcus %A Klockl, Claude %X We introduce a Hermitian generalization of Pauli matrices to higher dimensions which is based on Heisenberg-Weyl operators. The complete set of Heisenberg-Weyl observables allows us to identify a real-valued Bloch vector for an arbitrary density operator in discrete phase space, with a smooth transition to infinite dimensions. Furthermore, we derive bounds on the sum of expectation values of any set of anticommuting observables. Such bounds can be used in entanglement detection and we show that Heisenberg-Weyl observables provide a first nontrivial example beyond the dichotomic case. %B Physical Review A %V 94 %P 010301 %8 jul %U https://link.aps.org/doi/10.1103/PhysRevA.94.010301 %R 10.1103/PhysRevA.94.010301 %0 Journal Article %J Nature Communications %D 2016 %T Microcanonical and resource-theoretic derivations of the thermal state of a quantum system with noncommuting charges %A Yunger Halpern, Nicole %A Faist, Philippe %A Oppenheim, Jonathan %A Winter, Andreas %B Nature Communications %V 77 %P 12051 %8 Jul-07-2016 %U http://www.nature.com/doifinder/10.1038/ncomms12051 %! Nat Comms %R 10.1038/ncomms12051 %0 Journal Article %J Nature Photonics %D 2016 %T Multi-photon entanglement in high dimensions %A Malik, Mehul %A Erhard, Manuel %A Huber, Marcus %A Krenn, Mario %A Robert Fickler %A Anton Zeilinger %B Nature Photonics %I Nature Publishing Group %V 10 %P 248 - 252 %8 04/2016 %@ 1749-4885 %G eng %U http://dx.doi.org/10.1038/nphoton.2016.12 %N 4 %0 Journal Article %J IEEE Transactions on Information Theory %D 2016 %T No-Signalling-Assisted Zero-Error Capacity of Quantum Channels and an Information Theoretic Interpretation of the Lovász Number %A Duan, Runyao %A Winter, Andreas %B IEEE Transactions on Information Theory %V 62 %P 891-914 %8 2/2016 %N 2 %! IEEE Trans. Inform. Theory %R 10.1109/TIT.2015.2507979 %0 Journal Article %J Physical Review Letters %D 2016 %T Operational Resource Theory of Coherence %A Winter, Andreas %A Dong Yang %B Physical Review Letters %V 116 %P 120404 %8 3/2016 %N 12 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.116.120404 %0 Journal Article %J IEEE Transactions on Information Theory %D 2016 %T Polar Codes in Network Quantum Information Theory %A Christoph Hirche %A Ciara Morgan %A Wilde, Mark M. %B IEEE Transactions on Information Theory %V 62 %P 915 - 924 %8 02/2016 %N 2 %! IEEE Trans. Inform. Theory %R 10.1109/TIT.2016.2514319 %0 Journal Article %J IEEE Transactions on Information Theory %D 2016 %T Potential Capacities of Quantum Channels %A Winter, Andreas %A Dong Yang %B IEEE Transactions on Information Theory %V 62 %P 1415-1424 %8 3/2016 %N 3 %! IEEE Trans. Inform. Theory %R 10.1109/TIT.2016.2519920 %0 Journal Article %J IEEE Transactions on Information Theory %D 2016 %T The Private and Public Correlation Cost of Three Random Variables With Collaboration %A Eric Chitambar %A Hsieh, Min-Hsiu %A Winter, Andreas %B IEEE Transactions on Information Theory %V 62 %P 2034-2043 %8 4/2016 %N 4 %! IEEE Trans. Inform. Theory %R 10.1109/TIT.2016.2530086 %0 Journal Article %J New Journal of Physics %D 2016 %T Probabilistic metrology or how some measurement outcomes render ultra-precise estimates %A Calsamiglia, J %A Gendra, B %A Muñoz-Tapia, R %A Bagan, E %XWe show on theoretical grounds that, even in the presence of noise, probabilistic measurement strategies (which have a certain probability of failure or abstention) can provide, upon a heralded successful outcome, estimates with a precision that exceeds the deterministic bounds for the average precision. This establishes a new ultimate bound on the phase estimation precision of particular measurement outcomes (or sequence of outcomes). For probe systems subject to local dephasing, we quantify such precision limit as a function of the probability of failure that can be tolerated. Our results show that the possibility of abstaining can set back the detrimental effects of noise.

%B New Journal of Physics %V 18 %P 103049 %8 Jan-10-2016 %U http://stacks.iop.org/1367-2630/18/i=10/a=103049?key=crossref.fb78efa5084e8d9fdb51f574cf3bfed3http://stacks.iop.org/1367-2630/18/i=10/a=103049/pdfhttp://stacks.iop.org/1367-2630/18/i=10/a=103049?key=crossref.fb78efa5084e8d9fdb51f574cf3bfed3 %! New J. Phys. %R 10.1088/1367-2630/18/10/103049 %0 Journal Article %J Physical Review Letters %D 2016 %T Probabilistically Perfect Cloning of Two Pure States: Geometric Approach %A Yerokhin, V. %A Shehu, A. %A Feldman, E. %A E. Bagan %A Bergou, J. A. %B Physical Review Letters %V 116 %8 5/2016 %N 20 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.116.200401 %0 Journal Article %J Physical Review A %D 2016 %T Process output nonclassicality and nonclassicality depth of quantum-optical channels %A Sabapathy, Krishna Kumar %B Physical Review A %V 93 %8 4/2016 %N 4 %! Phys. Rev. A %R 10.1103/PhysRevA.93.042103 %0 Journal Article %J Physical Review Letters %D 2016 %T Quantifying {Entanglement} of {Maximal} {Dimension} in {Bipartite} {Mixed} {States} %A Sentís, Gael %A Eltschka, Christopher %A Gühne, Otfried %A Huber, Marcus %A Siewert, Jens %X The Schmidt coefficients capture all entanglement properties of a pure bipartite state and therefore determine its usefulness for quantum information processing. While the quantification of the corresponding properties in mixed states is important both from a theoretical and a practical point of view, it is considerably more difficult, and methods beyond estimates for the concurrence are elusive. In particular this holds for a quantitative assessment of the most valuable resource, the forms of entanglement that can only exist in high-dimensional systems. We derive a framework for lower bounding the appropriate measure of entanglement, the so-called G-concurrence, through few local measurements. Moreover, we show that these bounds have relevant applications also for multipartite states. %B Physical Review Letters %V 117 %P 190502 %U https://link.aps.org/doi/10.1103/PhysRevLett.117.190502 %R 10.1103/PhysRevLett.117.190502 %0 Journal Article %J Physical Review Letters %D 2016 %T Quantum Change Point %A Sentís, Gael %A Bagan, Emilio %A Calsamiglia, John %A Chiribella, Giulio %A Muñoz-Tapia, Ramon %B Physical Review Letters %V 117 %8 Jan-10-2016 %U http://link.aps.org/doi/10.1103/PhysRevLett.117.150502http://link.aps.org/article/10.1103/PhysRevLett.117.150502 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.117.150502 %0 Journal Article %J IEEE Transactions on Information Theory %D 2016 %T Quantum Channel Capacities With Passive Environment Assistance %A Karumanchi, Siddharth %A Mancini, Stefano %A Winter, Andreas %A Dong Yang %B IEEE Transactions on Information Theory %V 62 %P 1733-1747 %8 4/2016 %N 4 %! IEEE Trans. Inform. Theory %R 10.1109/TIT.2016.2522192 %0 Journal Article %J Journal of Mathematical Physics %D 2016 %T Quantum learning of classical stochastic processes: The completely positive realization problem %A Monràs, Alex %A Winter, Andreas %B Journal of Mathematical Physics %V 57 %P 015219 %8 01/2016 %N 1 %! J. Math. Phys. %R 10.1063/1.4936935 %0 Journal Article %J Quantum Science and Technology %D 2016 %T Realising a quantum absorption refrigerator with an atom-cavity system %A Mitchison, Mark T. %A Huber, Marcus %A Prior, Javier %A Woods, Mischa P. %A Plenio, Martin B. %X An autonomous quantum thermal machine comprising a trapped atom or ion placed inside an optical cavity is proposed and analysed. Such a machine can operate as a heat engine whose working medium is the quantised atomic motion or as an absorption refrigerator that cools without any work input. Focusing on the refrigerator mode, we predict that it is possible with state-of-the-art technology to cool a trapped ion almost to its motional ground state using a thermal light source such as sunlight. We nonetheless find that a laser or a similar reference system is necessary to stabilise the cavity frequencies. Furthermore, we establish a direct and heretofore unacknowledged connection between the abstract theory of quantum absorption refrigerators and practical sideband cooling techniques. We also highlight and clarify some assumptions underlying several recent theoretical studies on self-contained quantum engines and refrigerators. Our work indicates that cavity quantum electrodynamics is a promising and versatile experimental platform for the study of autonomous thermal machines in the quantum domain. %B Quantum Science and Technology %V 1 %P 015001 %U http://stacks.iop.org/2058-9565/1/i=1/a=015001 %R 10.1088/2058-9565/1/1/015001 %0 Journal Article %J Phys. Rev. Lett. %D 2016 %T Relations between Coherence and Path Information %A E. Bagan %A Bergou, János A. %A Cottrell, Seth S. %A Hillery, Mark %B Phys. Rev. Lett. %I American Physical Society %V 116 %P 160406 %8 Apr %G eng %U http://link.aps.org/doi/10.1103/PhysRevLett.116.160406 %R 10.1103/PhysRevLett.116.160406 %0 Journal Article %J Journal of Physics A: Mathematical and Theoretical %D 2016 %T The role of quantum information in thermodynamics—a topical review %A Goold, John %A Huber, Marcus %A Riera, Arnau %A Rio, Lídia del %A Paul Skrzypczyk %B Journal of Physics A: Mathematical and Theoretical %V 49 %P 143001 %G eng %U http://stacks.iop.org/1751-8121/49/i=14/a=143001?key=crossref.ce80cac2171964cc6d6c51ed12d1e014 %R 10.1088/1751-8113/49/14/143001 %0 Journal Article %J Physical Review Letters %D 2016 %T Schur Complement Inequalities for Covariance Matrices and Monogamy of Quantum Correlations %A Lami, Ludovico %A Hirche, Christoph %A Adesso, Gerardo %A Winter, Andreas %B Physical Review Letters %V 117 %8 Jan-11-2016 %U http://link.aps.org/doi/10.1103/PhysRevLett.117.220502http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevLett.117.220502/fulltexthttp://link.aps.org/article/10.1103/PhysRevLett.117.220502 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.117.220502 %0 Journal Article %J Physical Review Letters %D 2016 %T Should Entanglement Measures be Monogamous or Faithful? %A Lancien, Cécilia %A Di Martino, Sara %A Huber, Marcus %A Piani, Marco %A Adesso, Gerardo %A Winter, Andreas %B Physical Review Letters %V 117 %8 8/2016 %N 6 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.117.060501 %0 Journal Article %J Physical Review Letters %D 2016 %T Steering Maps and Their Application to Dimension-Bounded Steering %A Moroder, Tobias %A Gittsovich, Oleg %A Huber, Marcus %A Uola, Roope %A Gühne, Otfried %B Physical Review Letters %V 116 %G eng %U http://link.aps.org/doi/10.1103/PhysRevLett.116.090403 %R 10.1103/PhysRevLett.116.090403 %0 Journal Article %J Entropy %D 2016 %T Testing a Quantum Heat Pump with a Two-Level Spin %A Correa, Luis %A Mehboudi, Mohammad %B Entropy %V 18 %P 141 %8 4/2016 %U http://www.mdpi.com/1099-4300/18/4/141/htm %N 4 %! Entropy %R 10.3390/e18040141 %0 Journal Article %J Communications in Mathematical Physics %D 2016 %T Tight Uniform Continuity Bounds for Quantum Entropies: Conditional Entropy, Relative Entropy Distance and Energy Constraints %A Winter, Andreas %B Communications in Mathematical Physics %V 347 %P 291-313 %8 08/2016 %U http://arxiv.org/abs/1507.07775 %! Commun. Math. Phys. %R 10.1007/s00220-016-2609-8 %0 Journal Article %J Journal of Cryptology %D 2016 %T Weak Locking Capacity of Quantum Channels Can be Much Larger Than Private Capacity %A Winter, Andreas %B Journal of Cryptology %U http://arxiv.org/abs/1403.6361 %! J Cryptol %R 10.1007/s00145-015-9215-3 %0 Journal Article %J New Journal of Physics %D 2016 %T Witnessing entanglement by proxy %A Bäuml, Stefan %A Bruß, Dagmar %A Huber, Marcus %A Kampermann, Hermann %A Winter, Andreas %B New Journal of Physics %V 18 %P 015002 %8 01/2016 %N 1 %! New J. Phys. %R 10.1088/1367-2630/18/1/015002 %0 Journal Article %J IEEE Transactions on Information Theory %D 2016 %T On {Zero}-{Error} {Communication} via {Quantum} {Channels} in the {Presence} of {Noiseless} {Feedback} %A Duan, R. %A Severini, S. %A Winter, A. %K bipartite equivocation graph %K Bipartite graph %K Capacity planning %K Choi-Kraus operators %K coding theorem %K commutative bipartite graph %K Electronic mail %K encoding %K entanglement-assisted capacity %K feedback %K feedback-assisted capacity %K feedback-assisted zero-error capacity %K information theory %K noiseless feedback channel %K quantum channels %K quantum communication %K quantum entanglement %K Quantum information %K Receivers %K Shannon's zero-error communication %K telecommunication channels %K unlimited quantum capacity %K zero-error capacity %X We initiate the study of zero-error communication via quantum channels when the receiver and the sender have at their disposal a noiseless feedback channel of unlimited quantum capacity, generalizing Shannon's zero-error communication theory with instantaneous feedback. We first show that this capacity is only a function of the linear span of Choi-Kraus operators of the channel, which generalizes the bipartite equivocation graph of a classical channel, and which we dub non-commutative bipartite graph. Then, we go on to show that the feedback-assisted capacity is non-zero (allowing for a constant amount of activating noiseless communication) if and only if the non-commutative bipartite graph is non-trivial, and give a number of equivalent characterizations. This result involves a far-reaching extension of the conclusive exclusion of quantum states. We then present an upper bound on the feedback-assisted zero-error capacity, motivated by a conjecture originally made by Shannon and proved later by Ahlswede. We demonstrate that this bound to have many good properties, including being additive and given by a minimax formula. We also prove a coding theorem showing that this quantity is the entanglement-assisted capacity against an adversarially chosen channel from the set of all channels with the same Choi-Kraus span, which can also be interpreted as the feedback-assisted unambiguous capacity. The proof relies on a generalization of the Postselection Lemma (de Finetti reduction) that allows to reflect additional constraints, and which we believe to be of independent interest. This capacity is a relaxation of the feedback-assisted zero-error capacity; however, we have to leave open the question of whether they coincide in general. We illustrate our ideas with a number of examples, including classical-quantum channels and Weyl diagonal channels, and close with an extensive discussion of open questions. %B IEEE Transactions on Information Theory %V 62 %P 5260–5277 %8 sep %R 10.1109/TIT.2016.2562580 %0 Journal Article %J New Journal of Physics %D 2015 %T Autonomous quantum thermal machine for generating steady-state entanglement %A Bohr Brask, Jonatan %A Haack, Géraldine %A Nicolas Brunner %A Huber, Marcus %B New Journal of Physics %V 17 %P 113029 %8 nov %G eng %U http://stacks.iop.org/1367-2630/17/i=11/a=113029?key=crossref.0c6a75cb628f113c6cbd2a7b0d092f0d %R 10.1088/1367-2630/17/11/113029 %0 Journal Article %J Physical Review A %D 2015 %T Characterizing multipartite entanglement without shared reference frames %A Klöckl, C. %A Huber, M %B Physical Review A %V 91 %8 apr %G eng %U http://link.aps.org/doi/10.1103/PhysRevA.91.042339 %R 10.1103/PhysRevA.91.042339 %0 Journal Article %J New Journal of Physics %D 2015 %T Coherence-assisted single-shot cooling by quantum absorption refrigerators %A Mitchison, Mark T %A Woods, Mischa P %A Prior, Javier %A Huber, Marcus %B New Journal of Physics %V 17 %P 115013 %G eng %U http://stacks.iop.org/1367-2630/17/i=11/a=115013?key=crossref.3c2d7f1bc5de4bbea1cba38fd2236293 %R 10.1088/1367-2630/17/11/115013 %0 Web Page %D 2015 %T Decoupling with random diagonal-unitary matrices %A Yoshifumi Nakata %A Christoph Hirche %A Ciara Morgan %A Winter, Andreas %U http://arxiv.org/abs/1509.05155 %0 Journal Article %J Journal of Mathematical Physics %D 2015 %T Entanglement–breaking indices %A Lami, L. %A Giovannetti, V. %B Journal of Mathematical Physics %V 56 %P 092201 %8 09/2015 %N 9 %! J. Math. Phys. %R 10.1063/1.4931482 %0 Journal Article %J Optica %D 2015 %T Experimental access to higher-dimensional entangled quantum systems using integrated optics %A Christoph Schaeff %A Robert Polster %A Huber, Marcus %A Ramelow, Sven %A Anton Zeilinger %K Integrated optics %K quantum optics %X Integrated optics allows for the generation and control of increasingly complex photonic states on chip-based architectures. Here, we implement two entangled qutrits&\#x2014;a nine-dimensional quantum system&\#x2014;and demonstrate an exceptionally high degree of experimental control. The approach, which is conceptually different to common bulk optical implementations, is heavily based on methods of integrated in-fiber and on-chip technologies and further motivated by methods commonly used in today&\#x2019;s telecommunications industry. The system is composed of an in-fiber source creating entangled qutrit states of any amplitude and phase, and an on-chip integrated general Multiport enabling the realization of any desired local unitary transformation within the two qutrit nine-dimensional Hilbert space. The complete design is readily extendible toward higher dimensions with moderate increase in complexity. Ultimately, our scheme allows for complete on-chip integration. We demonstrate the flexibility and generality of our system by realizing a complete characterization of the two-qutrit space of higher-order Einstein&\#x2013;Podolsky&\#x2013;Rosen correlations. %B Optica %I OSA %V 2 %P 523–529 %8 Jun %G eng %U http://www.osapublishing.org/optica/abstract.cfm?URI=optica-2-6-523 %R 10.1364/OPTICA.2.000523 %0 Journal Article %J Physical Review X %D 2015 %T Extractable Work from Correlations %A Martí Perarnau-Llobet %A Karen V. Hovhannisyan %A Huber, Marcus %A Paul Skrzypczyk %A Nicolas Brunner %A Acín, Antonio %B Physical Review X %V 5 %8 oct %G eng %U http://link.aps.org/doi/10.1103/PhysRevX.5.041011 %R 10.1103/PhysRevX.5.041011 %0 Journal Article %J New Journal of Physics %D 2015 %T A geometric approach to quantum state separation %A E. Bagan %A Yerokhin, V. %A Shehu, A. %A Feldman, E. %A Bergou, J. A. %X Probabilistic quantum state transformations can be characterized by the degree of state separation they provide. This, in turn, sets limits on the success rate of these transformations. We consider optimum state separation of two known pure states in the general case where the known states have arbitrary a priori probabilities. The problem is formulated from a geometric perspective and shown to be equivalent to the problem of finding tangent curves within two families of conics that represent the unitarity constraints and the objective functions to be optimized, respectively. We present the corresponding analytical solutions in various forms. In the limit of perfect state separation, which is equivalent to unambiguous state discrimination, the solution exhibits a phenomenon analogous to a second order symmetry breaking phase transition. We also propose a linear optics implementation of separation which is based on the dual rail representation of qubits and single-photon multiport interferometry. %B New Journal of Physics %V 17 %P 123015 %G eng %U http://stacks.iop.org/1367-2630/17/i=12/a=123015 %0 Web Page %D 2015 %T Implementing unitary 2-designs using random diagonal-unitary matrices %A Yoshifumi Nakata %A Christoph Hirche %A Ciara Morgan %A Winter, Andreas %U http://arxiv.org/abs/1502.07514 %0 Journal Article %J Physical Review Letters %D 2015 %T Individual Quantum Probes for Optimal Thermometry %A Correa, Luis A. %A Mehboudi, Mohammad %A Adesso, Gerardo %A Sanpera, Anna %B Physical Review Letters %V 114 %8 6/2015 %N 22 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.114.220405 %0 Journal Article %J Nature Communications %D 2015 %T Limitations on quantum key repeaters %A Bäuml, Stefan %A Christandl, Matthias %A Horodecki, Karol %A Winter, Andreas %B Nature Communications %V 6 %P 6908 %8 4/2015 %! Nat Comms %R 10.1038/ncomms7908 %0 Journal Article %J Physical Review A %D 2015 %T Mesoscopic superpositions of Tonks-Girardeau states and the Bose-Fermi mapping %A M.A. García-March %A A.Yuste %A B.Juliá-Díaz %A A.Polls %X We study a one-dimensional gas of repulsively interacting ultracold bosons trapped in a double-well potential as the atom-atom interactions are tuned from zero to infinity. We concentrate on the properties of the excited states which evolve from the so-called NOON states to the NOON Tonks-Girardeau states. The relation between the latter and the Bose-Fermi mapping limit is explored. We state under which conditions NOON Tonks-Girardeau states, which are not predicted by the Bose-Fermi mapping, will appear in the spectrum. %B Physical Review A %R http://dx.doi.org/10.1103/PhysRevA.92.033621 %0 Journal Article %J Physical Review E %D 2015 %T Most energetic passive states %A Martí Perarnau-Llobet %A Karen V. Hovhannisyan %A Huber, Marcus %A Paul Skrzypczyk %A Tura, Jordi %A Acín, Antonio %B Physical Review E %V 92 %G eng %U http://link.aps.org/doi/10.1103/PhysRevE.92.042147 %R 10.1103/PhysRevE.92.042147 %0 Journal Article %J Physical Review A %D 2015 %T Quantum-optical channels that output only classical states %A Sabapathy, Krishna Kumar %B Physical Review A %V 92 %8 11/2015 %N 5 %! Phys. Rev. A %R 10.1103/PhysRevA.92.052301 %0 Journal Article %J Journal of Physics A: Mathematical and Theoretical %D 2015 %T Relaxations of separability in multipartite systems: Semidefinite programs, witnesses and volumes %A Lancien, Cécilia %A Gühne, Otfried %A Sengupta, Ritabrata %A Huber, Marcus %B Journal of Physics A: Mathematical and Theoretical %V 48 %P 505302 %G eng %U http://stacks.iop.org/1751-8121/48/i=50/a=505302?key=crossref.2e93a28de5494676a0b866fa95ccebfd %R 10.1088/1751-8113/48/50/505302 %0 Journal Article %J IEEE Transactions on Information Theory %D 2015 %T Strong Converse for the Classical Capacity of Optical Quantum Communication Channels %A Bardhan, Bhaskar Roy %A Garcia-Patron, Raul %A Wilde, Mark M. %A Winter, Andreas %B IEEE Transactions on Information Theory %V 61 %P 1842-1850 %8 4/2015 %N 4 %! IEEE Trans. Inform. Theory %R 10.1109/TIT.2015.2403840 %0 Journal Article %J New Journal of Physics %D 2015 %T Thermodynamic cost of creating correlations %A Huber, Marcus %A Martí Perarnau-Llobet %A Karen V. Hovhannisyan %A Paul Skrzypczyk %A Klöckl, Claude %A Nicolas Brunner %A Acín, Antonio %B New Journal of Physics %V 17 %P 065008 %G eng %U http://stacks.iop.org/1367-2630/17/i=6/a=065008?key=crossref.b9c20f4bb628b09efcb912347c01851e %R 10.1088/1367-2630/17/6/065008 %0 Journal Article %J Physical Review E %D 2015 %T Thermodynamics of creating correlations: {Limitations} and optimal protocols %A Bruschi, David Edward %A Martí Perarnau-Llobet %A Friis, Nicolai %A Karen V. Hovhannisyan %A Huber, Marcus %B Physical Review E %V 91 %8 mar %G eng %U http://link.aps.org/doi/10.1103/PhysRevE.91.032118 %R 10.1103/PhysRevE.91.032118 %0 Journal Article %J New Journal of Physics %D 2015 %T Thermometry precision in strongly correlated ultracold lattice gases %A Mehboudi, M %A Moreno-Cardoner, M. %A Chiara, G De %A Sanpera, A. %B New Journal of Physics %V 17 %P 055020 %8 05/2015 %N 5 %! New J. Phys. %R 10.1088/1367-2630/17/5/055020 %0 Journal Article %J Physical Review A %D 2014 %T Best separable approximation of multipartite diagonal symmetric states %A Quesada, Ruben %A Sanpera, Anna %X The structural study of entanglement in multipartite systems is hindered by the lack of necessary and sufficient operational criteria able to discriminate among the various entanglement properties of a given mixed state. Here, we pursue a different route to the study of multipartite entanglement based on the closeness of a multipartite state to the set of separable ones. In particular, we analyze multipartite diagonal symmetric N-qubit states and provide the analytical expression for their best separable approximation (BSA, [Phys. Rev. Lett. 80, 2261 (1998)]), that is, their unique convex decomposition into a separable part and an entangled part, with maximal weight of the separable part. %B Physical Review A %V 89 %8 5/2014 %N 5 %! Phys. Rev. A %R 10.1103/PhysRevA.89.052319 %0 Journal Article %J IEEE Transactions on Information Theory %D 2014 %T Bounds on entanglement assisted source-channel coding via the Lovasz theta number and its variants %A Toby S. Cubitt %A Laura Mancinska %A David Roberson %A Stahlke, Daniel %A Winter, Andreas %B IEEE Transactions on Information Theory %V 60 %P 7330-7344 %8 11/2014 %R 10.1109/TIT.2014.2349502 %0 Journal Article %J Physical Review B %D 2014 %T Case study of the uniaxial anisotropic spin-1 bilinear-biquadratic Heisenberg model on a triangular lattice %A Moreno-Cardoner, M. %A Perrin, H. %A Paganelli, S. %A De Chiara, G. %A Sanpera, A. %X We study the spin-1 model on a triangular lattice in the presence of a uniaxial anisotropy field using a cluster mean-field (CMF) approach. The interplay among antiferromagnetic exchange, lattice geometry, and anisotropy forces Gutzwiller mean-field approaches to fail in a certain region of the phase diagram. There, the CMF method yields two supersolid phases compatible with those present in the spin−1/2 XXZ model onto which the spin-1 system maps. Between these two supersolid phases, the three-sublattice order is broken and the results of the CMF approach depend heavily on the geometry and size of the cluster. We discuss the possible presence of a spin liquid in this region. %B Physical Review B %V 90 %8 10/2014 %N 14 %! Phys. Rev. B %R 10.1103/PhysRevB.90.144409 %0 Conference Proceedings %B 2014 IEEE International Symposium on Information Theory (ISIT) %D 2014 %T Constant compositions in the sphere packing bound for classical-quantum channels %A Dalai, Marco %A Winter, Andreas %B 2014 IEEE International Symposium on Information Theory (ISIT) %I IEEE %C Honolulu, HI, USA %P 151-155 %8 07/2014 %R 10.1109/ISIT.2014.6874813 %0 Thesis %D 2014 %T Dealing with ignorance: universal discrimination, learning and quantum correlations %A Sentís, Gael %X The problem of discriminating the state of a quantum system among a number of hypothetical states is usually addressed under the assumption that one has perfect knowledge of the possible states of the system. In this thesis, I analyze the role of the prior information available in facing such problems, and consider scenarios where the information regarding the possible states is incomplete. In front of a complete ignorance of the possible states' identity, I discuss a quantum "programmable" discrimination machine for qubit states that accepts this information as input programs using a quantum encoding, rather than as a classical description. The optimal performance of these machines is studied for general qubit states when several copies are provided, in the schemes of unambiguous, minimum-error, and error-margin discrimination. Then, this type of automation in discrimination tasks is taken further. By realizing a programmable machine as a device that is trained through quantum information to perform a specific task, I propose a quantum "learning" machine for classifying qubit states that does not require a quantum memory to store the qubit programs and, nevertheless, performs as good as quantum mechanics permits. Such learning machine thus allows for several optimal uses with no need for retraining. A similar learning scheme is also discussed for coherent states of light. I present it in the context of the readout of a classical memory by means of classically correlated coherent signals, when these are produced by an imperfect source. I show that, in this case, the retrieval of information stored in the memory can be carried out more accurately when fully general quantum measurements are used. Finally, as a transversal topic, I propose an efficient algorithmic way of decomposing any quantum measurement into convex combinations of simpler (extremal) measurements. %I Universitat Autònoma de Barcelona %8 06/2014 %U http://arxiv.org/abs/1407.4690 %0 Journal Article %J Physical Review E %D 2014 %T Entanglement enhances cooling in microscopic quantum refrigerators %A Nicolas Brunner %A Huber, Marcus %A Noah Linden %A Sandu Popescu %A Ralph Silva %A Paul Skrzypczyk %X Small self-contained quantum thermal machines function without external source of work or control, but using only incoherent interactions with thermal baths. Here we investigate the role of entanglement in a small self-contained quantum refrigerator. We first show that entanglement is detrimental as far as efficiency is concerned---fridges operating at efficiencies close to the Carnot limit do not feature any entanglement. Moving away from the Carnot regime, we show that entanglement can enhance cooling and energy transport. Hence a truly quantum refrigerator can outperform a classical one. Furthermore, the amount of entanglement alone quantifies the enhancement in cooling. %B Physical Review E %V 89 %8 3/2014 %N 3 %! Phys. Rev. E %R 10.1103/PhysRevE.89.032115 %0 Journal Article %J Journal of Statistical Mechanics: Theory and Experiment %D 2014 %T Entanglement properties of spin models in triangular lattices %A Moreno-Cardoner, M. %A Paganelli, S. %A De Chiara, G. %A Sanpera, A. %X The different quantum phases appearing in strongly correlated systems as well as their transitions are closely related to the entanglement shared between their constituents. In 1D systems, it is well established that the entanglement spectrum is linked to the symmetries that protect the different quantum phases. This relation extends even further at the phase transitions where a direct link associates the entanglement spectrum to the conformal field theory describing the former. For 2D systems much less is known. The lattice geometry becomes a crucial aspect to consider when studying entanglement and phase transitions. Here, we analyze the entanglement properties of triangular spin lattice models by also considering concepts borrowed from quantum information theory such as geometric entanglement. %B Journal of Statistical Mechanics: Theory and Experiment %V 2014 %P P10008 %8 10/2014 %N 10 %! J. Stat. Mech. %R 10.1088/1742-5468/2014/10/P10008 %0 Journal Article %J Communications in Mathematical Physics %D 2014 %T Everything You Always Wanted to Know About LOCC (But Were Afraid to Ask) %A Eric Chitambar %A Debbie Leung %A Mančinska, Laura %A Maris Ozols %A Winter, Andreas %B Communications in Mathematical Physics %V 328 %P 303-326 %8 5/2014 %N 1 %! Commun. Math. Phys. %R 10.1007/s00220-014-1953-9 %0 Journal Article %J Journal of Physics A: Mathematical and Theoretical %D 2014 %T Fifty years of Bell’s theorem %A Nicolas Brunner %A Gühne, Otfried %A Huber, Marcus %B Journal of Physics A: Mathematical and Theoretical %V 47 %P 420301 %8 oct %G eng %U http://stacks.iop.org/1751-8121/47/i=42/a=420301?key=crossref.9d548f14fc7338685392dcd6228de98d %R 10.1088/1751-8113/47/42/420301 %0 Journal Article %J IEEE Transactions on Information Theory %D 2014 %T Full Security of Quantum Key Distribution From No-Signaling Constraints %A Masanes, Lluis %A Renner, Renato %A Christandl, Matthias %A Winter, Andreas %A Barrett, Jonathan %B IEEE Transactions on Information Theory %V 60 %P 4973-4986 %8 8/2014 %N 8 %! IEEE Trans. Inform. Theory %R 10.1109/TIT.2014.2329417 %0 Journal Article %J Proceedings of the National Academy of Sciences %D 2014 %T Generation and confirmation of a (100 x 100)-dimensional entangled quantum system %A Krenn, M. %A Huber, M %A Fickler, R. %A Lapkiewicz, R. %A Ramelow, S. %A Zeilinger, A. %X Entangled quantum systems have properties that have fundamentally overthrown the classical worldview. Increasing the complexity of entangled states by expanding their dimensionality allows the implementation of novel fundamental tests of nature, and moreover also enables genuinely new protocols for quantum information processing. Here we present the creation of a (100x100)-dimensional entangled quantum system, using spatial modes of photons. For its verification we develop a novel nonlinear criterion which infers entanglement dimensionality of a global state by using only information about its subspace correlations. This allows very practical experimental implementation as well as highly efficient extraction of entanglement dimensionality information. Applications in quantum cryptography and other protocols are very promising. %B Proceedings of the National Academy of Sciences %V 111 %P 6243 - 6247 %8 04/2014 %N 17 %! Proceedings of the National Academy of Sciences %R 10.1073/pnas.1402365111 %0 Journal Article %J Physical Review Letters %D 2014 %T Graph-Theoretic Approach to Quantum Correlations %A Cabello, Adán %A Severini, Simone %A Winter, Andreas %B Physical Review Letters %V 112 %P 040401 %8 1/2014 %N 4 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.112.040401 %0 Journal Article %J Linear Algebra and its Applications %D 2014 %T Inequalities for the ranks of multipartite quantum states %A Josh Cadney %A Huber, Marcus %A Noah Linden %A Winter, Andreas %X We investigate relations between the ranks of marginals of multipartite quantum states. These are the Schmidt ranks across all possible bipartitions and constitute a natural quantification of multipartite entanglement dimensionality. We show that there exist inequalities constraining the possible distribution of ranks. This is analogous to the case of von Neumann entropy (\alpha-R\'enyi entropy for \alpha=1), where nontrivial inequalities constraining the distribution of entropies (such as e.g. strong subadditivity) are known. It was also recently discovered that all other \alpha-R\'enyi entropies for α∈(0,1)∪(1,∞) satisfy only one trivial linear inequality (non-negativity) and the distribution of entropies for α∈(0,1) is completely unconstrained beyond non-negativity. Our result resolves an important open question by showing that also the case of \alpha=0 (logarithm of the rank) is restricted by nontrivial linear relations and thus the cases of von Neumann entropy (i.e., \alpha=1) and 0-R\'enyi entropy are exceptionally interesting measures of entanglement in the multipartite setting. %B Linear Algebra and its Applications %V 452 %P 153 - 171 %8 07/2014 %! Linear Algebra and its Applications %R 10.1016/j.laa.2014.03.035 %0 Journal Article %J Nature Communications %D 2014 %T Interface between path and orbital angular momentum entanglement for high-dimensional photonic quantum information %A Robert Fickler %A Radek Lapkiewicz %A Huber, Marcus %A Lavery, Martin P.J. %A Padgett, Miles J. %A Anton Zeilinger %X Photonics has become a mature field of quantum information science, where integrated optical circuits offer a way to scale the complexity of the setup as well as the dimensionality of the quantum state. On photonic chips, paths are the natural way to encode information. To distribute those high-dimensional quantum states over large distances, transverse spatial modes, like orbital angular momentum (OAM) possessing Laguerre Gauss modes, are favourable as flying information carriers. Here we demonstrate a quantum interface between these two vibrant photonic fields. We create three-dimensional path entanglement between two photons in a non-linear crystal and use a mode sorter as the quantum interface to transfer the entanglement to the OAM degree of freedom. Thus our results show a novel, flexible way to create high-dimensional spatial mode entanglement. Moreover, they pave the way to implement broad complex quantum networks where high-dimensionally entangled states could be distributed over distant photonic chips. %B Nature Communications %V 5 %8 7/2014 %! Nat Comms %R 10.1038/ncomms5502 %0 Journal Article %J Physical Review A %D 2014 %T Long-range multipartite entanglement close to a first-order quantum phase transition %A Stasińska, J. %A Rogers, B. %A M. Paternostro %A De Chiara, G. %A Sanpera, A. %X We provide insight into the quantum correlations structure present in strongly correlated systems beyond the standard framework of bipartite entanglement. To this aim we first exploit rotationally invariant states as a test bed to detect genuine tripartite entanglement beyond the nearest neighbor in spin-1/2 models. Then we construct in a closed analytical form a family of entanglement witnesses which provides a sufficient condition to determine if a state of a many-body system formed by an arbitrary number of spin-1/2 particles possesses genuine tripartite entanglement, independently of the details of the model. We illustrate our method by analyzing in detail the anisotropic XXZ spin chain close to its phase transitions, where we demonstrate the presence of long-range multipartite entanglement near the critical point and the breaking of the symmetries associated with the quantum phase transition. %B Physical Review A %V 89 %8 3/2014 %N 3 %! Phys. Rev. A %R 10.1103/PhysRevA.89.032330 %0 Journal Article %J IEEE Transactions on Information Theory %D 2014 %T ‘Pretty strong’ converse for the quantum capacity of degradable channels %A Ciara Morgan %A Winter, Andreas %B IEEE Transactions on Information Theory %V 60 %P 317-333 %8 01/2014 %R 10.1109/TIT.2013.2288971 %0 Journal Article %J Physical Review Letters %D 2014 %T Probabilistic Metrology Attains Macroscopic Cloning of Quantum Clocks %A Gendra, B. %A Calsamiglia, John %A Muñoz-Tapia, R. %A E. Bagan %A Chiribella, G. %B Physical Review Letters %V 113 %G eng %U http://link.aps.org/doi/10.1103/PhysRevLett.113.260402 %R 10.1103/PhysRevLett.113.260402 %0 Journal Article %J Nature Physics %D 2014 %T Quantum information: The occasional super clock-cloner %A Calsamiglia, John %B Nature Physics %V 10 %P 91 %8 2/2014 %! Nat Phys %R 10.1038/nphys2853 %0 Journal Article %J IEEE Transactions on Information Theory %D 2014 %T The Quantum Reverse Shannon Theorem and Resource Tradeoffs for Simulating Quantum Channels %A Bennett, Charles H. %A Devetak, Igor %A Harrow, Aram W. %A Shor, Peter W. %A Winter, Andreas %B IEEE Transactions on Information Theory %V 60 %P 2926-2959 %8 5/2014 %N 5 %! IEEE Trans. Inform. Theory %R 10.1109/TIT.2014.2309968 %0 Journal Article %J New Journal of Physics %D 2014 %T Quantumness of correlations, quantumness of ensembles and quantum data hiding %A Piani, M %A Narasimhachar, V %A Calsamiglia, John %B New Journal of Physics %V 16 %P 113001 %G eng %U http://stacks.iop.org/1367-2630/16/i=11/a=113001?key=crossref.9f5154f8c85e7f9471e2931d9a02df88 %R 10.1088/1367-2630/16/11/113001 %0 Journal Article %J Communications in Mathematical Physics %D 2014 %T Relative Entropy and Squashed Entanglement %A Ke Li %A Winter, Andreas %B Communications in Mathematical Physics %V 326 %P 63-80 %8 2/2014 %N 1 %! Commun. Math. Phys. %R 10.1007/s00220-013-1871-2 %0 Journal Article %J Physical Review Letters %D 2014 %T Steering Bound Entangled States: A Counterexample to the Stronger Peres Conjecture %A Moroder, Tobias %A Gittsovich, Oleg %A Huber, Marcus %A Gühne, Otfried %X Quantum correlations are at the heart of many applications in quantum information science and, at the same time, they form the basis for discussions about genuine quantum effects and their difference to classical physics. On one hand, entanglement theory provides the tools to quantify correlations in information processing and many results have been obtained to discriminate useful entanglement, which can be distilled to a pure form, from bound entanglement, being of limited use in many applications. On the other hand, for discriminating quantum phenomena from their classical counterparts, Schr\"odinger and Bell introduced the notions of steering and local hidden variable models. We provide a method to generate systematically bound entangled quantum states which can still be used for steering and therefore to rule out local hidden state models. This sheds light on the relations between the various views on quantum correlations and disproves a widespread conjecture known as the stronger Peres conjecture. For practical applications, it implies that even the weakest form of entanglement can be certified in a semi-device independent way. %B Physical Review Letters %V 113 %8 8/2014 %N 5 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.113.050404 %0 Journal Article %J Communications in Mathematical Physics %D 2014 %T Strong Converse for the Classical Capacity of Entanglement-Breaking and Hadamard Channels via a Sandwiched Rényi Relative Entropy %A Wilde, Mark M. %A Winter, Andreas %A Dong Yang %B Communications in Mathematical Physics %V 331 %P 593-622 %8 10/2014 %N 2 %! Commun. Math. Phys. %R 10.1007/s00220-014-2122-x %0 Journal Article %J Problems of Information Transmission %D 2014 %T Strong converse for the classical capacity of the pure-loss bosonic channel %A Wilde, Mark M. %A Winter, Andreas %B Problems of Information Transmission %V 50 %P 117-132 %8 4/2014 %N 2 %! Probl Inf Transm %R 10.1134/S003294601402001X %0 Conference Proceedings %B 9th Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2014) %D 2014 %T Strong Converse for the Quantum Capacity of the Erasure Channel for Almost All Codes %A Wilde, Mark M. %A Winter, Andreas %E Steve T Flammia %E Harrow, Aram W. %B 9th Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2014) %I LIPICS %C Singapore %V 27 %P 52-66 %R 10.4230/LIPIcs.TQC.2014.52 %0 Journal Article %J Physical Review A %D 2014 %T Unified approach to entanglement criteria using the Cauchy-Schwarz and Hölder inequalities %A Wölk, Sabine %A Huber, Marcus %A Gühne, Otfried %X We present a unified approach to different recent entanglement criteria. Although they were developed in different ways, we show that they are all applications of a more general principle given by the Cauchy-Schwarz inequality. We explain this general principle and show how to use it to derive not only already known entanglement criteria, but also criteria which were unknown so far. We systematically investigate its potential and limitations to detect bipartite and multipartite entanglement. Furthermore, we describe how to apply our generalized entanglement detection scheme to find the measurement directions to verify the entanglement of a given state experimentally. %B Physical Review A %V 90 %8 8/2014 %N 2 %! Phys. Rev. A %R 10.1103/PhysRevA.90.022315 %0 Journal Article %J Journal of Physics A: Mathematical and Theoretical %D 2014 %T What does an experimental test of quantum contextuality prove or disprove? %A Winter, Andreas %B Journal of Physics A: Mathematical and Theoretical %V 47 %P 424031 %8 10/2014 %N 42 %! J. Phys. A: Math. Theor. %R 10.1088/1751-8113/47/42/424031 %0 Journal Article %J Physical Review Letters %D 2014 %T Witnessing Genuine Multipartite Entanglement with Positive Maps %A Huber, Marcus %A Sengupta, Ritabrata %X We derive a general framework that connects every positive map with a corresponding witness for partial separability in multipartite quantum systems. We show that many previous approaches were intimately connected to the witnesses derived from partial transposition and that such criteria can easily be outperformed in higher dimensions by non-decomposable maps. As an exemplary case we present a witness that is capable of detecting genuine multipartite entanglement in bound entangled states. %B Physical Review Letters %V 113 %8 9/2014 %N 10 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.113.100501 %0 Journal Article %J Physical Review Letters %D 2013 %T Bose-Glass Phases of Ultracold Atoms due to Cavity Backaction %A Hessam Habibian %A André Winter %A Paganelli, Simone %A Heiko Rieger %A Giovanna Morigi %X We determine the quantum ground-state properties of ultracold bosonic atoms interacting with the mode of a high-finesse resonator. The atoms are confined by an external optical lattice, whose period is incommensurate with the cavity mode wavelength, and are driven by a transverse laser, which is resonant with the cavity mode. While for pointlike atoms photon scattering into the cavity is suppressed, for sufficiently strong lasers quantum fluctuations can support the buildup of an intracavity field, which in turn amplifies quantum fluctuations. The dynamics is described by a Bose-Hubbard model where the coefficients due to the cavity field depend on the atomic density at all lattice sites. Quantum Monte Carlo simulations and mean-field calculations show that, for large parameter regions, cavity backaction forces the atoms into clusters with a checkerboard density distribution. Here, the ground state lacks superfluidity and possesses finite compressibility, typical of a Bose glass. This system constitutes a novel setting where quantum fluctuations give rise to effects usually associated with disorder. %B Physical Review Letters %V 110 %8 2/2013 %U http://prl.aps.org/abstract/PRL/v110/i7/e075304 %N 7 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.110.075304 %0 Journal Article %J Journal of Physics B: Atomic, Molecular and Optical Physics %D 2013 %T Bound entanglement in the Jaynes–Cummings model %A Quesada, Nicolás %A Sanpera, Anna %B Journal of Physics B: Atomic, Molecular and Optical Physics %V 46 %P 224002 %8 11/2013 %N 22 %! J. Phys. B: At. Mol. Opt. Phys. %R 10.1088/0953-4075/46/22/224002 %0 Journal Article %J Journal of Physics A: Mathematical and Theoretical %D 2013 %T Decomposition of any quantum measurement into extremals %A Sentís, G %A Gendra, B. %A Bartlett, S D %A Doherty, A C %B Journal of Physics A: Mathematical and Theoretical %V 46 %P 375302 %8 09/2013 %N 37 %! J. Phys. A: Math. Theor. %R 10.1088/1751-8113/46/37/375302 %0 Journal Article %J Communications in Mathematical Physics %D 2013 %T Distinguishing Multi-Partite States by Local Measurements %A Lancien, Cécilia %A Winter, Andreas %B Communications in Mathematical Physics %V 323 %P 555 - 573 %8 10/2013 %N 2 %! Commun. Math. Phys. %R 10.1007/s00220-013-1779-x %0 Journal Article %J Physical Review A %D 2013 %T Entangled singularity patterns of photons in {Ince}-{Gauss} modes %A Krenn, Mario %A Robert Fickler %A Huber, Marcus %A Radek Lapkiewicz %A Plick, William %A Ramelow, Sven %A Anton Zeilinger %B Physical Review A %V 87 %8 jan %G eng %U http://link.aps.org/doi/10.1103/PhysRevA.87.012326 %R 10.1103/PhysRevA.87.012326 %0 Journal Article %J Physical Review Letters %D 2013 %T Entanglement Generation is Not Necessary for Optimal Work Extraction %A Hovhannisyan, Karen %A Martí Perarnau-Llobet %A Huber, Marcus %A Acín, Antonio %X We consider reversible work extraction from identical quantum batteries. From an ensemble of individually passive states, work can be produced only via global unitary (and thus entangling) operations. However, we show here that there always exists a method to extract all possible work without creating any entanglement, at the price of generically requiring more operations (i.e. additional time). We then study faster methods to extract work and provide a quantitative relation between the amount of generated multipartite entanglement and extractable work. Our results suggest a general relation between entanglement generation and the power of work extraction. %B Physical Review Letters %V 111 %8 12/2013 %N 24 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.111.240401 %0 Journal Article %J Physical Review A %D 2013 %T Entropy vector formalism and the structure of multidimensional entanglement in multipartite systems %A Huber, Marcus %A Martí Perarnau-Llobet %A de Vicente, Julio I. %B Physical Review A %V 88 %8 10/2013 %U http://arxiv.org/abs/1307.3541 %N 4 %! Phys. Rev. A %R 10.1103/PhysRevA.88.042328 %0 Journal Article %J Quantum Information Processing %D 2013 %T Examining the dimensionality of genuine multipartite entanglement %A Spengler, Christoph %A Huber, Marcus %A Gabriel, Andreas %A Hiesmayr, Beatrix C. %B Quantum Information Processing %V 12 %P 269–278 %G eng %U http://link.springer.com/10.1007/s11128-012-0369-8 %R 10.1007/s11128-012-0369-8 %0 Book Section %B Information Theory, Combinatorics, and Search Theory: In Memory of Rudolf Ahlswede %D 2013 %T Identification via Quantum Channels %A Winter, Andreas %E Aydinian, Harout %E Cicalese, Ferdinando %E Deppe, Christian %B Information Theory, Combinatorics, and Search Theory: In Memory of Rudolf Ahlswede %S Lecture Notes in Computer Science %I Springer Berlin Heidelberg %C Berlin, Heidelberg %V LNCS 7777 %P 217 - 233 %@ 978-3-642-36899-8 %R 10.1007/978-3-642-36899-8_9 %0 Journal Article %J Physical Review A %D 2013 %T Programmable discrimination with an error margin %A Sentís, G %A E. Bagan %A Calsamiglia, John %A Muñoz-Tapia, R. %B Physical Review A %V 88 %P 052304 %8 11/2013 %N 5 %! Phys. Rev. A %R 10.1103/PhysRevA.88.052304 %0 Journal Article %J New Journal of Physics %D 2013 %T Proving the generation of genuine multipartite entanglement in a single-neutron interferometer experiment %A Erdösi, Daniel %A Huber, Marcus %A Hiesmayr, Beatrix C. %A Hasegawa, Yuji %B New Journal of Physics %V 15 %P 023033 %G eng %U http://stacks.iop.org/1367-2630/15/i=2/a=023033?key=crossref.b714f63bb49eb9e64ce9a70aa4a6ddfe %R 10.1088/1367-2630/15/2/023033 %0 Conference Proceedings %B 8th Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2013) %D 2013 %T The Quantum Entropy Cone of Stabiliser States %A Noah Linden %A Frantisek Matus %A Mary Beth Ruskai %A Winter, Andreas %E Severini, Simone %E Fernando Brandao %B 8th Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2013) %S LIPICS %C Guelph, ON %V 22 %P 270-284 %8 02/2013 %R 10.4230/LIPIcs.TQC.2013.270 %0 Journal Article %J New Journal of Physics %D 2013 %T Quantum hypergraph states %A Rossi, M %A Huber, M %A Bruß, D. %A Macchiavello, C. %B New Journal of Physics %V 15 %P 113022 %8 11/2013 %N 11 %! New J. Phys. %R 10.1088/1367-2630/15/11/113022 %0 Journal Article %J Physical Review Letters %D 2013 %T Quantum Metrology Assisted by Abstention %A Gendra, B. %A Ronco-Bonvehi, E. %A Calsamiglia, John %A Muñoz-Tapia, R. %A E. Bagan %X The main goal of quantum metrology is to obtain accurate values of physical parameters using quantum probes. In this context, we show that abstention, i.e., the possibility of getting an inconclusive answer at readout, can drastically improve the measurement precision and even lead to a change in its asymptotic behavior, from the shot-noise to the Heisenberg scaling. We focus on phase estimation and quantify the required amount of abstention for a given precision. We also develop analytical tools to obtain the asymptotic behavior of the precision and required rate of abstention for arbitrary pure states. %B Physical Review Letters %V 110 %P 100501 %8 3/2013 %U http://prl.aps.org/abstract/PRL/v110/i10/e100501 %N 10 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.110.100501 %0 Journal Article %J Phyiscal Review A %D 2013 %T Quantum phases of incommensurate optical lattices due to cavity backaction %A Hessam Habibian %A André Winter %A Paganelli, Simone %A Heiko Rieger %A Giovanna Morigi %B Phyiscal Review A %V 88 %P 043618 %0 Journal Article %J IEEE Transactions on Information Theory %D 2013 %T Quantum Rate-Distortion Coding With Auxiliary Resources %A Wilde, Mark M. %A Datta, Nilanjana %A Hsieh, Min-Hsiu %A Winter, Andreas %B IEEE Transactions on Information Theory %V 59 %P 6755 - 6773 %8 10/2013 %N 10 %! IEEE Trans. Inform. Theory %R 10.1109/TIT.2013.2271772 %0 Journal Article %J Physical Review Letters %D 2013 %T Quantum State Cloning Using Deutschian Closed Timelike Curves %A Brun, Todd A. %A Wilde, Mark M. %A Winter, Andreas %B Physical Review Letters %V 111 %P 190401 %8 11/2013 %N 19 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.111.190401 %0 Journal Article %J Journal of Mathematical Physics %D 2013 %T Quantum-to-classical rate distortion coding %A Datta, Nilanjana %A Hsieh, Min-Hsiu %A Wilde, Mark M. %A Winter, Andreas %B Journal of Mathematical Physics %V 54 %P 042201 %8 2013 %N 4 %! J. Math. Phys. %R 10.1063/1.4798396 %0 Journal Article %J Physical Review A %D 2013 %T Routing quantum information in spin chains %A Paganelli, Simone %A Lorenzo, Salvatore %A Apollaro, Tony J. G. %A Plastina, Francesco %A Giorgi, Gian Luca %B Physical Review A %V 87 %8 6/2013 %N 6 %! Phys. Rev. A %R 10.1103/PhysRevA.87.062309 %0 Journal Article %J Physical Review B %D 2013 %T Scaling of the entanglement spectrum near quantum phase transitions %A Lepori, L. %A De Chiara, G. %A Sanpera, A. %B Physical Review B %V 87 %8 6/2013 %N 23 %! Phys. Rev. B %R 10.1103/PhysRevB.87.235107 %0 Journal Article %J Physical Review Letters %D 2013 %T Structure of Multidimensional Entanglement in Multipartite Systems %A Huber, Marcus %A de Vicente, Julio I. %X We explore the structure of multipartite quantum systems which are entangled in multiple degrees of freedom. We find necessary and sufficient conditions for the characterization of tripartite systems and necessary conditions for any number of parties. Furthermore we develop a framework of multilevel witnesses for efficient discrimination and quantification of multidimensional entanglement that is applicable for an arbitrary number of systems and dimensions. %B Physical Review Letters %V 110 %8 1/2013 %N 3 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.110.030501 %0 Journal Article %J Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences %D 2013 %T The structure of Renyi entropic inequalities %A Linden, N. %A Mosonyi, M. %A Winter, A. %B Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences %V 469 %P 20120737 %8 10/2013 %N 2158472222 %! Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences %R 10.1098/rspa.2012.0737 %0 Journal Article %J Physical Review A %D 2013 %T Weak randomness in device-independent quantum key distribution and the advantage of using high-dimensional entanglement %A Huber, Marcus %A Pawłowski, Marcin %B Physical Review A %V 88 %8 9/2013 %N 3 %! Phys. Rev. A %R 10.1103/PhysRevA.88.032309 %0 Journal Article %J IEEE Transactions on Information Theory %D 2013 %T Zero-Error Communication via Quantum Channels, Noncommutative Graphs, and a Quantum Lovász Number %A Duan, Runyao %A Severini, Simone %A Winter, Andreas %B IEEE Transactions on Information Theory %V 59 %P 1164 - 1174 %8 02/2013 %N 2 %! IEEE Trans. Inform. Theory %R 10.1109/TIT.2012.2221677 %0 Journal Article %J New Journal of Physics %D 2012 %T Beating noise with abstention in state estimation %A Gendra, Bernat %A Ronco-Bonvehi, Elio %A Calsamiglia, John %A Muñoz-Tapia, Ramon %A Bagan, Emilio %B New Journal of Physics %V 14 %P 105015 %U http://iopscience.iop.org/1367-2630/14/10/105015 %R 10.1088/1367-2630/14/10/105015 %0 Journal Article %J New Journal of Physics %D 2012 %T Beyond pure state entanglement for atomic ensembles %A Julia Stasińska %A Paganelli, Simone %A Sanpera, Anna %X We analyze multipartite entanglement between atomic ensembles within quantum matter–light interfaces. In our proposal, a polarized light beam crosses sequentially several polarized atomic ensembles impinging on each of them at a given angle $\alpha$ i . These angles are crucial parameters for shaping the entanglement since they are directly connected to the appropriate combinations of the collective atomic spins that are squeezed. We exploit such a scheme to go beyond the pure state paradigm proposing realistic experimental settings to address multipartite mixed state entanglement in continuous variables. %B New Journal of Physics %V 14 %P 033034 %U http://stacks.iop.org/1367-2630/14/i=3/a=033034 %R 10.1088/1367-2630/14/3/033034 %0 Journal Article %J J. Math. Phys. %D 2012 %T Convergence of expansions in Schrödinger and Dirac eigenfunctions, with an application to the R-matrix theory %A Julia Stasińska %B J. Math. Phys. %V 53 %P 022101 %U http://arxiv.org/pdf/0912.2909v3 %R 10.1063/1.3679763 %0 Thesis %B Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics %D 2012 %T Entanglement distribution in quantum complex networks %A Cuquet, Martí %Y Calsamiglia, John %X This thesis deals with the study of quantum networks with a complex structure, the implications this structure has in the distribution of entanglement and how their functioning can be enhanced by operating in the quantum regime. We first consider a complex network of bipartite states, both pure and mixed, and study the distribution of long-distance entanglement. Then, we move to a network with noisy channels and study the creation and distribution of large, multipartite states. The work contained in this thesis is primarily motivated by the idea that the interplay between quantum information and complex networks may give rise to a new understanding and characterization of natural systems. Complex networks are of particular importance in communication infrastructures, as most present telecommunication networks have a complex structure. In the case of quantum networks, which are the necessary framework for distributed quantum processing and for quantum communication, it is very plausible that in the future they acquire a complex topology resembling that of existing networks, or even that methods will be developed to use current infrastructures in the quantum regime. A central task in quantum networks is to devise strategies to distribute entanglement among its nodes. In the first part of this thesis, we consider the distribution of bipartite entanglement as an entanglement percolation process in a complex network. Within this approach, perfect entanglement is established probabilistically between two arbitrary nodes. We see that for large networks, the probability of doing so is a constant strictly greater than zero (and independent of the size of the network) if the initial amount of entanglement is above a certain critical value. Quantum mechanics offer here the possibility to change the structure of the network without need to establish new, "physical" channels. By a proper local transformation of the network, the critical entanglement can be decreased and the probability increased. We apply this transformation to complex network models with arbitrary degree distribution. In the case of a noisy network of mixed states, we see that for some classes of states, the same approach of entanglement percolation can be used. For general mixed states, we consider a limited-path-length entanglement percolation constrained by the amount of noise in the connections. We see how complex networks still offer a great advantage in the probability of connecting two nodes. In the second part, we move to the multipartite scenario. We study the creation and distribution of graph states with a structure that mimic the underlying communication network. In this case, we use an arbitrary complex network of noisy channels, and consider that operations and measurements are also noisy. We propose an efficient scheme to distribute and purify small subgraphs, which are then merged to reproduce the desired state. We compare this approach with two bipartite protocols that rely on a central station and full knowledge of the network structure. We show that the fidelity of the generated graphs can be written as the partition function of a classical disordered spin system (a spin glass), and its decay rate is the analog of the free energy. Applying the three protocols to a one-dimensional network and to complex networks, we see that they are all comparable, and in some cases the proposed subgraph protocol, which needs only local information of the network, performs even better. %B Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics %I Universitat Autònoma de Barcelona %C Barcelona %V Doctorat en Física %8 11/2012 %9 PhD %0 Thesis %B Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics %D 2012 %T Entanglement percolation with graph-states' %A Franquet, Abert %Y Calsamiglia, John %B Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics %I Universitat Autònoma de Barcelona %C Barcelona %V Grau en Física %8 09/2012 %9 BSc Thesis %0 Journal Article %J Physical Review Letters %D 2012 %T Entanglement Spectrum, Critical Exponents, and Order Parameters in Quantum Spin Chains %A De Chiara, G. %A Lepori, L. %A Lewenstein, M. %A Sanpera, A. %X We investigate the entanglement spectrum near criticality in finite quantum spin chains. Using finite size scaling we show that when approaching a quantum phase transition, the Schmidt gap, i.e., the difference between the two largest eigenvalues of the reduced density matrix λ1, λ2, signals the critical point and scales with universal critical exponents related to the relevant operators of the corresponding perturbed conformal field theory describing the critical point. Such scaling behavior allows us to identify explicitly the Schmidt gap as a local order parameter. %B Physical Review Letters %V 109 %8 12/2012 %N 23 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.109.237208 %0 Journal Article %J Il Nuovo Cimento %D 2012 %T From Vector Meson Dominance to Quark-Hadron Duality %A Bramon, Albert %X A short review of the many contributions to hadron physics made by Mario Greco is presented. The review roughly covers his production between1971 and 1974, just before the advent of QCD, when quark-model ideas, duality principles and vector meson dominance were widely accepted, developed and applied. The present author had the privilege to collaborate with Mario in most of these contributions and looking backward in time to remember the good old days we spent together in Frascati has been a great pleasure. %B Il Nuovo Cimento %V 35C %P 9-14 %U http://arxiv.org/abs/1203.4205 %R 10.1393/ncc/i2012-11133-9 %0 Journal Article %J New J. Phys. %D 2012 %T Glass to superfluid transition in dirty bosons on a lattice %A Julia Stasińska %A Massignan, Pietro %A Bishop, Michael %A Wehr, Jan %A Sanpera, Anna %A Lewenstein, Maciej %X We investigate the interplay between disorder and interactions in a Bose gas on a lattice in presence of randomly localized impurities. We compare the performance of two theoretical methods, the simple version of multi-orbital Hartree-Fock and the common Gross-Pitaevskii approach, showing how the former gives a very good approximation to the ground state in the limit of weak interactions, where the superfluid fraction is small. We further prove rigorously that for this class of disorder the fractal dimension of the ground state d* tends to the physical dimension in the thermodynamic limit. This allows us to introduce a quantity, the fractional occupation, which gives insightful information on the crossover from a Lifshits to a Bose glass. Finally, we compare temperature and interaction effects, highlighting similarities and intrinsic differences. %B New J. Phys. %V 14 %P 043043 %U http://arxiv.org/abs/1111.3494 %R 10.1088/1367-2630/14/4/043043 %0 Journal Article %J Physical Review A %D 2012 %T Growth of graph states in quantum networks %A Cuquet, Martí %A Calsamiglia, John %X We propose a scheme to distribute graph states over quantum networks in the presence of noise in the channels and in the operations. The protocol can be implemented efficiently for large graph sates of arbitrary (complex) topology. We benchmark our scheme with two protocols where each connected component is prepared in a node belonging to the component and subsequently distributed via quantum repeaters to the remaining connected nodes. We show that the fidelity of the generated graphs can be written as the partition function of a classical Ising-type Hamiltonian. We give exact expressions of the fidelity of the linear cluster and results for its decay rate in random graphs with arbitrary (uncorrelated) degree distributions. %B Physical Review A %V 86 %P 042304 %U http://link.aps.org/doi/10.1103/PhysRevA.86.042304 %R 10.1103/PhysRevA.86.042304 %0 Journal Article %J Scientific Reports (Nature Publishing Group) %D 2012 %T Quantum learning without quantum memory %A Sentís, Gael %A Calsamiglia, John %A Muñoz-Tapia, Ramon %A Bagan, Emilio %X A quantum learning machine for binary classification of qubit states that does not require quantum memory is introduced and shown to perform with the minimum error rate allowed by quantum mechanics for any size of the training set. This result is shown to be robust under (an arbitrary amount of) noise and under (statistical) variations in the composition of the training set, provided it is large enough. This machine can be used an arbitrary number of times without retraining. Its required classical memory grows only logarithmically with the number of training qubits, while its excess risk decreases as the inverse of this number, and twice as fast as the excess risk of an “estimate-and-discriminate” machine, which estimates the states of the training qubits and classifies the data qubit with a discrimination protocol tailored to the obtained estimates. %B Scientific Reports (Nature Publishing Group) %I Nature Publishing Group %V 2 %P 708 %8 10/2012 %U http://www.nature.com/srep/2012/121005/srep00708/full/srep00708.html %R 10.1038/srep00708 %0 Journal Article %J Physical Review Letters %D 2012 %T Quantum Memory Assisted Probing of Dynamical Spin Correlations %A O. Romero-Isart %A Rizzi, M. %A Muschik, C. A. %A Polzik, E. S. %A Lewenstein, M. %A Sanpera, A. %X We propose a method to probe time-dependent correlations of nontrivial observables in many-body ultracold lattice gases. The scheme uses a quantum nondemolition matter-light interface, first to map the observable of interest on the many-body system into the light and then to store coherently such information into an external system acting as a quantum memory. Correlations of the observable at two (or more) instances of time are retrieved with a single final measurement that includes the readout of the quantum memory. Such a method brings to reach the study of dynamics of many-body systems in and out of equilibrium by means of quantum memories in the field of quantum simulators. %B Physical Review Letters %V 108 %8 2/2012 %N 6 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.108.065302 %0 Journal Article %J Physical Review A %D 2012 %T Spin-driven spatial symmetry breaking of spinor condensates in a double well %A Melé-Messeguer, M. %A Paganelli, Simone %A Juliá-Díaz, B. %A Ann Sanpera %A Polls, A. %B Physical Review A %I American Physical Society %V 86 %P 053626 - %8 2012/11/26/ %G eng %U http://link.aps.org/doi/10.1103/PhysRevA.86.053626 %N 5 %0 Book %D 2012 %T Ultracold atoms in optical lattices: simulating Quantum Many-Body systems %A Lewenstein Maciej %A Sanpera Anna %A Ahufinger Verònica %I Oxford University Press %@ 978-0199573127 %0 Journal Article %J Phys. Rev. Lett. %D 2011 %T All Nonclassical Correlations Can Be Activated into Distillable Entanglement %A Piani, Marco %A Gharibian, Sevag %A Adesso, Gerardo %A Calsamiglia, John %A Horodecki, Paweł %A Winter, Andreas %B Phys. Rev. Lett. %I American Physical Society %V 106 %P 220403 %8 Jun %U http://prl.aps.org/abstract/PRL/v106/i22/e220403 %) http://arxiv.org/abs/1103.4032 %R 10.1103/PhysRevLett.106.220403 %0 Journal Article %J Phys. Rev. Lett. %D 2011 %T Bell Inequalities with No Quantum Violation and Unextendable Product Bases %A Remigiusz Augusiak %A Julia Stasińska %A Hadley, Christopher %A Korbicz, Jarosław K. %A Lewenstein, Maciej %A Acín, Antonio %X The strength of classical correlations is subject to certain constraints, commonly known as Bell inequalities. Violation of these inequalities is the manifestation of nonlocality—displayed, in particular, by quantum mechanics, meaning that quantum mechanics can outperform classical physics at tasks associated with such Bell inequalities. Interestingly, however, there exist situations in which this is not the case. We associate an intriguing class of bound entangled states, constructed from unextendable product bases with a wide family of tasks, for which (i) quantum correlations do not outperform the classical ones but (ii) there exist supraquantum nonsignaling correlations that do provide an advantage. %B Phys. Rev. Lett. %I American Physical Society %V 107 %P 070401 %8 Aug %R 10.1103/PhysRevLett.107.070401 %0 Journal Article %J Physical Review B %D 2011 %T Bilinear-biquadratic spin-1 chain undergoing quadratic Zeeman effect %A De Chiara, G. %A Lewenstein, M. %A Sanpera, A. %X The Heisenberg model for spin-1 bosons in one dimension presents many different quantum phases, including the famous topological Haldane phase. Here we study the robustness of such phases in front of a SU(2) symmetry-breaking field as well as the emergence of unique phases. Previous studies have analyzed the effect of such uniaxial anisotropy in some restricted relevant points of the phase diagram. Here we extend those studies and present the complete phase diagram of the spin-1 chain with uniaxial anisotropy. To this aim, we employ the density-matrix renormalization group together with analytical approaches. The complete phase diagram can be realized using ultracold spinor gases in the Mott insulator regime under a quadratic Zeeman effect. %B Physical Review B %V 84 %8 8/2011 %U http://prb.aps.org/abstract/PRB/v84/i5/e054451 %N 5 %! Phys. Rev. B %R 10.1103/PhysRevB.84.054451 %0 Journal Article %J International Journal of Quantum Information %D 2011 %T Characterizing quantumness via entanglement creation %A Gharibian, Sevag %A Piani, Marco %A Adesso, Gerardo %A Calsamiglia, John %A Pawel Horodecki %B International Journal of Quantum Information %V 9 %P 1701-1713 %8 05 %U http://arxiv.org/abs/1105.3419 %R 10.1142/S0219749911008258 %0 Journal Article %J ArXiv e-prints %D 2011 %T A continuous-variable formalism for the Faraday atom-light interface %A Stasińska, J. %A Paganelli, Simone %A Rodó, C. %A Sanpera, A. %X Quantum interfaces between polarized atomic ensembles and coherent states of light, applied recently to manipulate bipartite and multipartite entanglement, are revisited by means of a continuous-variable formalism. The explicit use of the continuous-variable formalism facilitates significantly the analysis of entanglement between different modes, reducing it to the study of the properties of a final covariance matrix which can be found analytically. Furthermore, it allows to study matter-light interfaces for mixed states, adapting the formalism to the experimental situations in which the initial prepared Gaussian states are, unavoidably, affected by a certain amount of noise. A multipartite scenario, leading to the generation of macroscopic cluster states is presented and analyzed in detail within this formalism. %B ArXiv e-prints %G eng %U http://arxiv.org/abs/1007.0403 %0 Journal Article %J Journal of Low Temperature Physics %D 2011 %T Detection of Entanglement in Ultracold Lattice Gases %A Chiara, G. %A Sanpera, A. %X Probing non trivial magnetic ordering in quantum magnets realized with ultracold lattice gases demands detection methods with some spatial resolution built on it. Here we demonstrate that the Faraday matter-light interface provides an experimentally feasible tool to distinguish indubitably different quantum phases of a given many-body system in a non-demolishing way. We illustrate our approach by focussing on the Heisenberg chain for spin-1 bosons in the presence of a SU(2) symmetry breaking field. We explain how using the light signal obtained via homodyne detection one can reconstruct the phase diagram of the model. Further we show that the very same technique that provides a direct experimentally measurable signal of different order parameters can be extended to detect also the presence of multipartite entanglement in such systems. %B Journal of Low Temperature Physics %V 165 %P 292 - 305 %8 12/2011 %U link.springer.com/article/10.1007%2Fs10909-011-0403-8 %N 5-6 %! J Low Temp Phys %R 10.1007/s10909-011-0403-8 %0 Journal Article %J Phys. Rev. A %D 2011 %T Disordered spinor Bose-Hubbard model %A Ł{\k a}cki, M. %A Paganelli, Simone %A Ahufinger, V. %A Sanpera, Anna %A Zakrzewski, J. %K Condensed Matter - Other Condensed Matter %K Quantum Physics %X We study the zero-temperature phase diagram of the disordered spin-1 Bose-Hubbard model in a two-dimensional square lattice. To this aim, we use a mean-field Gutzwiller ansatz and a probabilistic mean-field perturbation theory. The spin interaction induces two different regimes, corresponding to a ferromagnetic and antiferromagnetic order. In the ferromagnetic case, the introduction of disorder reproduces analogous features of the disordered scalar Bose-Hubbard model, consisting in the formation of a Bose glass phase between Mott insulator lobes. In the antiferromagnetic regime, the phase diagram differs more from the scalar case. Disorder in the chemical potential can lead to the disappearance of Mott insulator lobes with an odd-integer filling factor and, for sufficiently strong spin coupling, to Bose glass of singlets between even-filling Mott insulator lobes. Disorder in the spinor coupling parameter results in the appearance of a Bose glass phase only between the n and the n+1 lobes for n odd. Disorder in the scalar Hubbard interaction inhibits Mott insulator regions for occupation larger than a critical value. %B Phys. Rev. A %V 83 %P 013605 %G eng %U http://link.aps.org/doi/10.1103/PhysRevA.83.013605 %R 10.1103/PhysRevA.83.013605 %0 Journal Article %J Phys. Rev. A %D 2011 %T Entanglement detection in hybrid optomechanical systems %A De Chiara, Gabriele %A Paternostro, Mauro %A Palma, G. Massimo %X We study a device formed by a Bose Einstein condensate (BEC) coupled to the field of a cavity with a moving end-mirror and find a working point such that the mirror-light entanglement is reproduced by the BEC-light quantum correlations. This provides an experimentally viable tool for inferring mirror-light entanglement with only a limited set of assumptions. We prove the existence of tripartite entanglement in the hybrid device, persisting up to temperatures of a few milli-Kelvin, and discuss a scheme to detect it. %B Phys. Rev. A %V 83 %P 052324 %U http://link.aps.org/doi/10.1103/PhysRevA.83.052324 %R 10.1103/PhysRevA.83.052324 %0 Journal Article %J EPL (Europhysics Letters) %D 2011 %T Entangling two distant oscillators with a quantum reservoir %A Wolf, A %A De Chiara, G. %A Kajari, E %A Lutz, E %A G. Morigi %X The generation of entanglement between two oscillators that interact via a common reservoir is theoretically studied. The reservoir is modeled by a one-dimensional harmonic crystal initially in thermal equilibrium. Starting from a separable state, the oscillators can become entangled after a transient time, that is of the order of the thermalization time scale. This behaviour is observed at finite temperature even when the oscillators are at a distance significantly larger than the crystal's interparticle spacing. The underlying physical mechanisms can be explained by the dynamical properties of the collective variables of the two oscillators which may decouple from or be squeezed by the reservoir. Our predictions can be tested with an ion chain in a linear Paul trap. %B EPL (Europhysics Letters) %V 95 %P 60008 %8 09/2011 %U http://epljournal.edpsciences.org/index.php?option=com_article&access=standard&Itemid=129&url=/articles/epl/abs/2011/18/epl13816/epl13816.html %N 6 %! EPL %R 10.1209/0295-5075/95/60008 %0 Journal Article %J Physical Review A %D 2011 %T Limited-path-length entanglement percolation in quantum complex networks %A Cuquet, Martí %A Calsamiglia, John %X We study entanglement distribution in quantum complex networks where nodes are connected by bipartite entangled states. These networks are characterized by a complex structure, which dramatically affects how information is transmitted through them. For pure quantum state links, quantum networks exhibit a remarkable feature absent in classical networks: it is possible to effectively rewire the network by performing local operations on the nodes. We propose a family of such quantum operations that decrease the entanglement percolation threshold of the network and increase the size of the giant connected component. We provide analytic results for complex networks with an arbitrary (uncorrelated) degree distribution. These results are in good agreement with numerical simulations, which also show enhancement in correlated and real-world networks. The proposed quantum preprocessing strategies are not robust in the presence of noise. However, even when the links consist of (noisy) mixed-state links, one can send quantum information through a connecting path with a fidelity that decreases with the path length. In this noisy scenario, complex networks offer a clear advantage over regular lattices, namely, the fact that two arbitrary nodes can be connected through a relatively small number of steps, known as the small-world effect. We calculate the probability that two arbitrary nodes in the network can successfully communicate with a fidelity above a given threshold. This amounts to working out the classical problem of percolation with a limited path length. We find that this probability can be significant even for paths limited to few connections and that the results for standard (unlimited) percolation are soon recovered if the path length exceeds by a finite amount the average path length, which in complex networks generally scales logarithmically with the size of the network. %B Physical Review A %V 83 %P 032319-14 %8 03/2011 %U http://pra.aps.org/abstract/PRA/v83/i3/e032319 %R 10.1103/PhysRevA.83.032319 %0 Journal Article %J Phys. Rev. A %D 2011 %T Probing magnetic order in ultracold lattice gases %A De Chiara, Gabriele %A O. Romero-Isart %A Sanpera, A. %X A forthcoming challenge in ultracold lattice gases is the simulation of quantum magnetism. That involves both the preparation of the lattice atomic gas in the desired spin state and the probing of the state. Here we demonstrate how a probing scheme based on atom-light interfaces gives access to the order parameters of nontrivial quantum magnetic phases, allowing us to characterize univocally strongly correlated magnetic systems produced in ultracold gases. This method, which is also nondemolishing, yields spatially resolved spin correlations and can be applied to bosons or fermions. As a proof of principle, we apply this method to detect the complete phase diagram displayed by a chain of (rotationally invariant) spin-1 bosons. %B Phys. Rev. A %V 83 %P 021604 %U http://pra.aps.org/abstract/PRA/v83/i2/e021604 %R 10.1103/PhysRevA.83.021604 %0 Journal Article %J Physical Review A %D 2011 %T Quantifying, characterizing, and controlling information flow in ultracold atomic gases %A Haikka, P. %A McEndoo, S. %A De Chiara, G. %A G. M. Palma %A Maniscalco, S. %X We study quantum information flow in a model comprised of a trapped impurity qubit immersed in a Bose-Einstein-condensed reservoir. We demonstrate how information flux between the qubit and the condensate can be manipulated by engineering the ultracold reservoir within experimentally realistic limits. We show that this system undergoes a transition from Markovian to non-Markovian dynamics, which can be controlled by changing key parameters such as the condensate scattering length. In this way, one can realize a quantum simulator of both Markovian and non-Markovian open quantum systems, the latter ones being characterized by a reverse flow of information from the background gas (reservoir) to the impurity (system). %B Physical Review A %V 84 %8 9/2011 %U http://pra.aps.org/abstract/PRA/v84/i3/e031602 %N 3 %! Phys. Rev. A %R 10.1103/PhysRevA.84.031602 %0 Thesis %B Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics %D 2011 %T Quantum Accuracy Limits and Benchmarks in Continuos Variable Systems %A Bracons, Mariona (formerly Mariona Aspachs) %Y Calsamiglia, John %Y Muñoz-Tapia, Ramon %B Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics %I Universitat Autònoma de Barcelona %C Barcelona %V Doctorat en Física %8 10/2011 %9 PhD %0 Journal Article %J J Low Temp Phys %D 2011 %T Spin Effects in Bose-Glass Phases %A Paganelli, Simone %A Ła̧cki, Mateusz %A Ahufinger, Veronica %A Zakrzewski, Jakub %A Sanpera, Anna %B J Low Temp Phys %V 165 %P 227 %U http://www.springerlink.com/content/10w5036068935773/ %R 10.1007/s10909-011-0392-7 %0 Journal Article %J Phys. Rev. Lett. %D 2010 %T Adiabatic Markovian Dynamics %A Oreshkov, Ognyan %A Calsamiglia, John %K Quantum Physics %X We propose a theory of adiabaticity in quantum Markovian dynamics based on a structural decomposition of the Hilbert space induced by the asymptotic behavior of the Lindblad semigroup. A central idea of our approach is that the natural generalization of the concept of eigenspace of the Hamiltonian in the case of Markovian dynamics is a noiseless subsystem with a minimal noisy cofactor. Unlike previous attempts to define adiabaticity for open systems, our approach deals exclusively with physical entities and provides a simple, intuitive picture at the underlying Hilbert-space level, linking the notion of adiabaticity to the theory of noiseless subsystems. As an application of our theory, we propose a framework for decoherence-assisted computation in noiseless codes under general Markovian noise. We also formulate a dissipation-driven approach to holonomic computation based on adiabatic dragging of subsystems that is generally not achievable by non-dissipative means. %B Phys. Rev. Lett. %V 105 %P 050503 %U http://arxiv.org/abs/1002.2219 %R 10.1103/PhysRevLett.105.050503 %0 Journal Article %J Phys. Rev. Lett. %D 2010 %T Cold-atom induced control of an opto-mechanical device %A M. Paternostro %A De Chiara, Gabriele %A G. M. Palma %X We consider a cavity with a vibrating end mirror and coupled to a Bose-Einstein condensate. The cavity field mediates the interplay between mirror and collective oscillations of the atomic density. We study the implications of this dynamics and the possibility of an indirect diagnostic. Our predictions can be observed in a realistic setup that is central to the current quest for mesoscopic quantumness. %B Phys. Rev. Lett. %V 104 %P 243602 %8 06/2010 %U http://link.aps.org/doi/10.1103/PhysRevLett.104.243602 %R 10.1103/PhysRevLett.104.243602 %0 Journal Article %J Phys. Rev. A %D 2010 %T Creating p-wave superfluids and topological excitations in optical lattices %A Massignan, P. %A Sanpera, A. %A Lewenstein, M. %X We propose to realize a p-wave superfluid using bosons mixed with a single species of fermions in a deep optical lattice. We analyze with a self-consistent method its excitation spectrum in presence of a vortex, and we point out the range of interaction strengths in which the zero-energy mode with topological character exists on a finite optical lattice. Lattice effects are strongest close to fermionic half filling: here the linearity of the low-lying spectrum is lost, and a new class of extended zero-energy modes with checkerboard structure and d-wave symmetry appears. %B Phys. Rev. A %I American Physical Society %V 81 %P 031607 %8 Mar/2010 %U http://link.aps.org/doi/10.1103/PhysRevA.81.031607 %R 10.1103/PhysRevA.81.031607 %0 Journal Article %J Phys. Rev. Lett. %D 2010 %T Decay of Polarons and Molecules in a Strongly Polarized Fermi Gas %A Bruun, G. M. %A Massignan, P. %X The ground state of an impurity immersed in a Fermi sea changes from a polaron to a molecule as the interaction strength is increased. We show here that the coupling between these two states is strongly suppressed due to a combination of phase-space effects and Fermi statistics, and that it vanishes much faster than the energy difference between the two states, thereby confirming the first order nature of the polaron-molecule transition. In the regime where each state is metastable, we find quasiparticle lifetimes which are much longer than what is expected for a usual Fermi liquid. Our analysis indicates that the decay rates are sufficiently slow to be experimentally observable. %B Phys. Rev. Lett. %I American Physical Society %V 105 %P 020403 %8 Jul %U http://link.aps.org/doi/10.1103/PhysRevLett.105.020403 %R 10.1103/PhysRevLett.105.020403 %0 Thesis %B Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics %D 2010 %T Discriminació quàntica %A Arnau Pujol %Y Muñoz-Tapia, Ramon %X S'estudia la discriminació de diferents sistemes quàntics. S'analitza el procés de discriminació entre qúbits, i es compara amb la discriminació de mesures projectives. Seguidament, es busquen les condicions en les quals la discriminació entre mesures projectives és perfecta, i es realitza el mateix per a la discriminació de transformacions unitàries, per tal de comparar ambdós resultats. %B Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics %I Universitat Autònoma de Barcelona %C Barcelona %V Grau en Física %8 07/2010 %9 BSc Thesis %0 Journal Article %J Physical Review A %D 2010 %T Estimation of quantum finite mixtures %A de Vicente, J. I. %A Calsamiglia, John %A Muñoz-Tapia, Ramon %A Bagan, Emilio %X We consider the problem of determining the weights of a quantum ensemble. That is to say, given a quantum system that is in a set of possible known states according to an unknown probability law, we give strategies to estimate the individual probabilities, weights, or mixing proportions. Such strategies can be used to estimate the frequencies at which different independent signals are emitted by a source. They can also be used to estimate the weights of particular terms in a canonical decomposition of a quantum channel. The quality of these strategies is quantified by a covariance-type error matrix. According with this cost function, we give optimal strategies in both the single-shot and multiple-copy scenarios. The latter is also analyzed in the asymptotic limit of large number of copies. We give closed expressions of the error matrix for two-component quantum mixtures of qubit systems. The Fisher information plays an unusual role in the problem at hand, providing exact expressions of the minimum covariance matrix for any number of copies. %B Physical Review A %V 81 %P 012332 %8 1/2010 %U http://link.aps.org/doi/10.1103/PhysRevA.81.012332 %R 10.1103/PhysRevA.81.012332 %0 Journal Article %J Phys. Rev. A %D 2010 %T Ion-trap simulation of the quantum phase transition in an exactly solvable model of spins coupled to bosons %A Giorgi, Gian Luca %A Paganelli, Simone %A Galve, Fernando %K Quantum Physics %X It is known that arrays of trapped ions can be used to efficiently simulate a variety of many-body quantum systems. Here we show how it is possible to build a model representing a spin chain interacting with bosons that is exactly solvable. The exact spectrum of the model at zero temperature and the ground-state properties are studied. We show that a quantum phase transition occurs when the coupling between spins and bosons reaches a critical value, which corresponds to a level crossing in the energy spectrum. Once the critical point is reached, the number of bosonic excitations in the ground state, which can be assumed as an order parameter, starts to be different from zero. The population of the bosonic mode is accompanied by a macroscopic magnetization of the spins. This double effect could represent a useful resource for phase transition detection since a measure of the phonon can give information about the phase of the spin system. A finite-temperature phase diagram is also given in the adiabatic regime. %B Phys. Rev. A %V 81 %P 052118 %8 2010 %G eng %R 10.1103/PhysRevA.81.052118 %0 Journal Article %J Phys. Rev. Lett. %D 2010 %T Local Discrimination of Mixed States %A Calsamiglia, John %A de Vicente, J. I. %A Muñoz-Tapia, R. %A E. Bagan %B Phys. Rev. Lett. %I American Physical Society %V 105 %P 080504 %8 Aug %) http://arxiv.org/abs/1004.5522 %R 10.1103/PhysRevLett.105.080504 %0 Thesis %B Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics %D 2010 %T Localitat i separabilitat en discriminació d'hipòtesis quàntiques %A Caminal Armadans, Roger %Y Calsamiglia, John %X En aquest treball s'estudiarà la probabilitat òptima d'obtenir l'estat correcte en una mesura sobre un sistema de N còpies de tres estats quàntics amb probabilitats a priori idèntiques i separats 120º. S'estudiaran diferents estratègies, posant especial èmfasi en la localitat i l'entrellaçament de les mesures. Finalment es farà una generalització d'aquestes estratègies per a estats mescla, estudiant la probabilitat de discriminar-los en funció de la seva puresa i l'estratègia portada a terme. %B Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics %I Universitat Autònoma de Barcelona %C Barcelona %V Grau en Física %8 07/2010 %9 BSc Thesis %0 Thesis %B Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics %D 2010 %T Localització en xarxes complexes %A Perarnau Llobet, Martí %Y Calsamiglia, John %X En els anys 1950s, P.W. Anderson va suggerir la possibilitat d'un nou fenomen en física de l'estat sòlid: la localització. Aquest fenomen consisteix en que la funció d'ona dels electrons es queda totalment localitzada en l'espai provocant que el material passi de ser un conductor a un aïllant. Ha estat extensament estudiat en xarxes ordenades i és conseqüència de les impureses del material. En aquest treball, pretenem estudiar la localització en xarxes amb naturalesa aleatòria, en particular, en xarxes complexes. La idea bàsica és, tenint en compte que l'aleatorietat de les impureses del material provoca localització, veure si l'aleatorietat en l'estructura del material produeix el mateix efecte. %B Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics %I Universitat Autònoma de Barcelona %C Barcelona %V Grau en Física %8 07/2010 %9 BSc Thesis %0 Journal Article %J Physical Review A %D 2010 %T Multicopy programmable discrimination of general qubit states %A Sentís, Gael %A Bagan, Emili %A Calsamiglia, John %A Muñoz-Tapia, Ramon %X Quantum state discrimination is a fundamental primitive in quantum statistics where one has to correctly identify the state of a system that is in one of two possible known states. A programmable discrimination machine performs this task when the pair of possible states is not a priori known but instead the two possible states are provided through two respective program ports. We study optimal programmable discrimination machines for general qubit states when several copies of states are available in the data or program ports. Two scenarios are considered: One in which the purity of the possible states is a priori known, and the fully universal one where the machine operates over generic mixed states of unknown purity. We find analytical results for both the unambiguous and minimum error discrimination strategies. This allows us to calculate the asymptotic performance of programmable discrimination machines when a large number of copies are provided and to recover the standard state discrimination and state comparison values as different limiting cases. %B Physical Review A %V 82 %P 042312 %8 10/2010 %U http://link.aps.org/doi/10.1103/PhysRevA.82.042312 %R 10.1103/PhysRevA.82.042312 %0 Journal Article %J Phys. Rev. A %D 2010 %T The quantum ground state of self-organized atomic crystals in optical resonators %A S. Fernández-Vidal %A De Chiara, Gabriele %A J. Larson %A G. Morigi %X Cold atoms, driven by a laser and simultaneously coupled to the quantum field of an optical resonator, may self-organize in periodic structures. These structures are supported by the optical lattice, which emerges from the laser light they scatter into the cavity mode and form when the laser intensity exceeds a threshold value. We study theoretically the quantum ground state of these structures above the pump threshold of self-organization by mapping the atomic dynamics of the self-organized crystal to a Bose-Hubbard model. We find that the quantum ground state of the self-organized structure can be the one of a Mott insulator, depending on the pump strength of the driving laser. For very large pump strengths, where the intracavity-field intensity is maximum and one would expect a Mott-insulator state, we find intervals of parameters where the phase is compressible. These states could be realized in existing experimental setups. %B Phys. Rev. A %V 81 %P 043407 %8 04/2010 %U http://link.aps.org/doi/10.1103/PhysRevA.81.043407 %R 10.1103/PhysRevA.81.043407 %0 Thesis %B Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics %D 2010 %T Quantum Information with continuous variable systems %A Rodó, Carles %Y Sanpera, Anna %X This thesis deals with the study of quantum communication protocols with Continuous Variable (CV) systems. CV systems are those described by canonical conjugated coordinates $x$ and $p$ endowed with infinite dimensional Hilbert spaces, thus involving a complex mathematical structure. A special class of CV states, are the so-called Gaussian states. We present a protocol that permits to extract quantum keys from entangled Gaussian states. Differently from discrete systems, Gaussian entangled states cannot be distilled with Gaussian operations only. However it was already shown, that it is still possible to extract perfectly correlated classical bits to establish secret random keys. We properly modify the protocol using bipartite Gaussian entanglement to perform quantum key distribution in an efficient and realistic way. We describe and demonstrate security in front of different possible attacks on the communication, detailing the resources demanded. We also consider a simple 3-partite protocol known as Byzantine Agreement. It is an old classical communication problem in which parties (with possible traitors among them) can only communicate pairwise, while trying to reach a common decision. Classically, there is a bound in the maximal number of possible traitors that can be involved in the game. Nevertheless, a quantum solution exist. We show that solution within CV using multipartite entangled Gaussian states and Gaussian operations. Furthermore, we show under which premises concerning entanglement content of the state, noise, inefficient homodyne detectors, our protocol is efficient and applicable with present technology. It is known that in spite of their exceptional role within the space of all CV states, in fact, Gaussian states are not always the best candidates to perform quantum information tasks. Thus, we tackle the problem of quantification of correlations (quantum and/or classical) between two CV modes (Gaussian and non-Gaussian). We propose to define correlations between the two modes as the maximal number of correlated bits extracted via local quadrature measurements on each mode. On Gaussian states, where entanglement is accessible via their covariance matrix our quantification majorizes entanglement, reducing to an entanglement monotone for pure states. For non-Gaussian states, such as photonic Bell states, photon subtracted states and mixtures of Gaussian states, the bit quadrature correlations are shown to be also a monotonic function of the negativity. This quantification yields a feasible, operational way to measure non-Gaussian entanglement in current experiments by means of direct homodyne detection, without needing a complete state tomography with the same complexity as if dealing with Gaussian states. Finally we focus to atomic ensembles described as CV. Measurement induced entanglement between two macroscopical atomic samples was reported experimentally in 2001. There, the interaction between a single laser pulse propagating through two spatially separated atomic samples combined with a final projective measurement on the light led to the creation of pure EPR entanglement between the two samples. We show how to generate, manipulate and detect mesoscopic entanglement between an arbitrary number of atomic samples through a quantum non-demolition matter-light interface. Our proposal extends in a non-trivial way for multipartite entanglement (GHZ and cluster-like) without needing local magnetic fields. Moreover, we show quite surprisingly that given the irreversible character of a measurement, the interaction of the atomic sample with a second pulse light can modify and even reverse the entangling action of the first one leaving the samples in a separable state. %B Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics %I Universitat Autònoma de Barcelona %C Barcelona %V PhD %8 04/2010 %9 PhD Thesis %0 Journal Article %J Physica Scripta %D 2010 %T Recycling of qubits %A P Rapcan %A Calsamiglia, John %A Muñoz-Tapia, Ramon %A Bagan, Emilio %A V Buzek %X We consider a finite number, N, of qubits that encode a pure single qubit state SU(2) covariantly. Given the N-qubit state has already been measured optimally to estimate the single-qubit state, we analyse the maximum information obtainable by a second, and subsequent observers ignorant of important details of the previous measurements. We quantify the information acquired by each observer as a function of N and of the number of independent observers that in succession have independently measured the same ensemble of qubits before him. %B Physica Scripta %V T140 %P 014059 %U http://iopscience.iop.org/1402-4896/2010/T140/014059?fromSearchPage=true %R 10.1088/0031-8949/2010/T140/014059 %0 Journal Article %J New. J. Phys. %D 2010 %T Spontaneous nucleation of structural defects in inhomogeneous ion chains %A De Chiara, Gabriele %A del Campo, Adolfo %A G. Morigi %A M. B. Plenio %A A. Retzker %X Structural defects in ion crystals can be formed during a linear quench of the transverse trapping frequency across the mechanical instability from a linear chain to the zigzag structure. The density of defects after the sweep can be conveniently described by the Kibble-Zurek mechanism. In particular, the number of kinks in the zigzag ordering can be derived from a time-dependent Ginzburg-Landau equation for the order parameter, here the zigzag transverse size, under the assumption that the ions are continuously laser cooled. In a linear Paul trap the transition becomes inhomogeneous, being the charge density larger in the center and more rarefied at the edges. During the linear quench the mechanical instability is first crossed in the center of the chain, and a front, at which the mechanical instability is crossed during the quench, is identified which propagates along the chain from the center to the edges. If the velocity of this front is smaller than the sound velocity, the dynamics becomes adiabatic even in the thermodynamic limit and no defect is produced. Otherwise, the nucleation of kinks is reduced with respect to the case in which the charges are homogeneously distributed, leading to a new scaling of the density of kinks with the quenching rate. The analytical predictions are verified numerically by integrating the Langevin equations of motion of the ions, in presence of a time-dependent transverse confinement. We argue that the non-equilibrium dynamics of an ion chain in a Paul trap constitutes an ideal scenario to test the inhomogeneous extension of the Kibble-Zurek mechanism, which lacks experimental evidence to date. %B New. J. Phys. %V 12 %P 115003 %8 12/2010 %U http://iopscience.iop.org/1367-2630/12/11/115003?fromSearchPage=true %R 10.1088/1367-2630/12/11/115003 %0 Journal Article %J Phys. Rev. Lett. %D 2010 %T Structural defects in ion crystals by quenching the external potential: the inhomogeneous Kibble-Zurek mechanism %A del Campo, Adolfo %A De Chiara, Gabriele %A G. Morigi %A M. B. Plenio %A A. Retzker %B Phys. Rev. Lett. %V 105 %P 075701 %8 08/2010 %U http://link.aps.org/doi/10.1103/PhysRevLett.105.075701 %R 10.1103/PhysRevLett.105.075701 %0 Journal Article %J Europhysics Letters %D 2010 %T Topological superfluids on a lattice with non-Abelian gauge fields %A A. Kubasiak %A Massignan, P. %A Lewenstein, M. %B Europhysics Letters %V 92 %P 46004 %8 12/2010 %U http://arxiv.org/abs/1007.4827 %R 10.1209/0295-5075/92/46004 %0 Journal Article %J Physical Review Letters %D 2009 %T Comment on “Nongeometric Conditional Phase Shift via Adiabatic Evolution of Dark Eigenstates: A New Approach to Quantum Computation” %A Oreshkov, Ognyan %A Calsamiglia, John %B Physical Review Letters %V 103 %P 188901 %G eng %U http://prl.aps.org/abstract/PRL/v103/i18/e188901 %R 10.1103/PhysRevLett.103.188901 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2009 %T Distinguishability measures between ensembles of quantum states %A Oreshkov, Ognyan %A Calsamiglia, John %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 79 %P 032336–29 %G eng %U http://link.aps.org/doi/10.1103/PhysRevA.79.032336 %R 10.1103/PhysRevA.79.032336 %0 Journal Article %J Physical Review Letters %D 2009 %T Entanglement Percolation in Quantum Complex Networks %A Cuquet, Martí %A Calsamiglia, John %K complex networks %K entanglement distribution %K entanglement percolation %K erdos-renyi %K generating function %K quantum complex networks %K quantum networks %K scale-free %K small-world %X Quantum networks are essential to quantum information distributed applications, and communicatingover them is a key challenge. Complex networks have richand intriguing properties, which are as yet unexplored in thequantum setting. Here, we study the effect of entanglement percolationas a means to establish long-distance entanglement between arbitrary nodesof quantum complex networks. We develop a theory to analyticallystudy random graphs with arbitrary degree distribution and give exactresults for some models. Our findings are in good agreementwith numerical simulations and show that the proposed quantum strategiesenhance the percolation threshold substantially. Simulations also show a clearenhancement in small-world and other real-world networks. %B Physical Review Letters %I APS %V 103 %P 240503–4 %8 12/2009 %G eng %U http://link.aps.org/abstract/PRL/v103/e240503 %R 10.1103/PhysRevLett.103.240503 %0 Journal Article %J Physical Review Letters %D 2009 %T Holonomic Quantum Computation in Subsystems %A Oreshkov, Ognyan %X We introduce a generalized method of holonomic quantum computation (HQC) based on encoding in subsystems. As an application, we propose a scheme for applying holonomic gates to unencoded qubits by the use of a noisy ancillary qubit. This scheme does not require initialization in a subspace since all dynamical effects factor out as a transformation on the ancilla. We use this approach to show how fault-tolerant HQC can be realized via 2-local Hamiltonians with perturbative gadgets. %B Physical Review Letters %V 103 %P 090502 %G eng %U http://prl.aps.org/abstract/PRL/v103/i9/e090502 %R 10.1103/PhysRevLett.103.090502 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2009 %T Manipulating mesoscopic multipartite entanglement with atom-light interfaces %A Julia Stasińska %A Rodó, Carles %A Paganelli, Simone %A Sanpera, Anna %A Birkl, G. %K atom-photon collisions %K EPR paradox %K measurement theory %K mesoscopic entanglement %K multipartite entanglement %K quantum entanglement %K quantum optics %X Entanglementbetween two macroscopic atomic ensembles induced by measurement on anancillary light system has proven to be a powerful methodfor engineering quantum memories and quantum state transfer. Here weinvestigate the feasibility of such methods for generation, manipulation, anddetection of genuine multipartite entanglement (Greenberger-Horne-Zeilinger and clusterlike states) betweenmesoscopic atomic ensembles without the need of individual addressing ofthe samples. Our results extend in a nontrivial way theEinstein-Podolsky-Rosen entanglement between two macroscopic gas samples reported experimentally in[B. Julsgaard, A. Kozhekin, and E. Polzik, Nature (London) 413,400 (2001)]. We find that under realistic conditions, a secondorthogonal light pulse interacting with the atomic samples, can modifyand even reverse the entangling action of the first oneleaving the samples in a separable state. ©2009 The American Physical Society %B Physical Review A (Atomic, Molecular, and Optical Physics) %I APS %V 80 %P 062304–8 %8 12/2009 %G eng %U http://link.aps.org/abstract/PRA/v80/e062304 %0 Journal Article %J Fortschritte der Physik %D 2009 %T Optimized electron propagation on a quantum chain by a topological phase %A Paganelli, Simone %A Giorgi, G. L. %A de Pasquale, F. %X We study the quantum diffusion of an electron in a quantum chain starting from an initial state localized around a given site. As the wavepacket diffuses, the probability of reconstructing the initial state on another site diminishes drastically with the distance. In order to optimize the state transmission we find that a topological quantum phase can be introduced. The effect of this phase is the reduction of wavepacket spreading together with almost coherent group propagation. In this regime, the electron has a quasi-linear dispersion and high fidelity can be achieved also over large distances in terms of lattice spacing. %B Fortschritte der Physik %V 57 %P 1094–1102 %G eng %U http://doi.wiley.com/10.1002/prop.200900087 %R 10.1002/prop.200900087 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2009 %T Phase estimation for thermal Gaussian states %A Aspachs, Mariona %A Calsamiglia, John %A Muñoz-Tapia, Ramon %A Bagan, Emili %K gaussian noise %K gaussian states %K optical losses %K optical squeezing %K phase estimation %K quantum noise %K thermal noise %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 79 %P 033834–12 %8 03/2009 %G eng %U http://link.aps.org/abstract/PRA/v79/e033834 %R 10.1103/PhysRevA.79.033834 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2009 %T Phase-covariant quantum benchmarks %A Calsamiglia, John %A Aspachs, Mariona %A Muñoz-Tapia, Ramon %A Bagan, Emili %K quantum benchmarks %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 79 %P 050301–4 %G eng %U http://link.aps.org/doi/10.1103/PhysRevA.79.050301 %R 10.1103/PhysRevA.79.050301 %0 Journal Article %J New Journal of Physics %D 2009 %T Positive maps, majorization, entropic inequalities and detection of entanglement %A Remigiusz Augusiak %A Julia Stasińska %B New Journal of Physics %V 11 %P 053018 %8 05/2009 %U http://stacks.iop.org/1367-2630/11/i=5/a=053018 %R 10.1088/1367-2630/11/5/053018 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2009 %T Preparation of decoherence-free cluster states with optical superlattices %A Jiang, Liang %A Rey, Ana Maria %A Romero-Isart, Oriol %A Garcia-Ripoll, Juan Jose %A Sanpera, Anna %A Lukin, Mikhail D. %K atom-photon collisions %K ground states %K optical multilayers %K quantum optics %K superlattices %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 79 %P 022309–16 %8 02/2009 %G eng %U http://link.aps.org/abstract/PRA/v79/e022309 http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PLRAAN000079000002022309000001&idtype=cvips&gifs=yes http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PLRAAN000079000002 %R 10.1103/PhysRevA.79.022309 %0 Journal Article %J Physical Review A %D 2009 %T Scheme for fault-tolerant holonomic computation on stabilizer codes %A Oreshkov, Ognyan %A Brun, Todd %A Lidar, Daniel %B Physical Review A %V 80 %P 022325 %G eng %U http://pra.aps.org/abstract/PRA/v80/i2/e022325 %R 10.1103/PhysRevA.80.022325 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2008 %T Double-barrier potentials for matter-wave gap solitons %A Ahufinger, Veronica %A Malomed, B. A. %A Birkl, G. %A Corbalán, Ramon %A Sanpera, Anna %K Bose-Einstein condensation %K Fabry-Perot interferometers %K matter waves %K optical solitons %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 78 %P 013608–5 %8 07/2008 %G eng %U http://link.aps.org/abstract/PRA/v78/e013608 http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PLRAAN000078000001013608000001&idtype=cvips&gifs=yes %R 10.1103/PhysRevA.78.013608 %0 Journal Article %J Physical Review Letters %D 2008 %T Frustration, Area Law, and Interference in Quantum Spin Models %A Sen(De), Aditi %A Sen, Ujjwal %A Dziarmaga, Jacek %A Sanpera, Anna %A Lewenstein, Maciej %B Physical Review Letters %V 101 %P 187202–4 %8 10/2008 %G eng %U http://link.aps.org/abstract/PRL/v101/e187202 %R 10.1103/PhysRevLett.101.187202 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2008 %T Manipulation of single-photon states encoded in transverse spatial modes: Possible and impossible tasks %A Calvo, Gabriel F. %A Picon, Antonio %K SU(3) theory %X Controlled generation and manipulation of photon states encoded in their spatial degrees of freedom is a crucial ingredient in many quantum-information tasks exploiting higher-than-two dimensional encoding. Here, we prove the impossibility to arbitrarily modify d-level state superpositions (qudits) for d>2, encoded in the transverse modes of light, with optical components associated to the group of symplectic transforms (Gaussian operations). Surprisingly, we also provide an explicit construction of how non-Gaussian operations acting on mode subspaces do enable one to overcome the limit d=2. In addition, this set of operations realizes the full SU(3) algebra. %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 77 %P 012302–7 %8 01/2008 %G eng %U http://link.aps.org/doi/10.1103/PhysRevA.77.012302 %R 10.1103/PhysRevA.77.012302 %0 Journal Article %J Physical Review Letters %D 2008 %T Measuring the Complete Transverse Spatial Mode Spectrum of a Wave Field %A Calvo, Gabriel F. %A Picon, Antonio %A Zambrini, Roberta %B Physical Review Letters %V 100 %P 173902–4 %8 05/2008 %G eng %U http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PRLTAO000100000017173902000001&idtype=cvips&prog=normal %R 10.1103/PhysRevLett.100.173902 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2008 %T Multipartite continuous-variable solution for the Byzantine agreement problem %A Neigovzen, Rodion %A Rodó, Carles %A Adesso, Gerardo %A Sanpera, Anna %K Gaussian processes %K homodyne detection %K quantum cryptography %K quantum entanglement %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 77 %P 062307–11 %8 06/2008 %G eng %U http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PLRAAN000077000006062307000001&idtype=cvips&prog=normal %R 10.1103/PhysRevA.77.062307 %0 Journal Article %J Physical Review Letters %D 2008 %T Operational Interpretation for Global Multipartite Entanglement %A Boixo, Sergio %A Monras, Alex %K multipartite entanglement %B Physical Review Letters %V 100 %P 100503–4 %8 03/2008 %G eng %U http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PRLTAO000100000010100503000001&idtype=cvips&prog=normal http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000100000010100503000001&idtype=cvips&gifs=yes http:// %R 10.1103/PhysRevLett.100.100503 %0 Journal Article %J Physical Review Letters %D 2008 %T Operational Quantification of Continuous-Variable Correlations %A Rodó, Carles %A Adesso, Gerardo %A Sanpera, Anna %B Physical Review Letters %V 100 %P 110505–4 %8 03/2008 %G eng %U http://link.aps.org/abstract/PRL/v100/e110505 %R 10.1103/PhysRevLett.100.110505 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2008 %T Operator quantum error correction for continuous dynamics %A Oreshkov, Ognyan %A Lidar, Daniel A. %A Brun, Todd A. %K error correction codes %K quantum noise %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 78 %P 022333–10 %G eng %U http://link.aps.org/abstract/PRA/v78/e022333 %R 10.1103/PhysRevA.78.022333 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2008 %T Phase variance of squeezed vacuum states %A Bagan, Emili %A Monras, Alex %A Muñoz-Tapia, Ramon %K optical squeezing %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 78 %P 043829–7 %8 10/2008 %G eng %U http://link.aps.org/abstract/PRA/v78/e043829 %R 10.1103/PhysRevA.78.043829 %0 Journal Article %J Science %D 2008 %T Probing Quantum Magnetism with Cold Atoms %A Lewenstein, Maciej %A Sanpera, Anna %B Science %V 319 %P 292–293 %8 01/2008 %G eng %U http://www.sciencemag.org/cgi/content/full/319/5861/292?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=probing+quantum+magnetism&searchid=1&FIRSTINDEX=0&resourcetype=HWCIT %R 10.1126/science.1153723 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2008 %T Quantum Chernoff bound as a measure of distinguishability between density matrices: Application to qubit and Gaussian states %A Calsamiglia, John %A Muñoz-Tapia, Ramon %A Masanes, Lluís %A Acín, Antonio %A Bagan, Emili %K Bayes methods %K error statistics %K quantum chernoff bound %K quantum statistical mechanics %K quantum theory %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 77 %P 032311–15 %8 03/2008 %G eng %U http://link.aps.org/abstract/PRA/v77/e032311 %R 10.1103/PhysRevA.77.032311 %0 Journal Article %J arXiv %D 2008 %T Quantum information processing in spin chains via quantum Zeno dynamics %A Monras, Alex %A Romero-Isart, Oriol %K spin chain %X We show how the quantum Zeno effect can be exploited to control quantum information in a spin chain in a flexible way. In particular, we consider a one dimensional array of three level systems interacting via a swap operator, an interaction found in a generalized Heisenberg Hamiltonian. By encoding the qubit into two levels and using simple projective frequent measurements, the dynamics of the chain is guided to achieve basic quantum information manipulation tools, $\backslash$emph\{i.e.\}, quantum registers, single qubit operations and quantum state transfer on demand, while local addressability is not required. State transfer is perfect, robust against errors and delivers qubits at a constant rate, unconditional of the length of the transfer. Moreover, two dimensional lattices with tunable global interactions in different directions offer the possibility of performing two-qubit gates. %B arXiv %8 01/2008 %G eng %U http://arxiv.org/abs/0801.1959 %0 Journal Article %J Nature Physics %D 2008 %T Quantum non-demolition detection of strongly correlated systems %A Eckert, Kai %A Romero-Isart, Oriol %A Rodriguez, Mirta %A Lewenstein, Maciej %A Polzik, Eugene S. %A Sanpera, Anna %B Nature Physics %V 4 %P 50–54 %8 01/2008 %G eng %U http://www.nature.com/nphys/journal/v4/n1/full/nphys776.html %R 10.1038/nphys776 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2008 %T Quantum-limited metrology with product states %A Boixo, Sergio %A Datta, Animesh %A Flammia, Steven T. %A Shaji, Anil %A Bagan, Emili %A Caves, Carlton M. %K angular momentum theory %K Bose-Einstein condensation %K bound states %K protocols %K quantum entanglement %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 77 %P 012317–15 %8 01/2008 %G eng %U http://link.aps.org/abstract/PRA/v77/e012317 %R 10.1103/PhysRevA.77.012317 %0 Journal Article %J Physical Review Letters %D 2007 %T Discriminating States: The Quantum Chernoff Bound %A K. M. R. Audenaert %A Calsamiglia, John %A Muñoz-Tapia, Ramon %A Bagan, Emili %A Masanes, Lluís %A Acín, Antonio %A Verstraete, Frank %K quantum chernoff bound %K state discrimination %B Physical Review Letters %V 98 %P 160501–4 %8 04/2007 %G eng %U http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000098000016160501000001&idtype=cvips&gifs=yes %R 10.1103/PhysRevLett.98.160501 %0 Journal Article %J Physical Review Letters %D 2007 %T Discriminating States: The Quantum Chernoff Bound %A K. M. R. Audenaert %A J. Calsamiglia %A R. {Munoz-Tapia} %A E. Bagan %A Ll. Masanes %A A. Acin %A F. Verstraete %B Physical Review Letters %V 98 %P 160501–4 %U http://link.aps.org/abstract/PRL/v98/e160501 %R 10.1103/PhysRevLett.98.160501 %0 Journal Article %J Open Systems & Information Dynamics %D 2007 %T Efficiency in Quantum Key Distribution Protocols with Entangled Gaussian States %A Rodó, Carles %A Romero-Isart, Oriol %A Eckert, Kai %A Sanpera, Anna %K gaussian states %K quantum cryptography %K quantum key distribution %X Abstract Quantum key distribution (QKD) refers to specific quantum strategies which permit the secure distribution of a secret key between two parties that wish to communicate secretly. Quantum cryptography has proven unconditionally secure in ideal scenarios and has been successfully implemented using quantum states with finite (discrete) as well as infinite (continuous) degrees of freedom. Here, we analyze the efficiency of QKD protocols that use as a resource entangled gaussian states and gaussian operations only. In this framework, it has already been shown that QKD is possible [1] but the issue of its efficiency has not been considered. We propose a figure of merit (the efficiency E) to quantify the number of classical correlated bits that can be used to distill a key from a sample of N entangled states. We relate the efficiency of the protocol to the entanglement and purity of the states shared between the parties. %B Open Systems & Information Dynamics %V 14 %P 69–80 %8 03/2007 %G eng %U http://dx.doi.org/10.1007/s11080-007-9030-x %R 10.1007/s11080-007-9030-x %0 Journal Article %J New Journal of Physics %D 2007 %T Efficient quantum-state transfer in spin chains via adiabatic passage %A Eckert, Kai %A Romero-Isart, Oriol %A Sanpera, Anna %K spin chain %K state transfer %B New Journal of Physics %V 9 %P 155 %G eng %U http://iopscience.iop.org/1367-2630/9/5/155/pdf/1367-2630\_9\_5\_155.pdf %0 Journal Article %J New Journal of Physics %D 2007 %T How to hide a secret direction %A Bagan, Emili %A Calsamiglia, John %A Demkowicz-Dobrzanski, R. %A Muñoz-Tapia, Ramon %X We present a procedure to share a secret spatial direction in the absence of a common reference frame using a multipartite quantum state. The procedure guarantees that the parties can determine the direction if they perform joint measurements on the state, but fail to do so if they restrict themselves to local operations and classical communication (LOCC). We calculate the fidelity for joint measurements, give bounds on the fidelity achievable by LOCC, and prove that there is a non-vanishing gap between the two of them, even in the limit of infinitely many copies. The robustness of the procedure under particle loss is also studied. As a by-product we find bounds on the probability of discriminating by LOCC between the invariant subspaces of total angular momentum N/2 and N/2-1 in a system of N elementary spins. %B New Journal of Physics %V 9 %P 244 %G eng %U http://iopscience.iop.org/1367-2630/9/8/244/pdf/1367-2630\_9\_8\_244.pdf %R doi:10.1088/1367-2630/9/8/244 %0 Conference Paper %B AIP Conference Proceedings %D 2007 %T Kaonic Quantitative Complementarity and Quantum Erasers %A Bramon, Albert %A Garbarino, G. %A Hiesmayr, B. C. %B AIP Conference Proceedings %V 962 %P 242 %G eng %U http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=APCPCS000962000001000242000001&idtype=cvips&gifs=yes %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2007 %T Measuring two-photon orbital angular momentum entanglement %A Calvo, Gabriel F. %A Picon, Antonio %A Bramon, Albert %K orbital angular momentum %K quantum entanglement %K quantum optics %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 75 %P 012319–8 %8 01/2007 %G eng %U http://link.aps.org/abstract/PRA/v75/e012319 %R 10.1103/PhysRevA.75.012319 %0 Journal Article %J Physical Review Letters %D 2007 %T Optimal Quantum Estimation of Loss in Bosonic Channels %A Monras, Alex %A Paris, Matteo G. A. %B Physical Review Letters %V 98 %P 160401–4 %8 04/2007 %G eng %U http://link.aps.org/abstract/PRL/v98/e160401 %R 10.1103/PhysRevLett.98.160401 %0 Journal Article %J Physical Review Letters %D 2007 %T Predicting Spinor Condensate Dynamics from Simple Principles %A Moreno-Cardoner, M. %A Mur-Petit, J. %A Guilleumas, M. %A Polls, Artur %A Sanpera, Anna %A Lewenstein, Maciej %K Bose-Einstein condensation %K entropy %B Physical Review Letters %V 99 %P 020404–4 %8 07/2007 %G eng %U http://link.aps.org/abstract/PRL/v99/e020404 %R 10.1103/PhysRevLett.99.020404 %0 Journal Article %J Acta Physica Polonica B %D 2007 %T Quantum Mechanics with Neutral Kaons %A Bramon, Albert %A Garbarino, G. %A Hiesmayr, B. C. %B Acta Physica Polonica B %V 38 %P 2763–2776 %G eng %U http://arxiv.org/abs/hep-ph/0703152 %0 Journal Article %J Physical Review Letters %D 2007 %T Quantum Polarization Spectroscopy of Ultracold Spinor Gases %A Eckert, Kai %A Zawitkowski, L. %A Sanpera, Anna %A Lewenstein, Maciej %A Polzik, Eugene S. %B Physical Review Letters %V 98 %P 100404–4 %8 03/2007 %G eng %U http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PRLTAO000098000010100404000001&idtype=cvips&prog=normal %R 10.1103/PhysRevLett.98.100404 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2007 %T Quantum ratchets for quantum communication with optical superlattices %A Romero-Isart, Oriol %A Garcia-Ripoll, Juan Jose %K atom-photon collisions %K optical modulation %K quantum entanglement %K quantum optics %K superlattices %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 76 %P 052304–8 %8 11/2007 %G eng %U http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PLRAAN000076000005052304000001&idtype=cvips&prog=normal %R 10.1103/PhysRevA.76.052304 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2007 %T Quantum state transfer in spin-1 chains %A Romero-Isart, Oriol %A Eckert, Kai %A Sanpera, Anna %K ferromagnetic materials %K ground states %K Heisenberg model %K magnetic transitions %K quantum entanglement %K spin chain %K spin systems %K state transfer %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 75 %P 050303–4 %8 05/2007 %G eng %U http://link.aps.org/abstract/PRA/v75/e050303 %R 10.1103/PhysRevA.75.050303 %0 Journal Article %J New Journal of Physics %D 2007 %T Quantum switches and quantum memories for matter-wave lattice solitons %A Ahufinger, Veronica %A Mebrahtu, A. %A Corbalán, Ramon %A Sanpera, Anna %B New Journal of Physics %V 9 %P 4 %G eng %U http://iopscience.iop.org/1367-2630/9/1/004/pdf/1367-2630\_9\_1\_004.pdf %0 Journal Article %J arXiv %D 2007 %T Recycling of quantum information: Multiple observations of quantum systems %A Rapcan, Peter %A Calsamiglia, John %A Muñoz-Tapia, Ramon %A Bagan, Emili %A Bużek, Vladimir %X Given a finite number of copies of an unknown qubit state that have already been measured optimally, can one still extract any information about the original unknown state? We give a positive answer to this question and quantify the information obtainable by a given observer as a function of the number of copies in the ensemble, and of the number of independent observers that, one after the other, have independently measured the same ensemble of qubits before him. The optimality of the protocol is proven and extensions to other states and encodings are also studied. According to the general lore, the state after a measurement has no information about the state before the measurement. Our results manifestly show that this statement has to be taken with a grain of salt, specially in situations where the quantum states encode confidential information. %B arXiv %8 08/2007 %G eng %U http://arxiv.org/abs/0708.1086 %0 Journal Article %J Optics Letters %D 2007 %T Spin-induced angular momentum switching %A Calvo, Gabriel F. %A Picon, Antonio %K angular momentum %K anisotropic optical materials %K polarization %K polarization-selective devices %K quantum optics %X When light is transmitted through optically inhomogeneous and anisotropic media the spatial distribution of light can be modified according to its input polarization state. A complete analysis of this process, based on the paraxial approximation, is presented, and we show how it can be exploited to produce a spin-controlled change in the orbital angular momentum of light beams propagating in patterned space-variant optical axis phase plates. We also unveil a new effect: the development of a strong modulation in the angular momentum change upon variation of the optical path through the phase plates. %B Optics Letters %V 32 %P 838–840 %8 04/2007 %G eng %U http://ol.osa.org/abstract.cfm?URI=ol-32-7-838 %R 10.1364/OL.32.000838 %0 Journal Article %J Journal of Physics A: Mathematical and General %D 2007 %T Transport and entanglement generation in the Bose-Hubbard model %A Romero-Isart, Oriol %A Eckert, Kai %A Rodó, Carles %A Sanpera, Anna %B Journal of Physics A: Mathematical and General %V 40 %P 8019–8032 %G eng %U http://www.iop.org/EJ/article/1751-8121/40/28/S11/a7\_28\_s11.pdf %0 Journal Article %J Physical Review Letters %D 2007 %T Trapped Ion Chain as a Neural Network: Error Resistant Quantum Computation %A Pons, Marisa %A Ahufinger, Veronica %A Wunderlich, Christof %A Sanpera, Anna %A Braungardt, Sibylle %A Sen(De), Aditi %A Sen, Ujjwal %A Lewenstein, Maciej %K neural networks %K quantum computation %B Physical Review Letters %V 98 %P 023003–4 %8 01/2007 %G eng %U http://link.aps.org/abstract/PRL/v98/e023003 %R 10.1103/PhysRevLett.98.023003 %0 Journal Article %J New Journal of Physics %D 2007 %T Ultracold atomic Bose and Fermi spinor gases in optical lattices %A Eckert, Kai %A Zawitkowski, L. %A Leskinen, M. J. %A Sanpera, Anna %A Lewenstein, Maciej %K optical lattices %B New Journal of Physics %V 9 %P 133 %G eng %U http://iopscience.iop.org/1367-2630/9/5/133/pdf/1367-2630\_9\_5\_133.pdf %0 Journal Article %J Advances in Physics %D 2007 %T Ultracold atomic gases in optical lattices: mimicking condensed matter physics and beyond %A Lewenstein, Maciej %A Sanpera, Anna %A Ahufinger, Veronica %A Damski, B. %A Sen(De), Aditi %A Sen, Ujjwal %K optical lattices %B Advances in Physics %V 56 %P 243 %G eng %U http://www.informaworld.com/smpp/ftinterface\~content=a778175433\~fulltext=713240930 %0 Journal Article %J Journal of Physics B: Atomic, Molecular and Optical Physics %D 2007 %T Weighted graph states and applications to spin chains, lattices and gases %A Hartmann, L. %A Calsamiglia, John %A Dür, Wolfgang %A Briegel, Hans J. %B Journal of Physics B: Atomic, Molecular and Optical Physics %V 40 %P S1 %G eng %U http://www.iop.org/EJ/abstract/0953-4075/40/9/S01 %R 10.1088/0953-4075/40/9/S01 %0 Journal Article %J International Journal of Quantum Information %D 2006 %T Aligning spatial frames through quantum channels %A Bagan, Emili %A Muñoz-Tapia, Ramon %B International Journal of Quantum Information %V 4 %P 5 %G eng %0 Journal Article %J Foundations of Physics %D 2006 %T Bell’s Inequality Tests with Meson–Antimeson Pairs %A Bramon, Albert %A Escribano, Rafel %A Garbarino, G. %K bell inequalities %X Recent proposals to test Bell’s inequalities with entangled pairs of pseudoscalar mesons are reviewed. This includes pairs of neutral kaons or B-mesons and offers some hope to close both the locality and the detection loopholes. Specific difficulties, however, appear thus invalidating most of those proposals. The best option requires the use of kaon regeneration effects and could lead to a successful test if moderate K0 and k̄0 detection efficiencies are achieved. %B Foundations of Physics %V 36 %P 563–584 %8 04/2006 %G eng %U http://www.springerlink.com/content/2028326rqh24qgk6/fulltext.pdf %R 10.1007/s10701-005-9030-z %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2006 %T Dynamics of F = 1 [sup 87]Rb condensates at finite temperatures %A Mur-Petit, J. %A Guilleumas, M. %A Polls, Artur %A Sanpera, Anna %A Lewenstein, Maciej %A Bongs, K. %A Sengstock, K. %K atom optics %K atom-photon collisions %K Bose-Einstein condensation %K radiation pressure %K rubidium %K spin dynamics %K vectors %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 73 %P 013629–6 %8 01/2006 %G eng %U http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PLRAAN000073000001013629000001&idtype=cvips&gifs=yes %R 10.1103/PhysRevA.73.013629 %0 Journal Article %J arXiv %D 2006 %T Kaonic Quantum Erasers at KLOE 2: "Erasing the Present, changing the Past'' %A Bramon, Albert %A Garbarino, G. %A Hiesmayr, B. H %K Quantum Physics %X Neutral kaons are unique quantum systems to show some of the most puzzling peculiarities of quantum mechanics. Here we focus on a quantitative version of Bohr's complementary principle and on quantum marking and eraser concepts. In detail we show that neutral kaons (1) are kind of double slit devices encapsulating Bohr's complementarity principle in a simple and transparent way, and (2) offer marking and eraser options which are not afforded by other quantum systems and which can be performed at the DAPHNE machine. %B arXiv %8 11/2006 %G eng %U http://www.arxiv.org/pdf/hep-ph/0609314.pdf http://arxiv.org/abs/hep-ph/0609314 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2006 %T Optimal full estimation of qubit mixed states %A Bagan, Emili %A Ballester, M. A. %A Gill, R. D. %A Monras, Alex %A Muñoz-Tapia, Ramon %K mixed states %K probability %K state estimation %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 73 %P 032301–18 %8 03/2006 %G eng %U http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PLRAAN000073000003032301000001&idtype=cvips&gifs=yes %R 10.1103/PhysRevA.73.032301 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2006 %T Optimal phase measurements with pure Gaussian states %A Monras, Alex %K gaussian states %K optical squeezing %K optical tomography %K quantum theory %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 73 %P 033821–8 %8 03/2006 %G eng %U http://scitation.aip.org/vsearch/servlet/VerityServlet?KEY=ALL&possible1=Monras%2C+Alex&possible1zone=author&maxdisp=25&smode=strresults&aqs=true %R 10.1103/PhysRevA.73.033821 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2006 %T Quantum field theory of photons with orbital angular momentum %A Calvo, Gabriel F. %A Picon, Antonio %A Bagan, Emili %K information theory %K orbital angular momentum %K quantum field theory %K quantum optics %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 73 %P 013805–10 %8 01/2006 %G eng %U http://link.aps.org/abstract/PRA/v73/e013805 %R 10.1103/PhysRevA.73.013805 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2006 %T Quantum-information processing in disordered and complex quantum systems %A Sen(De), Aditi %A Sen, Ujjwal %A Ahufinger, Veronica %A Briegel, Hans J. %A Sanpera, Anna %A Lewenstein, Maciej %K information theory %K quantum entanglement %K quantum gates %K radiation pressure %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 74 %P 062309–8 %8 12/2006 %G eng %U http://link.aps.org/abstract/PRA/v74/e062309 %R 10.1103/PhysRevA.74.062309 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2006 %T Relative states, quantum axes, and quantum references %A Bagan, Emili %A Iblisdir, Sofyan %A Muñoz-Tapia, Ramon %K quantum theory %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 73 %P 022341–6 %8 02/2006 %G eng %U http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PLRAAN000073000002022341000001&idtype=cvips&gifs=yes %R 10.1103/PhysRevA.73.022341 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2006 %T Secrecy properties of quantum channels %A Acín, Antonio %A Bae, J. %A Bagan, Emili %A Baig, Marià %A Masanes, Lluís %A Muñoz-Tapia, Ramon %K information theory %K protocols %K quantum cryptography %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 73 %P 012327–5 %8 01/2006 %G eng %U http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PLRAAN000073000001012327000001&idtype=cvips&gifs=yes %R 10.1103/PhysRevA.73.012327 %0 Journal Article %J Physical Review Letters %D 2006 %T Separable Measurement Estimation of Density Matrices and its Fidelity Gap with Collective Protocols %A Bagan, Emili %A Ballester, M. A. %A Gill, R. D. %A Muñoz-Tapia, Ramon %A Romero-Isart, Oriol %K mixed states %K state estimation %B Physical Review Letters %V 97 %P 130501–4 %G eng %U http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000097000013130501000001&idtype=cvips&gifs=yes %R 10.1103/PhysRevLett.97.130501 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2006 %T Splitting and merging an elongated Bose-Einstein condensate at finite temperature %A Mebrahtu, A. %A Sanpera, Anna %A Lewenstein, Maciej %K Bose-Einstein condensation %K critical points %K fluctuations %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 73 %P 033601–7 %8 03/2006 %G eng %U http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PLRAAN000073000003033601000001&idtype=cvips&gifs=yes %R 10.1103/PhysRevA.73.033601 %0 Journal Article %J JOURNAL OF PHYSICS-LONDON-B ATOMIC MOLECULAR AND OPTICAL PHYSICS %D 2006 %T Strongly correlated Fermi-Bose mixtures in disordered optical lattices %A Sanchez-Palencia, L. %A Ahufinger, Veronica %A Kantian, A. %A Zakrzewski, J. %A Sanpera, Anna %A Lewenstein, Maciej %K optical lattices %B JOURNAL OF PHYSICS-LONDON-B ATOMIC MOLECULAR AND OPTICAL PHYSICS %V 39 %P 121 %G eng %U http://iopscience.iop.org/0953-4075/39/10/S12/pdf/0953-4075\_39\_10\_S12.pdf %0 Journal Article %J Foundations of Physics %D 2006 %T Unitarity as Preservation of Entropy and Entanglement in Quantum Systems %A Hulpke, Florian %A Poulsen, Uffe %A Sanpera, Anna %A Sen(De), Aditi %A Sen, Ujjwal %A Lewenstein, Maciej %X Abstract The logical structure of Quantum Mechanics (QM) and its relation to other fundamental principles of Nature has been for decades a subject of intensive research. In particular, the question whether the dynamical axiom of QM can be derived from other principles has been often considered. In this contribution, we show that unitary evolutions arise as a consequences of demanding preservation of entropy in the evolution of a single pure quantum system, and preservation of entanglement in the evolution of composite quantum systems. 6 %B Foundations of Physics %V 36 %P 477–499 %8 04/2006 %G eng %U http://www.springerlink.com/content/tjg6r668u462848r/fulltext.pdf %R 10.1007/s10701-005-9035-7 %0 Journal Article %J Journal of Modern Optics %D 2005 %T Bell's inequality tests: from photons to B-mesons %A Bramon, Albert %A Escribano, Rafel %A Garbarino, G. %B Journal of Modern Optics %V 52 %P 1681–1684 %G eng %U http://dx.doi.org/10.1080/09500340500072893 %R 10.1080/09500340500072893 %0 Journal Article %J Physical Review Letters %D 2005 %T Cold Atoms in Non-Abelian Gauge Potentials: From the Hofstadter "Moth" to Lattice Gauge Theory %A Osterloh, K. %A Baig, Marià %A Santos, L. %A Zoller, P. %A Lewenstein, Maciej %X We demonstrate how to create artificial external non-Abelian gauge potentials acting on cold atoms in optical lattices. The method employs atoms with k internal states, and laser assisted state sensitive tunneling, described by unitary k×k matrices. The single-particle dynamics in the case of intense U(2) vector potentials lead to a generalized Hofstadter butterfly spectrum which shows a complex mothlike structure. We discuss the possibility to realize non-Abelian interferometry (Aharonov-Bohm effect) and to study many-body dynamics of ultracold matter in external lattice gauge fields. %B Physical Review Letters %I American Physical Society %V 95 %P 010403 %8 06/2005 %G eng %U http://link.aps.org/doi/10.1103/PhysRevLett.95.010403 %R 10.1103/PhysRevLett.95.010403 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2005 %T Comprehensive analysis of quantum pure-state estimation for two-level systems %A Bagan, Emili %A Monras, Alex %A Muñoz-Tapia, Ramon %X Given N identical copies of the state of a quantum two-level system, we analyze its optimal estimation. We consider two situations: general pure states and (pure) states restricted to lie on the equator of the Bloch sphere. We perform a complete and comprehensive analysis of the optimal schemes based on local measurements, and give results (optimal measurements, maximum fidelity, etc.) for arbitrary N, not necessarily large, within the Bayesian framework. We also make a comparative analysis of the asymptotic limit of these results with those derived from a (pointwise) Cramér-Rao type of approach. We give explicit schemes based on local measurements and no classical communication that saturate the fidelity bounds of the most general collective schemes. %B Physical Review A (Atomic, Molecular, and Optical Physics) %I American Physical Society %V 71 %P 062318 %8 06/2005 %G eng %U http://link.aps.org/doi/10.1103/PhysRevA.71.062318 %R 10.1103/PhysRevA.71.062318 %0 Journal Article %J arXiv %D 2005 %T Continuous Variable Solution for Byzantine Agreement %A Neigovzen, Rodion %A Sanpera, Anna %B arXiv %G eng %U http://arxiv.org/abs/quant-ph/0507249 %0 Conference Paper %B Laser Spectroscopy: Proceedings of the XVII International Conference, Aviemore, Scotland UK, 19-24 June 2005 %D 2005 %T Disordered complex systems using cold gases and trapped ions %A Sen(De), Aditi %A Sen, Ujjwal %A Lewenstein, Maciej %A Ahufinger, Veronica %A Pons, Marisa %A Sanpera, Anna %E Hinds, E. A. %E Ferguson, A. %E Riis, E. %B Laser Spectroscopy: Proceedings of the XVII International Conference, Aviemore, Scotland UK, 19-24 June 2005 %I World Scientific %C Singapore %P 158–166 %G eng %U http://arxiv.org/abs/quant-ph/0508018 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2005 %T Disordered ultracold atomic gases in optical lattices: A case study of Fermi-Bose mixtures %A Ahufinger, Veronica %A Sanchez-Palencia, L. %A Kantian, A. %A Sanpera, Anna %A Lewenstein, Maciej %X We present a review of properties of ultracold atomic Fermi-Bose mixtures in inhomogeneous and random optical lattices. In the strong interacting limit and at very low temperatures, fermions form, together with bosons or bosonic holes, composite fermions. Composite fermions behave as a spinless interacting Fermi gas, and in the presence of local disorder they interact via random couplings and feel effective random local potential. This opens a wide variety of possibilities of realizing various kinds of ultracold quantum disordered systems. In this paper we review these possibilities, discuss the accessible quantum disordered phases, and methods for their detection. The discussed quantum phases include Fermi glasses, quantum spin glasses, “dirty” superfluids, disordered metallic phases, and phases involving quantum percolation. %B Physical Review A (Atomic, Molecular, and Optical Physics) %I American Physical Society %V 72 %P 063616 %8 12/2005 %G eng %U http://link.aps.org/doi/10.1103/PhysRevA.72.063616 %R 10.1103/PhysRevA.72.063616 %0 Journal Article %J Physical Review Letters %D 2005 %T Lattice Solitons in Quasicondensates %A Ahufinger, Veronica %A Sanpera, Anna %X We analyze finite temperature effects in the generation of bright solitons in condensates in optical lattices. We show that even in the presence of strong phase fluctuations solitonic structures with a well defined phase profile can be created. We propose a novel family of variational functions which describe well the properties of these solitons and account for the nonlinear effects in the band structure. We discuss also the mobility and collisions of these localized wave packets. %B Physical Review Letters %I American Physical Society %V 94 %P 130403 %8 04/2005 %G eng %U http://link.aps.org/doi/10.1103/PhysRevLett.94.130403 %R 10.1103/PhysRevLett.94.130403 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2005 %T Multiple-copy two-state discrimination with individual measurements %A Acín, Antonio %A Bagan, Emili %A Baig, Marià %A Masanes, Lluís %A Muñoz-Tapia, Ramon %K state discrimination %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 71 %P 032338 %G eng %U http://link.aps.org/abstract/PRA/v71/e032338 %R 10.1103/PhysRevA.71.032338 %0 Journal Article %J arXiv %D 2005 %T Popper's test of quantum mechanics and two-photon "ghost" diffraction %A Bramon, Albert %A Escribano, Rafel %B arXiv %G eng %U http://arxiv.org/abs/quant-ph/0507040 %0 Journal Article %J Physical Review Letters %D 2005 %T Purity Estimation with Separable Measurements %A Bagan, Emili %A Ballester, M. A. %A Muñoz-Tapia, Ramon %A Romero-Isart, Oriol %X Given a large number N of copies of a qubit state of which we wish to estimate its purity, we prove that separable-measurement protocols can be as efficient as the optimal joint-measurement one if classical communication is used. This shows that the optimal estimation of the entanglement of a two-qubit state can also be achieved asymptotically with fully separable measurements. Thus, quantum memories provide no advantage in this situation. The relationship between our global Bayesian approach and the quantum Cramér-Rao bound is discussed. %B Physical Review Letters %I American Physical Society %V 95 %P 110504 %8 09/2005 %G eng %U http://link.aps.org/doi/10.1103/PhysRevLett.95.110504 %R 10.1103/PhysRevLett.95.110504 %0 Journal Article %J Physical Review Letters %D 2005 %T Quantum Key Distillation from Gaussian States by Gaussian Operations %A Navascués, M. %A Bae, J. %A Cirac, J. Ignacio %A Lewenstein, Maciej %A Sanpera, Anna %A Acín, Antonio %X We study the secrecy properties of Gaussian states under Gaussian operations. Although such operations are useless for quantum distillation, we prove that it is possible to distill a secret key secure against any attack from sufficiently entangled Gaussian states with nonpositive partial transposition. Moreover, all such states allow for key distillation, when Eve is assumed to perform finite-size coherent attacks before the reconciliation process. %B Physical Review Letters %I American Physical Society %V 94 %P 010502 %8 01/2005 %G eng %U http://link.aps.org/doi/10.1103/PhysRevLett.94.010502 %R 10.1103/PhysRevLett.94.010502 %0 Book Section %B Lectures on Quantum Information %D 2005 %T The separability versus entanglement problem %A Sen(De), Aditi %A Sen, Ujjwal %A Lewenstein, Maciej %A Sanpera, Anna %E Bruss, Dagmar %E Leuchs, G. %B Lectures on Quantum Information %I Wiley %P 123–146 %@ 9783527405275 %G eng %U http://www3.wileyinterscience.com/cgi-bin/bookhome/117903246?CRETRY=1&SRETRY=0 %R 10.1002/9783527618637 %0 Journal Article %J Physical Review Letters %D 2005 %T Spin Gases: Quantum Entanglement Driven by Classical Kinematics %A Calsamiglia, John %A Hartmann, L. %A Dür, Wolfgang %A Briegel, Hans J. %K entanglement distribution %K spin gas %X A spin gas is a natural extension of a classical gas. It consists of a large number of particles whose (random) motion is described classically, but, in addition, have internal (quantum mechanical) degrees of freedom that interact during collisions. For specific types of quantum interactions we determine the entanglement that occurs naturally in such systems. We analyze how the evolution of the quantum state is determined by the underlying classical kinematics of the gas. For the Boltzmann gas, we calculate the rate at which entanglement is produced and characterize the entanglement properties of the equilibrium state. %B Physical Review Letters %V 95 %P 180502 %8 10/2005 %G eng %U http://link.aps.org/abstract/PRL/v95/e180502 %R 10.1103/PhysRevLett.95.180502 %0 Journal Article %J Optics Letters %D 2005 %T Wigner representation and geometric transformations of optical orbital angular momentum spatial modes %A Calvo, Gabriel F. %K Berry's phase %K Mathematical methods (general) %K Modes %K Physical optics %K Propagation %X An exact Wigner representation of optical spatial modes carrying orbital angular momentum is found in closed form by exploiting the underlying SU(2) Lie-group algebra of their associated Poincaré sphere. Orthogonality relations and observables of these states are obtained within the phase space picture. Development of geometric phases on mode transformations is also elucidated. %B Optics Letters %I OSA %V 30 %P 1207 %8 05/2005 %G eng %U http://ol.osa.org/abstract.cfm?URI=ol-30-10-1207 %R 10.1364/OL.30.001207 %0 Journal Article %J Physical Review A %D 2004 %T Active and passive quantum erasers for neutral kaons %A Bramon, A. %A Garbarino, G. %A Hiesmayr, B. %B Physical Review A %V 69 %G eng %U http://pra.aps.org/abstract/PRA/v69/i6/e062111 %R 10.1103/PhysRevA.69.062111 %0 Journal Article %J Physical Review Letters %D 2004 %T Atomic Fermi-Bose Mixtures in Inhomogeneous and Random Lattices: From Fermi Glass to Quantum Spin Glass and Quantum Percolation %A Sanpera, A. %A Kantian, A. %A Sanchez-Palencia, L. %A Zakrzewski, J. %A Lewenstein, M. %B Physical Review Letters %V 93 %G eng %U http://prl.aps.org/abstract/PRL/v93/i4/e040401 %R 10.1103/PhysRevLett.93.040401 %0 Journal Article %J Physical Review Letters %D 2004 %T Coherence Properties of Guided-Atom Interferometers %A Kreutzmann, H. %A Poulsen, U. %A Lewenstein, M. %A Dumke, R. %A Ertmer, W. %A Birkl, G. %A Sanpera, A. %B Physical Review Letters %V 92 %G eng %U http://prl.aps.org/abstract/PRL/v92/i16/e163201 %R 10.1103/PhysRevLett.92.163201 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2004 %T Collective versus local measurements in a qubit mixed-state estimation %A Bagan, Emili %A Baig, Marià %A Muñoz-Tapia, Ramon %A Rodriguez, A. %X We discuss the problem of estimating a qubit mixed state. We give the optimal estimation that can be inferred from any given set of measurements. For collective measurements and for a large number N of copies, we show that the error in the estimation varies as 1/N. For local measurements, we focus on the simpler case of states lying on the equatorial plane of the Bloch sphere. We show that the error using plain tomography varies as 1/N1/4, while our approach leads to an error proportional to 1/N3/4. %B Physical Review A (Atomic, Molecular, and Optical Physics) %I American Physical Society %V 69 %P 010304 %8 01/2004 %G eng %U http://link.aps.org/doi/10.1103/PhysRevA.69.010304 %R 10.1103/PhysRevA.69.010304 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2004 %T Creation and mobility of discrete solitons in Bose-Einstein condensates %A Ahufinger, Veronica %A Sanpera, Anna %A Pedri, P. %A Santos, L. %A Lewenstein, Maciej %X We analyze the generation and mobility of discrete solitons in Bose-Einstein condensates confined in an optical lattice under realistic experimental conditions. We discuss first the creation of one-dimensional discrete solitons, for both attractive and repulsive interatomic interactions. We then address the issue of their mobility, focusing our attention on the conditions for the experimental observability of the Peierls-Nabarro barrier. Finally we report on the generation of self-trapped structures in two and three dimensions. Discrete solitons may open alternative routes for the manipulation and transport of Bose-Einstein condensates. %B Physical Review A (Atomic, Molecular, and Optical Physics) %I American Physical Society %V 69 %P 053604 %8 05/2004 %G eng %U http://link.aps.org/doi/10.1103/PhysRevA.69.053604 %R 10.1103/PhysRevA.69.053604 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2004 %T Entanglement-assisted alignment of reference frames using a dense covariant coding %A Bagan, Emili %A Baig, Marià %A Muñoz-Tapia, Ramon %X We present a procedure inspired by dense coding, which enables a highly efficient transmission of information of a continuous nature. The procedure requires the sender and the recipient to share a maximally entangled state. We deal with the concrete problem of aligning reference frames or trihedra by means of a quantum system. We find the optimal covariant measurement and compute the corresponding average error, which has a remarkably simple close form. The connection of this procedure with that of estimating unitary transformations on qubits is briefly discussed. %B Physical Review A (Atomic, Molecular, and Optical Physics) %I American Physical Society %V 69 %P 050303 %8 05/2004 %G eng %U http://link.aps.org/doi/10.1103/PhysRevA.69.050303 %R 10.1103/PhysRevA.69.050303 %0 Journal Article %J Physical Review Letters %D 2004 %T Experimental Detection of Multipartite Entanglement using Witness Operators %A Bourennane, Mohamed %A Eibl, Manfred %A Kurtsiefer, Christian %A Gaertner, Sascha %A Weinfurter, Harald %A Gühne, Otfried %A Hyllus, Philipp %A Bruß, Dagmar %A Lewenstein, Maciej %A Sanpera, Anna %B Physical Review Letters %V 92 %G eng %U http://prl.aps.org/abstract/PRL/v92/i8/e087902 %R 10.1103/PhysRevLett.92.087902 %0 Journal Article %J Physics Letters B %D 2004 %T Fixed boundary conditions analysis of the 3d gonihedric Ising model with k=0 %A Baig, Marià %A Clua, J. %A Johnston, D. A. %A Villanova, R. %K Author Keywords: Spin systems %K Fixed boundary conditions %K Gonihedric models %K Phase transitions %X The gonihedric Ising model is a particular case of the class of models defined by Savvidy and Wegner intended as discrete versions of string theories on cubic lattices. In this Letter we perform a high statistics analysis of the phase transition exhibited by the 3d gonihedric Ising model with k=0 in the light of a set of recently stated scaling laws applicable to first order phase transitions with fixed boundary conditions. Even though qualitative evidence was presented in a previous paper to support the existence of a first order phase transition at k=0, only now are we capable of pinpointing the transition inverse temperature at $\beta$c=0.54757(63) and of checking the scaling of standard observables. %B Physics Letters B %V 585 %P 180–186 %G eng %U http://www.sciencedirect.com/science?\_ob=ArticleURL&\_udi=B6TVN-4BSN95Y-3&\_user=1517286&\_coverDate=04%2F08%2F2004&\_rdoc=1&\_fmt=high&\_orig=search&\_sort=d&\_docanchor=&view=c&\_acct=C000053449&\_version=1&\_urlVersion=0&\_userid=1517286&md5=b8fb559f9 %R 10.1016/j.physletb.2004.02.002 %0 Book Section %B Symmetries in Gravity and Field Theory %D 2004 %T Passive Quantum Erasure for Neutral Kaons %A Bramon, Albert %A Garbarino, G. %A Hiesmayr, B. C. %E Aldaya, V. %E Cervero, J. M. %B Symmetries in Gravity and Field Theory %I Ediciones Universidad de Salamanca %C Salamanca %P 223 %G eng %U http://arxiv.org/abs/quant-ph/0404086 %0 Book Section %B Fundamental Physics Meeting "Alberto Galindo" %D 2004 %T Popper's test of Quantum Mechanics %A Bramon, Albert %A Escribano, Rafel %E Alvarez-Estrada, R. F. %E Dobado, A. %E Fernandez, L. A. %E Mart\'ın-Delgado, M. A. %E {Munoz Sudupe}, A. %B Fundamental Physics Meeting "Alberto Galindo" %I Aula Documental de Investigacion %C Madrid %P 201–207 %G eng %U http://arxiv.org/abs/quant-ph/0501134 %0 Journal Article %J Physical Review A %D 2004 %T Quantitative complementarity in two-path interferometry %A Bramon, A. %A Garbarino, G. %A Hiesmayr, B. %B Physical Review A %V 69 %G eng %U http://pra.aps.org/abstract/PRA/v69/i2/e022112 %R 10.1103/PhysRevA.69.022112 %0 Journal Article %J Physical Review Letters %D 2004 %T Quantum Marking and Quantum Erasure for Neutral Kaons %A Bramon, A. %A Garbarino, G. %A Hiesmayr, B. %B Physical Review Letters %V 92 %G eng %U http://prl.aps.org/abstract/PRL/v92/i2/e020405 %R 10.1103/PhysRevLett.92.020405 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2004 %T Quantum reverse engineering and reference-frame alignment without nonlocal correlations %A Bagan, Emili %A Baig, Marià %A Muñoz-Tapia, Ramon %X The estimation of unknown qubit elementary gates and the alignment of reference frames are formally the same problem. Using quantum states made out of N qubits, we show that the theoretical precision limit for both problems, which behaves as 1∕N2, can be asymptotically attained with a covariant protocol that exploits the quantum correlation of internal degrees of freedom instead of the more fragile entanglement between distant parties. This cuts by half the number of qubits needed to achieve the precision of the dense covariant coding protocol. %B Physical Review A (Atomic, Molecular, and Optical Physics) %I American Physical Society %V 70 %P 030301 %8 09/2004 %G eng %U http://link.aps.org/doi/10.1103/PhysRevA.70.030301 %R 10.1103/PhysRevA.70.030301 %0 Journal Article %J Quantum Information and Computation %D 2004 %T Simplifying Schmidt number witnesses via higher-dimensional embeddings %A Hulpke, F. %A Bruss, Dagmar %A Lewenstein, Maciej %A Sanpera, Anna %B Quantum Information and Computation %V 4 %P 207 %G eng %0 Journal Article %J Physical Review Letters %D 2004 %T Soliton Eigenvalue Control in Optical Lattices %A Kartashov, Yaroslav %A Crasovan, Lucian-Cornel %A Zelenina, Anna %A Vysloukh, Victor %A Sanpera, Anna %A Lewenstein, Maciej %A Torner, Lluis %B Physical Review Letters %V 93 %G eng %U http://prl.aps.org/abstract/PRL/v93/i14/e143902 %R 10.1103/PhysRevLett.93.143902 %0 Journal Article %J Physics Letters A %D 2004 %T Violation of a Bell inequality in particle physics experimentally verified? %A Bertlmann, R. A. %A Bramon, Albert %A Garbarino, G. %A Hiesmayr, B. C. %K Bell inequality %K Entanglement %X Relevant aspects for testing Bell inequalities with entangled meson–antimeson systems are analyzed. In particular, we argue that the results of Go [J. Mod. Opt. 51 (2004) 991], which nicely illustrate the quantum entanglement of B-meson pairs, cannot be considered as a Bell-test refuting local realism. %B Physics Letters A %V 332 %P 355–360 %G eng %U http://www.sciencedirect.com/science?\_ob=ArticleURL&\_udi=B6TVM-4DK6240-1&\_user=1517286&\_coverDate=11%2F22%2F2004&\_rdoc=1&\_fmt=high&\_orig=search&\_sort=d&\_docanchor=&view=c&\_searchStrId=1201715030&\_rerunOrigin=google&\_acct=C000053449&\_version=1 %R 10.1016/j.physleta.2004.10.006 %0 Book Section %B Quantum Information Processing %D 2003 %T Entanglement properties of composite quantum systems %A Eckert, Kai %A Guhne, O. %A Hulpke, Florian %A Hyllus, Philipp %A Korbicz, J. %A Mompart, Jordi %A Bruss, Dagmar %A Lewenstein, Maciej %A Sanpera, Anna %E Leuchs, G. %E Beth, T. %B Quantum Information Processing %I Wilew-VCH %C Berlin %P 87–95 %@ 352740371X %G eng %0 Journal Article %J Journal of Modern Optics %D 2003 %T Experimental detection of entanglement via witness operators and local measurements %A Gühne, Otfried %A Hyllus, Philipp %A Bruss, Dagmar %A Ekert, Artur K. %A Lewenstein, Maciej %A Macchiavello, Chiara %A Sanpera, Anna %B Journal of Modern Optics %V 50 %P 1079–1102 %G eng %U http://www.informaworld.com/smpp/36433790-99697256/content\~{}db=all\~{}content=a713826204 %R 10.1080/09500340308234554 %0 Journal Article %J Physical Review A %D 2003 %T Minimal measurements of the gate fidelity of a qudit map %A E. Bagan %A Baig, M. %A Muñoz-Tapia, R. %B Physical Review A %V 67 %G eng %U http://pra.aps.org/abstract/PRA/v67/i1/e014303 %R 10.1103/PhysRevA.67.014303 %0 Conference Paper %B Proceedings of the 8th ICSSUR %D 2003 %T Optimal reconstruction of a pure qubit state with local measurements %A Bagan, Emili %A Baig, Marià %A Monras, Alex %A Muñoz-Tapia, Ramon %B Proceedings of the 8th ICSSUR %I Rinton Press (2003) %G eng %0 Journal Article %J The European Physical Journal C - Particles and Fields %D 2003 %T Quantitative duality and neutral kaon interferometry %A Bramon, A. %A Garbarino, Gianni %A Hiesmayr, B. C. %X A quantitative formulation of Bohr&\#8217;s complementarity principle and interferometric duality is discussed and applied to the neutral kaon system. Recent measurements by the CPLEAR Collaboration can be easily interpreted in terms of neutral kaon interferometry, illustrating and confirming those basic principles of quantum mechanics. The subject can be further investigated at the operational \$\backslash phi\$ -factory Daphne. %B The European Physical Journal C - Particles and Fields %V 32 %P 377–380 %8 01/2003 %G eng %U http://www.springerlink.com/content/4l12acw6kn9xkfbt %R 10.1140/epjc/s2003-01444-5 %0 Conference Paper %B eConf C0309101:THWP005 %D 2003 %T Quantitative duality and quantum erasure for neutral kaons %A Bramon, Albert %A Garbarino, Gianni %A Hiesmayr, Beatrix C. %B eConf C0309101:THWP005 %G eng %0 Journal Article %J Physical Review A %D 2002 %T Detection of entanglement with few local measurements %A Gühne, O. %A Hyllus, P. %A Bruß, D. %A Ekert, A. %A Lewenstein, M. %A Macchiavello, C. %A Sanpera, A. %B Physical Review A %V 66 %G eng %U http://pra.aps.org/abstract/PRA/v66/i6/e062305 %R 10.1103/PhysRevA.66.062305 %0 Journal Article %J Physical Review Letters %D 2002 %T Novel Bell’s Inequalities for Entangled K0K¯0 Pairs %A Bramon, Albert %A Garbarino, Gianni %B Physical Review Letters %V 88 %G eng %U http://prl.aps.org/abstract/PRL/v88/i4/e040403 %R 10.1103/PhysRevLett.88.040403 %0 Journal Article %J Physical Review Letters %D 2002 %T Optimal Scheme for Estimating a Pure Qubit State via Local Measurements %A E. Bagan %A Baig, M. %A Muñoz-Tapia, R. %B Physical Review Letters %V 89 %G eng %U http://prl.aps.org/abstract/PRL/v89/i27/e277904 %R 10.1103/PhysRevLett.89.277904 %0 Journal Article %J Physical Review B %D 2002 %T Scaling laws for the two-dimensional eight-state Potts model with fixed boundary conditions %A Baig, M. %A Villanova, R. %B Physical Review B %V 65 %G eng %U http://prb.aps.org/abstract/PRB/v65/i9/e094428 %R 10.1103/PhysRevB.65.094428 %0 Journal Article %J Physical Review Letters %D 2002 %T Test of Local Realism with Entangled Kaon Pairs and without Inequalities %A Bramon, Albert %A Garbarino, Gianni %B Physical Review Letters %V 89 %G eng %U http://prl.aps.org/abstract/PRL/v89/i16/e160401 %R 10.1103/PhysRevLett.89.160401 %0 Journal Article %J arXiv %D 2001 %T Aligning Reference Frames with Quantum States %A Bagan, Emili %A Baig, Marià %A Muñoz-Tapia, Ramon %B arXiv %G eng %U http://arxiv.org/abs/quant-ph/0106155 %0 Journal Article %J Physical Review Letters %D 2001 %T Aligning Reference Frames with Quantum States %A E. Bagan %A Baig, M. %A Muñoz-Tapia, R. %B Physical Review Letters %V 87 %G eng %U http://prl.aps.org/abstract/PRL/v87/i25/e257903 %R 10.1103/PhysRevLett.87.257903 %0 Journal Article %J Physical Review Letters %D 2001 %T Classification of Mixed Three-Qubit States %A Acín, A. %A Bruß, D. %A Lewenstein, M. %A Sanpera, A. %B Physical Review Letters %V 87 %G eng %U http://prl.aps.org/abstract/PRL/v87/i4/e040401 %R 10.1103/PhysRevLett.87.040401 %0 Journal Article %J Physical Review A %D 2001 %T Communication of spin directions with product states and finite measurements %A E. Bagan %A Baig, M. %A Muñoz-Tapia, R. %B Physical Review A %V 64 %G eng %U http://pra.aps.org/abstract/PRA/v64/i2/e022305 %R 10.1103/PhysRevA.64.022305 %0 Journal Article %J Science (New York, N.Y.) %D 2001 %T Feynman's path-integral approach for intense-laser-atom interactions %A Salieres, P. %A Carre, B. %A Le Deroff, L. %A Grasbon, F. %A Paulus, G. G. %A Walther, H. %A Kopold, R. %A Becker, W. %A Milosevic, D. B. %A Sanpera, A. %A Lewenstein, M. %X Atoms interacting with intense laser fields can emit electrons and photons of very high energies. An intuitive and quantitative explanation of these highly nonlinear processes can be found in terms of a generalization of classical Newtonian particle trajectories, the so-called quantum orbits. Very few quantum orbits are necessary to reproduce the experimental results. These orbits are clearly identified, thus opening the way for an efficient control as well as previously unknown applications of these processes. %B Science (New York, N.Y.) %V 292 %P 902–5 %G eng %U http://www.sciencemag.org/cgi/content/abstract/292/5518/902 %R 10.1126/science.108836 %0 Journal Article %J Physical Review A %D 2001 %T Optimal encoding and decoding of a spin direction %A E. Bagan %A Baig, M. %A Brey, A. %A Muñoz-Tapia, R. %A Tarrach, R. %B Physical Review A %V 63 %G eng %U http://pra.aps.org/abstract/PRA/v63/i5/e052309 %R 10.1103/PhysRevA.63.052309 %0 Journal Article %J Physical Review A %D 2001 %T Schmidt-number witnesses and bound entanglement %A Sanpera, Anna %A Bruß, Dagmar %A Lewenstein, Maciej %B Physical Review A %V 63 %G eng %U http://pra.aps.org/abstract/PRA/v63/i5/e050301 %R 10.1103/PhysRevA.63.050301 %0 Journal Article %J Physical Review Letters %D 2000 %T Optimal Strategies for Sending Information through A Quantum Channel %A Bagan, E. %A Baig, M. %A Brey, A. %A Muñoz-Tapia, R. %A Tarrach, R. %B Physical Review Letters %V 85 %P 5230 - 5233 %8 Jan-12-2000 %U http://link.aps.org/doi/10.1103/PhysRevLett.85.5230http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevLett.85.5230/fulltexthttp://link.aps.org/article/10.1103/PhysRevLett.85.5230 %! Phys. Rev. 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