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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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. 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 (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 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 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 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 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 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 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. Lett. %R 10.1103/PhysRevLett.85.5230