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 %XTime 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 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 LETTERS %D 2017 %T Exact Identification of a Quantum Change Point %A Sentís, Gael %A Calsamiglia, John %A Muñoz-Tapia, Ramon %XThe 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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. 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 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 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 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 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 (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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 Journal of Modern Optics %D 2000 %T Separability and distillability in composite quantum systems - a primer %A Lewenstein, Maciej %A Bruß, Dagmar %A Cirac, J. Ignacio %A Kraus, Barbara %A Kuś, M. %A Samsonowicz, J. %A Sanpera, Anna %A Tarrach, R. %B Journal of Modern Optics %V 47 %P 2481–2499 %G eng %U http://www.informaworld.com/smpp/6084610-44811133/content\~{}db=all\~{}content=a755833577 %R 10.1080/09500340008232176 %0 Journal Article %J {PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES} %D 0 %T {Quantum reference frames and their applications to thermodynamics} %A Popescu, Sandu %A Belen Sainz, Ana %A Short, Anthony J. %A Winter, Andreas %X{We construct a quantum reference frame, which can be used to approximately implement arbitrary unitary transformations on a system in the presence of any number of extensive conserved quantities, by absorbing any back action provided by the conservation laws. Thus, the reference frame at the same time acts as a battery for the conserved quantities. Our construction features a physically intuitive, clear and implementation-friendly realization. Indeed, the reference system is composed of the same types of subsystems as the original system and is finite for any desired accuracy. In addition, the interaction with the reference frame can be broken down into two-body terms coupling the system to one of the reference frame subsystems at a time. We apply this construction to quantum thermodynamic set-ups with multiple, possibly non-commuting conserved quantities, which allows for the definition of explicit batteries in such cases. This article is part of a discussion meeting issue `Foundations of quantum mechanics and their impact on contemporary society'.}

%B {PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES} %V {376} %8 {JUL 13} %R {10.1098/rsta.2018.0111}