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.

VL - 7 UR - https://link.aps.org/doi/10.1103/PhysRevX.7.031022 ER - TY - JOUR T1 - Decoupling with random diagonal unitaries JF - Quantum Y1 - 2017 A1 - Nakata, Yoshifumi A1 - Hirche, Christoph A1 - Morgan, Ciara A1 - Winter, Andreas AB - 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… VL - 1 UR - https://quantum-journal.org/papers/q-2017-07-21-18/ ER - TY - JOUR T1 - Enhancement of low-temperature thermometry by strong coupling JF - PHYSICAL REVIEW A Y1 - 2017 A1 - Correa, Luis A. A1 - Perarnau-Llobet, Marti A1 - Hovhannisyan, Karen V. A1 - Hernandez-Santana, Senaida A1 - Mehboudi, Mohammad A1 - Sanpera, Anna AB - 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. VL - 96 ER - TY - JOUR T1 - Equilibration Time Scales of Physically Relevant Observables JF - PHYSICAL REVIEW X Y1 - 2017 A1 - Garcia-Pintos, Luis Pedro A1 - Linden, Noah A1 - Malabarba, Artur S. L. A1 - Short, Anthony J. A1 - Winter, Andreas AB -We 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.

VL - 7 ER - TY - JOUR T1 - 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.

VL - 119 ER - TY - JOUR T1 - Genuine-multipartite entanglement criteria based on positive maps JF - JOURNAL OF MATHEMATICAL PHYSICS Y1 - 2017 A1 - Clivaz, Fabien A1 - Huber, Marcus A1 - Lami, Ludovico A1 - Murta, Glaucia AB - 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. VL - 58 ER - TY - JOUR T1 - Global and local thermometry schemes in coupled quantum systems JF - NEW JOURNAL OF PHYSICS Y1 - 2017 A1 - Campbell, Steve A1 - Mehboudi, Mohammad A1 - De Chiara, Gabriele A1 - Paternostro, Mauro AB - 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. VL - 19 ER - TY - JOUR T1 - Heating without heat: Thermodynamics of passive energy filters between finite systems JF - PHYSICAL REVIEW E Y1 - 2017 A1 - Muñoz-Tapia, R. A1 - Brito, R. A1 - Parrondo, J. M. R. AB -Passive 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.

VL - 96 ER - TY - JOUR T1 - Magnetic phases of spin-1 lattice gases with random interactions JF - PHYSICAL REVIEW B Y1 - 2017 A1 - McAlpine, Kenneth D. A1 - Paganelli, Simone A1 - Ciuchi, Sergio A1 - Sanpera, Anna A1 - De Chiara, Gabriele AB - 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. VL - 95 ER - TY - JOUR T1 - Resource theory of coherence: Beyon} states JF - PHYSICAL REVIEW A Y1 - 2017 A1 - Ben Dana, Khaled A1 - Garcia Diaz, Maria A1 - Mejatty, Mohamed A1 - Winter, Andreas AB -We 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.

VL - 95 ER - TY - JOUR T1 - Unitary 2-designs from random X- and Z-diagonal unitaries JF - JOURNAL OF MATHEMATICAL PHYSICS Y1 - 2017 A1 - Nakata, Yoshifumi A1 - Hirche, Christoph A1 - Morgan, Ciara A1 - Winter, Andreas AB -Unitary 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.

VL - 58 ER - TY - JOUR T1 - Achieving sub-shot-noise sensing at finite temperatures JF - Physical Review A Y1 - 2016 A1 - Mehboudi, Mohammad A1 - Correa, Luis A. A1 - Sanpera, Anna VL - 94 IS - 4 JO - Phys. Rev. A ER - TY - JOUR T1 - Classical capacities of quantum channels with environment assistance JF - Problems of Information Transmission Y1 - 2016 A1 - Karumanchi, S. A1 - Mancini, S. A1 - Winter, A. A1 - Yang, D. AB - 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. VL - 52 UR - https://link.springer.com/article/10.1134/S0032946016030029 ER - TY - JOUR T1 - Multi-photon entanglement in high dimensions JF - Nature Photonics Y1 - 2016 A1 - Malik, Mehul A1 - Erhard, Manuel A1 - Huber, Marcus A1 - Krenn, Mario A1 - Robert Fickler A1 - Anton Zeilinger PB - Nature Publishing Group VL - 10 SN - 1749-4885 UR - http://dx.doi.org/10.1038/nphoton.2016.12 IS - 4 ER - TY - JOUR T1 - Polar Codes in Network Quantum Information Theory JF - IEEE Transactions on Information Theory Y1 - 2016 A1 - Christoph Hirche A1 - Ciara Morgan A1 - Wilde, Mark M. VL - 62 IS - 2 JO - IEEE Trans. Inform. Theory ER - TY - JOUR T1 - Probabilistic metrology or how some measurement outcomes render ultra-precise estimates JF - New Journal of Physics Y1 - 2016 A1 - Calsamiglia, J A1 - Gendra, B A1 - Muñoz-Tapia, R A1 - Bagan, E AB -We 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.

VL - 18 UR - 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 JO - New J. Phys. ER - TY - JOUR T1 - Quantum Change Point JF - Physical Review Letters Y1 - 2016 A1 - Sentís, Gael A1 - Bagan, Emilio A1 - Calsamiglia, John A1 - Chiribella, Giulio A1 - Muñoz-Tapia, Ramon VL - 117 UR - http://link.aps.org/doi/10.1103/PhysRevLett.117.150502http://link.aps.org/article/10.1103/PhysRevLett.117.150502 JO - Phys. Rev. Lett. ER - TY - JOUR T1 - Quantum Channel Capacities With Passive Environment Assistance JF - IEEE Transactions on Information Theory Y1 - 2016 A1 - Karumanchi, Siddharth A1 - Mancini, Stefano A1 - Winter, Andreas A1 - Dong Yang VL - 62 IS - 4 JO - IEEE Trans. Inform. Theory ER - TY - JOUR T1 - Quantum learning of classical stochastic processes: The completely positive realization problem JF - Journal of Mathematical Physics Y1 - 2016 A1 - Monràs, Alex A1 - Winter, Andreas VL - 57 IS - 1 JO - J. Math. Phys. ER - TY - JOUR T1 - Realising a quantum absorption refrigerator with an atom-cavity system JF - Quantum Science and Technology Y1 - 2016 A1 - Mitchison, Mark T. A1 - Huber, Marcus A1 - Prior, Javier A1 - Woods, Mischa P. A1 - Plenio, Martin B. AB - 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. VL - 1 UR - http://stacks.iop.org/2058-9565/1/i=1/a=015001 ER - TY - JOUR T1 - Steering Maps and Their Application to Dimension-Bounded Steering JF - Physical Review Letters Y1 - 2016 A1 - Moroder, Tobias A1 - Gittsovich, Oleg A1 - Huber, Marcus A1 - Uola, Roope A1 - Gühne, Otfried VL - 116 UR - http://link.aps.org/doi/10.1103/PhysRevLett.116.090403 ER - TY - JOUR T1 - Testing a Quantum Heat Pump with a Two-Level Spin JF - Entropy Y1 - 2016 A1 - Correa, Luis A1 - Mehboudi, Mohammad VL - 18 UR - http://www.mdpi.com/1099-4300/18/4/141/htm IS - 4 JO - Entropy ER - TY - JOUR T1 - Coherence-assisted single-shot cooling by quantum absorption refrigerators JF - New Journal of Physics Y1 - 2015 A1 - Mitchison, Mark T A1 - Woods, Mischa P A1 - Prior, Javier A1 - Huber, Marcus VL - 17 UR - http://stacks.iop.org/1367-2630/17/i=11/a=115013?key=crossref.3c2d7f1bc5de4bbea1cba38fd2236293 ER - TY - ICOMM T1 - Decoupling with random diagonal-unitary matrices Y1 - 2015 A1 - Yoshifumi Nakata A1 - Christoph Hirche A1 - Ciara Morgan A1 - Winter, Andreas UR - http://arxiv.org/abs/1509.05155 ER - TY - ICOMM T1 - Implementing unitary 2-designs using random diagonal-unitary matrices Y1 - 2015 A1 - Yoshifumi Nakata A1 - Christoph Hirche A1 - Ciara Morgan A1 - Winter, Andreas UR - http://arxiv.org/abs/1502.07514 ER - TY - JOUR T1 - Individual Quantum Probes for Optimal Thermometry JF - Physical Review Letters Y1 - 2015 A1 - Correa, Luis A. A1 - Mehboudi, Mohammad A1 - Adesso, Gerardo A1 - Sanpera, Anna VL - 114 IS - 22 JO - Phys. Rev. Lett. ER - TY - JOUR T1 - Thermometry precision in strongly correlated ultracold lattice gases JF - New Journal of Physics Y1 - 2015 A1 - Mehboudi, M A1 - Moreno-Cardoner, M. A1 - Chiara, G De A1 - Sanpera, A. VL - 17 IS - 5 JO - New J. Phys. ER - TY - JOUR T1 - Bounds on entanglement assisted source-channel coding via the Lovasz theta number and its variants JF - IEEE Transactions on Information Theory Y1 - 2014 A1 - Toby S. Cubitt A1 - Laura Mancinska A1 - David Roberson A1 - Stahlke, Daniel A1 - Winter, Andreas VL - 60 ER - TY - JOUR T1 - Case study of the uniaxial anisotropic spin-1 bilinear-biquadratic Heisenberg model on a triangular lattice JF - Physical Review B Y1 - 2014 A1 - Moreno-Cardoner, M. A1 - Perrin, H. A1 - Paganelli, S. A1 - De Chiara, G. A1 - Sanpera, A. AB - 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. VL - 90 IS - 14 JO - Phys. Rev. B ER - TY - JOUR T1 - Entanglement properties of spin models in triangular lattices JF - Journal of Statistical Mechanics: Theory and Experiment Y1 - 2014 A1 - Moreno-Cardoner, M. A1 - Paganelli, S. A1 - De Chiara, G. A1 - Sanpera, A. AB - 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. VL - 2014 IS - 10 JO - J. Stat. Mech. ER - TY - JOUR T1 - Everything You Always Wanted to Know About LOCC (But Were Afraid to Ask) JF - Communications in Mathematical Physics Y1 - 2014 A1 - Eric Chitambar A1 - Debbie Leung A1 - Mančinska, Laura A1 - Maris Ozols A1 - Winter, Andreas VL - 328 IS - 1 JO - Commun. Math. Phys. ER - TY - JOUR T1 - Full Security of Quantum Key Distribution From No-Signaling Constraints JF - IEEE Transactions on Information Theory Y1 - 2014 A1 - Masanes, Lluis A1 - Renner, Renato A1 - Christandl, Matthias A1 - Winter, Andreas A1 - Barrett, Jonathan VL - 60 IS - 8 JO - IEEE Trans. Inform. Theory ER - TY - JOUR T1 - ‘Pretty strong’ converse for the quantum capacity of degradable channels JF - IEEE Transactions on Information Theory Y1 - 2014 A1 - Ciara Morgan A1 - Winter, Andreas VL - 60 ER - TY - JOUR T1 - Probabilistic Metrology Attains Macroscopic Cloning of Quantum Clocks JF - Physical Review Letters Y1 - 2014 A1 - Gendra, B. A1 - Calsamiglia, John A1 - Muñoz-Tapia, R. A1 - E. Bagan A1 - Chiribella, G. VL - 113 UR - http://link.aps.org/doi/10.1103/PhysRevLett.113.260402 ER - TY - JOUR T1 - Steering Bound Entangled States: A Counterexample to the Stronger Peres Conjecture JF - Physical Review Letters Y1 - 2014 A1 - Moroder, Tobias A1 - Gittsovich, Oleg A1 - Huber, Marcus A1 - Gühne, Otfried AB - 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. VL - 113 IS - 5 JO - Phys. Rev. Lett. ER - TY - JOUR T1 - Bose-Glass Phases of Ultracold Atoms due to Cavity Backaction JF - Physical Review Letters Y1 - 2013 A1 - Hessam Habibian A1 - André Winter A1 - Paganelli, Simone A1 - Heiko Rieger A1 - Giovanna Morigi AB - 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. VL - 110 UR - http://prl.aps.org/abstract/PRL/v110/i7/e075304 IS - 7 JO - Phys. Rev. Lett. ER - TY - JOUR T1 - Programmable discrimination with an error margin JF - Physical Review A Y1 - 2013 A1 - Sentís, G A1 - E. Bagan A1 - Calsamiglia, John A1 - Muñoz-Tapia, R. VL - 88 IS - 5 JO - Phys. Rev. A ER - TY - Generic T1 - The Quantum Entropy Cone of Stabiliser States T2 - 8th Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2013) Y1 - 2013 A1 - Noah Linden A1 - Frantisek Matus A1 - Mary Beth Ruskai A1 - Winter, Andreas ED - Severini, Simone ED - Fernando Brandao JF - 8th Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2013) T3 - LIPICS CY - Guelph, ON VL - 22 ER - TY - JOUR T1 - Quantum hypergraph states JF - New Journal of Physics Y1 - 2013 A1 - Rossi, M A1 - Huber, M A1 - Bruß, D. A1 - Macchiavello, C. VL - 15 IS - 11 JO - New J. Phys. ER - TY - JOUR T1 - Quantum Metrology Assisted by Abstention JF - Physical Review Letters Y1 - 2013 A1 - Gendra, B. A1 - Ronco-Bonvehi, E. A1 - Calsamiglia, John A1 - Muñoz-Tapia, R. A1 - E. Bagan AB - 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. VL - 110 UR - http://prl.aps.org/abstract/PRL/v110/i10/e100501 IS - 10 JO - Phys. Rev. Lett. ER - TY - JOUR T1 - Quantum phases of incommensurate optical lattices due to cavity backaction JF - Phyiscal Review A Y1 - 2013 A1 - Hessam Habibian A1 - André Winter A1 - Paganelli, Simone A1 - Heiko Rieger A1 - Giovanna Morigi VL - 88 ER - TY - JOUR T1 - The structure of Renyi entropic inequalities JF - Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences Y1 - 2013 A1 - Linden, N. A1 - Mosonyi, M. A1 - Winter, A. VL - 469 IS - 2158472222 JO - Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences ER - TY - JOUR T1 - Beating noise with abstention in state estimation JF - New Journal of Physics Y1 - 2012 A1 - Gendra, Bernat A1 - Ronco-Bonvehi, Elio A1 - Calsamiglia, John A1 - Muñoz-Tapia, Ramon A1 - Bagan, Emilio VL - 14 UR - http://iopscience.iop.org/1367-2630/14/10/105015 ER - TY - JOUR T1 - Glass to superfluid transition in dirty bosons on a lattice JF - New J. Phys. Y1 - 2012 A1 - Julia Stasińska A1 - Massignan, Pietro A1 - Bishop, Michael A1 - Wehr, Jan A1 - Sanpera, Anna A1 - Lewenstein, Maciej AB - 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. VL - 14 UR - http://arxiv.org/abs/1111.3494 ER - TY - JOUR T1 - Quantum learning without quantum memory JF - Scientific Reports (Nature Publishing Group) Y1 - 2012 A1 - Sentís, Gael A1 - Calsamiglia, John A1 - Muñoz-Tapia, Ramon A1 - Bagan, Emilio AB - 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. PB - Nature Publishing Group VL - 2 UR - http://www.nature.com/srep/2012/121005/srep00708/full/srep00708.html ER - TY - JOUR T1 - Quantum Memory Assisted Probing of Dynamical Spin Correlations JF - Physical Review Letters Y1 - 2012 A1 - O. Romero-Isart A1 - Rizzi, M. A1 - Muschik, C. A. A1 - Polzik, E. S. A1 - Lewenstein, M. A1 - Sanpera, A. AB - 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. VL - 108 IS - 6 JO - Phys. Rev. Lett. ER - TY - JOUR T1 - Spin-driven spatial symmetry breaking of spinor condensates in a double well JF - Physical Review A Y1 - 2012 A1 - Melé-Messeguer, M. A1 - Paganelli, Simone A1 - Juliá-Díaz, B. A1 - Ann Sanpera A1 - Polls, A. PB - American Physical Society VL - 86 UR - http://link.aps.org/doi/10.1103/PhysRevA.86.053626 IS - 5 ER - TY - BOOK T1 - Ultracold atoms in optical lattices: simulating Quantum Many-Body systems Y1 - 2012 A1 - Lewenstein Maciej A1 - Sanpera Anna A1 - Ahufinger Verònica PB - Oxford University Press SN - 978-0199573127 ER - TY - JOUR T1 - Entangling two distant oscillators with a quantum reservoir JF - EPL (Europhysics Letters) Y1 - 2011 A1 - Wolf, A A1 - De Chiara, G. A1 - Kajari, E A1 - Lutz, E A1 - G. Morigi AB - 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. VL - 95 UR - http://epljournal.edpsciences.org/index.php?option=com_article&access=standard&Itemid=129&url=/articles/epl/abs/2011/18/epl13816/epl13816.html IS - 6 JO - EPL ER - TY - JOUR T1 - Quantifying, characterizing, and controlling information flow in ultracold atomic gases JF - Physical Review A Y1 - 2011 A1 - Haikka, P. A1 - McEndoo, S. A1 - De Chiara, G. A1 - G. M. Palma A1 - Maniscalco, S. AB - 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). VL - 84 UR - http://pra.aps.org/abstract/PRA/v84/i3/e031602 IS - 3 JO - Phys. Rev. A ER - TY - THES T1 - Quantum Accuracy Limits and Benchmarks in Continuos Variable Systems T2 - Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics Y1 - 2011 A1 - Bracons, Mariona (formerly Mariona Aspachs) JF - Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics PB - Universitat Autònoma de Barcelona CY - Barcelona VL - Doctorat en Física ER - TY - JOUR T1 - Creating p-wave superfluids and topological excitations in optical lattices JF - Phys. Rev. A Y1 - 2010 A1 - Massignan, P. A1 - Sanpera, A. A1 - Lewenstein, M. AB - 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. PB - American Physical Society VL - 81 UR - http://link.aps.org/doi/10.1103/PhysRevA.81.031607 ER - TY - JOUR T1 - Decay of Polarons and Molecules in a Strongly Polarized Fermi Gas JF - Phys. Rev. Lett. Y1 - 2010 A1 - Bruun, G. M. A1 - Massignan, P. AB - 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. PB - American Physical Society VL - 105 UR - http://link.aps.org/doi/10.1103/PhysRevLett.105.020403 ER - TY - THES T1 - Discriminació quàntica T2 - Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics Y1 - 2010 A1 - Arnau Pujol AB - 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. JF - Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics PB - Universitat Autònoma de Barcelona CY - Barcelona VL - Grau en Física ER - TY - JOUR T1 - Estimation of quantum finite mixtures JF - Physical Review A Y1 - 2010 A1 - de Vicente, J. I. A1 - Calsamiglia, John A1 - Muñoz-Tapia, Ramon A1 - Bagan, Emilio AB - 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. VL - 81 UR - http://link.aps.org/doi/10.1103/PhysRevA.81.012332 ER - TY - JOUR T1 - Local Discrimination of Mixed States JF - Phys. Rev. Lett. Y1 - 2010 A1 - Calsamiglia, John A1 - de Vicente, J. I. A1 - Muñoz-Tapia, R. A1 - E. Bagan PB - American Physical Society VL - 105 ER - TY - JOUR T1 - Multicopy programmable discrimination of general qubit states JF - Physical Review A Y1 - 2010 A1 - Sentís, Gael A1 - Bagan, Emili A1 - Calsamiglia, John A1 - Muñoz-Tapia, Ramon AB - 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. VL - 82 UR - http://link.aps.org/doi/10.1103/PhysRevA.82.042312 ER - TY - JOUR T1 - The quantum ground state of self-organized atomic crystals in optical resonators JF - Phys. Rev. A Y1 - 2010 A1 - S. Fernández-Vidal A1 - De Chiara, Gabriele A1 - J. Larson A1 - G. Morigi AB - 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. VL - 81 UR - http://link.aps.org/doi/10.1103/PhysRevA.81.043407 ER - TY - JOUR T1 - Recycling of qubits JF - Physica Scripta Y1 - 2010 A1 - P Rapcan A1 - Calsamiglia, John A1 - Muñoz-Tapia, Ramon A1 - Bagan, Emilio A1 - V Buzek AB - 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. VL - T140 UR - http://iopscience.iop.org/1402-4896/2010/T140/014059?fromSearchPage=true ER - TY - JOUR T1 - Spontaneous nucleation of structural defects in inhomogeneous ion chains JF - New. J. Phys. Y1 - 2010 A1 - De Chiara, Gabriele A1 - del Campo, Adolfo A1 - G. Morigi A1 - M. B. Plenio A1 - A. Retzker AB - 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. VL - 12 UR - http://iopscience.iop.org/1367-2630/12/11/115003?fromSearchPage=true ER - TY - JOUR T1 - Structural defects in ion crystals by quenching the external potential: the inhomogeneous Kibble-Zurek mechanism JF - Phys. Rev. Lett. Y1 - 2010 A1 - del Campo, Adolfo A1 - De Chiara, Gabriele A1 - G. Morigi A1 - M. B. Plenio A1 - A. Retzker VL - 105 UR - http://link.aps.org/doi/10.1103/PhysRevLett.105.075701 ER - TY - JOUR T1 - Topological superfluids on a lattice with non-Abelian gauge fields JF - Europhysics Letters Y1 - 2010 A1 - A. Kubasiak A1 - Massignan, P. A1 - Lewenstein, M. VL - 92 UR - http://arxiv.org/abs/1007.4827 ER - TY - JOUR T1 - Phase estimation for thermal Gaussian states JF - Physical Review A (Atomic, Molecular, and Optical Physics) Y1 - 2009 A1 - Aspachs, Mariona A1 - Calsamiglia, John A1 - Muñoz-Tapia, Ramon A1 - Bagan, Emili KW - gaussian noise KW - gaussian states KW - optical losses KW - optical squeezing KW - phase estimation KW - quantum noise KW - thermal noise VL - 79 UR - http://link.aps.org/abstract/PRA/v79/e033834 ER - TY - JOUR T1 - Phase-covariant quantum benchmarks JF - Physical Review A (Atomic, Molecular, and Optical Physics) Y1 - 2009 A1 - Calsamiglia, John A1 - Aspachs, Mariona A1 - Muñoz-Tapia, Ramon A1 - Bagan, Emili KW - quantum benchmarks VL - 79 UR - http://link.aps.org/doi/10.1103/PhysRevA.79.050301 ER - TY - JOUR T1 - Double-barrier potentials for matter-wave gap solitons JF - Physical Review A (Atomic, Molecular, and Optical Physics) Y1 - 2008 A1 - Ahufinger, Veronica A1 - Malomed, B. A. A1 - Birkl, G. A1 - Corbalán, Ramon A1 - Sanpera, Anna KW - Bose-Einstein condensation KW - Fabry-Perot interferometers KW - matter waves KW - optical solitons VL - 78 UR - http://link.aps.org/abstract/PRA/v78/e013608 http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PLRAAN000078000001013608000001&idtype=cvips&gifs=yes ER - TY - JOUR T1 - Operational Interpretation for Global Multipartite Entanglement JF - Physical Review Letters Y1 - 2008 A1 - Boixo, Sergio A1 - Monras, Alex KW - multipartite entanglement VL - 100 UR - 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:// ER - TY - JOUR T1 - Phase variance of squeezed vacuum states JF - Physical Review A (Atomic, Molecular, and Optical Physics) Y1 - 2008 A1 - Bagan, Emili A1 - Monras, Alex A1 - Muñoz-Tapia, Ramon KW - optical squeezing VL - 78 UR - http://link.aps.org/abstract/PRA/v78/e043829 ER - TY - JOUR T1 - Quantum Chernoff bound as a measure of distinguishability between density matrices: Application to qubit and Gaussian states JF - Physical Review A (Atomic, Molecular, and Optical Physics) Y1 - 2008 A1 - Calsamiglia, John A1 - Muñoz-Tapia, Ramon A1 - Masanes, Lluís A1 - Acín, Antonio A1 - Bagan, Emili KW - Bayes methods KW - error statistics KW - quantum chernoff bound KW - quantum statistical mechanics KW - quantum theory VL - 77 UR - http://link.aps.org/abstract/PRA/v77/e032311 ER - TY - JOUR T1 - Quantum information processing in spin chains via quantum Zeno dynamics JF - arXiv Y1 - 2008 A1 - Monras, Alex A1 - Romero-Isart, Oriol KW - spin chain AB - 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. UR - http://arxiv.org/abs/0801.1959 ER - TY - JOUR T1 - Discriminating States: The Quantum Chernoff Bound JF - Physical Review Letters Y1 - 2007 A1 - K. M. R. Audenaert A1 - J. Calsamiglia A1 - R. {Munoz-Tapia} A1 - E. Bagan A1 - Ll. Masanes A1 - A. Acin A1 - F. Verstraete VL - 98 UR - http://link.aps.org/abstract/PRL/v98/e160501 ER - TY - JOUR T1 - Discriminating States: The Quantum Chernoff Bound JF - Physical Review Letters Y1 - 2007 A1 - K. M. R. Audenaert A1 - Calsamiglia, John A1 - Muñoz-Tapia, Ramon A1 - Bagan, Emili A1 - Masanes, Lluís A1 - Acín, Antonio A1 - Verstraete, Frank KW - quantum chernoff bound KW - state discrimination VL - 98 UR - http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000098000016160501000001&idtype=cvips&gifs=yes ER - TY - JOUR T1 - How to hide a secret direction JF - New Journal of Physics Y1 - 2007 A1 - Bagan, Emili A1 - Calsamiglia, John A1 - Demkowicz-Dobrzanski, R. A1 - Muñoz-Tapia, Ramon AB - 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. VL - 9 UR - http://iopscience.iop.org/1367-2630/9/8/244/pdf/1367-2630\_9\_8\_244.pdf ER - TY - JOUR T1 - Optimal Quantum Estimation of Loss in Bosonic Channels JF - Physical Review Letters Y1 - 2007 A1 - Monras, Alex A1 - Paris, Matteo G. A. VL - 98 UR - http://link.aps.org/abstract/PRL/v98/e160401 ER - TY - JOUR T1 - Predicting Spinor Condensate Dynamics from Simple Principles JF - Physical Review Letters Y1 - 2007 A1 - Moreno-Cardoner, M. A1 - Mur-Petit, J. A1 - Guilleumas, M. A1 - Polls, Artur A1 - Sanpera, Anna A1 - Lewenstein, Maciej KW - Bose-Einstein condensation KW - entropy VL - 99 UR - http://link.aps.org/abstract/PRL/v99/e020404 ER - TY - JOUR T1 - Quantum switches and quantum memories for matter-wave lattice solitons JF - New Journal of Physics Y1 - 2007 A1 - Ahufinger, Veronica A1 - Mebrahtu, A. A1 - Corbalán, Ramon A1 - Sanpera, Anna VL - 9 UR - http://iopscience.iop.org/1367-2630/9/1/004/pdf/1367-2630\_9\_1\_004.pdf ER - TY - JOUR T1 - Recycling of quantum information: Multiple observations of quantum systems JF - arXiv Y1 - 2007 A1 - Rapcan, Peter A1 - Calsamiglia, John A1 - Muñoz-Tapia, Ramon A1 - Bagan, Emili A1 - Bużek, Vladimir AB - 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. UR - http://arxiv.org/abs/0708.1086 ER - TY - JOUR T1 - Aligning spatial frames through quantum channels JF - International Journal of Quantum Information Y1 - 2006 A1 - Bagan, Emili A1 - Muñoz-Tapia, Ramon VL - 4 ER - TY - JOUR T1 - Dynamics of F = 1 [sup 87]Rb condensates at finite temperatures JF - Physical Review A (Atomic, Molecular, and Optical Physics) Y1 - 2006 A1 - Mur-Petit, J. A1 - Guilleumas, M. A1 - Polls, Artur A1 - Sanpera, Anna A1 - Lewenstein, Maciej A1 - Bongs, K. A1 - Sengstock, K. KW - atom optics KW - atom-photon collisions KW - Bose-Einstein condensation KW - radiation pressure KW - rubidium KW - spin dynamics KW - vectors VL - 73 UR - http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PLRAAN000073000001013629000001&idtype=cvips&gifs=yes ER - TY - JOUR T1 - Optimal full estimation of qubit mixed states JF - Physical Review A (Atomic, Molecular, and Optical Physics) Y1 - 2006 A1 - Bagan, Emili A1 - Ballester, M. A. A1 - Gill, R. D. A1 - Monras, Alex A1 - Muñoz-Tapia, Ramon KW - mixed states KW - probability KW - state estimation VL - 73 UR - http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PLRAAN000073000003032301000001&idtype=cvips&gifs=yes ER - TY - JOUR T1 - Optimal phase measurements with pure Gaussian states JF - Physical Review A (Atomic, Molecular, and Optical Physics) Y1 - 2006 A1 - Monras, Alex KW - gaussian states KW - optical squeezing KW - optical tomography KW - quantum theory VL - 73 UR - http://scitation.aip.org/vsearch/servlet/VerityServlet?KEY=ALL&possible1=Monras%2C+Alex&possible1zone=author&maxdisp=25&smode=strresults&aqs=true ER - TY - JOUR T1 - Relative states, quantum axes, and quantum references JF - Physical Review A (Atomic, Molecular, and Optical Physics) Y1 - 2006 A1 - Bagan, Emili A1 - Iblisdir, Sofyan A1 - Muñoz-Tapia, Ramon KW - quantum theory VL - 73 UR - http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PLRAAN000073000002022341000001&idtype=cvips&gifs=yes ER - TY - JOUR T1 - Secrecy properties of quantum channels JF - Physical Review A (Atomic, Molecular, and Optical Physics) Y1 - 2006 A1 - Acín, Antonio A1 - Bae, J. A1 - Bagan, Emili A1 - Baig, Marià A1 - Masanes, Lluís A1 - Muñoz-Tapia, Ramon KW - information theory KW - protocols KW - quantum cryptography VL - 73 UR - http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PLRAAN000073000001012327000001&idtype=cvips&gifs=yes ER - TY - JOUR T1 - Separable Measurement Estimation of Density Matrices and its Fidelity Gap with Collective Protocols JF - Physical Review Letters Y1 - 2006 A1 - Bagan, Emili A1 - Ballester, M. A. A1 - Gill, R. D. A1 - Muñoz-Tapia, Ramon A1 - Romero-Isart, Oriol KW - mixed states KW - state estimation VL - 97 UR - http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000097000013130501000001&idtype=cvips&gifs=yes ER - TY - JOUR T1 - Splitting and merging an elongated Bose-Einstein condensate at finite temperature JF - Physical Review A (Atomic, Molecular, and Optical Physics) Y1 - 2006 A1 - Mebrahtu, A. A1 - Sanpera, Anna A1 - Lewenstein, Maciej KW - Bose-Einstein condensation KW - critical points KW - fluctuations VL - 73 UR - http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PLRAAN000073000003033601000001&idtype=cvips&gifs=yes ER - TY - JOUR T1 - Comprehensive analysis of quantum pure-state estimation for two-level systems JF - Physical Review A (Atomic, Molecular, and Optical Physics) Y1 - 2005 A1 - Bagan, Emili A1 - Monras, Alex A1 - Muñoz-Tapia, Ramon AB - 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. PB - American Physical Society VL - 71 UR - http://link.aps.org/doi/10.1103/PhysRevA.71.062318 ER - TY - JOUR T1 - Multiple-copy two-state discrimination with individual measurements JF - Physical Review A (Atomic, Molecular, and Optical Physics) Y1 - 2005 A1 - Acín, Antonio A1 - Bagan, Emili A1 - Baig, Marià A1 - Masanes, Lluís A1 - Muñoz-Tapia, Ramon KW - state discrimination VL - 71 UR - http://link.aps.org/abstract/PRA/v71/e032338 ER - TY - JOUR T1 - Purity Estimation with Separable Measurements JF - Physical Review Letters Y1 - 2005 A1 - Bagan, Emili A1 - Ballester, M. A. A1 - Muñoz-Tapia, Ramon A1 - Romero-Isart, Oriol AB - 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. PB - American Physical Society VL - 95 UR - http://link.aps.org/doi/10.1103/PhysRevLett.95.110504 ER - TY - JOUR T1 - Collective versus local measurements in a qubit mixed-state estimation JF - Physical Review A (Atomic, Molecular, and Optical Physics) Y1 - 2004 A1 - Bagan, Emili A1 - Baig, Marià A1 - Muñoz-Tapia, Ramon A1 - Rodriguez, A. AB - 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. PB - American Physical Society VL - 69 UR - http://link.aps.org/doi/10.1103/PhysRevA.69.010304 ER - TY - JOUR T1 - Entanglement-assisted alignment of reference frames using a dense covariant coding JF - Physical Review A (Atomic, Molecular, and Optical Physics) Y1 - 2004 A1 - Bagan, Emili A1 - Baig, Marià A1 - Muñoz-Tapia, Ramon AB - 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. PB - American Physical Society VL - 69 UR - http://link.aps.org/doi/10.1103/PhysRevA.69.050303 ER - TY - CHAP T1 - Popper's test of Quantum Mechanics T2 - Fundamental Physics Meeting "Alberto Galindo" Y1 - 2004 A1 - Bramon, Albert A1 - Escribano, Rafel ED - Alvarez-Estrada, R. F. ED - Dobado, A. ED - Fernandez, L. A. ED - Mart\'ın-Delgado, M. A. ED - {Munoz Sudupe}, A. JF - Fundamental Physics Meeting "Alberto Galindo" PB - Aula Documental de Investigacion CY - Madrid UR - http://arxiv.org/abs/quant-ph/0501134 ER - TY - JOUR T1 - Quantum reverse engineering and reference-frame alignment without nonlocal correlations JF - Physical Review A (Atomic, Molecular, and Optical Physics) Y1 - 2004 A1 - Bagan, Emili A1 - Baig, Marià A1 - Muñoz-Tapia, Ramon AB - 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. PB - American Physical Society VL - 70 UR - http://link.aps.org/doi/10.1103/PhysRevA.70.030301 ER - TY - CHAP T1 - Entanglement properties of composite quantum systems T2 - Quantum Information Processing Y1 - 2003 A1 - Eckert, Kai A1 - Guhne, O. A1 - Hulpke, Florian A1 - Hyllus, Philipp A1 - Korbicz, J. A1 - Mompart, Jordi A1 - Bruss, Dagmar A1 - Lewenstein, Maciej A1 - Sanpera, Anna ED - Leuchs, G. ED - Beth, T. JF - Quantum Information Processing PB - Wilew-VCH CY - Berlin SN - 352740371X ER - TY - JOUR T1 - Experimental detection of entanglement via witness operators and local measurements JF - Journal of Modern Optics Y1 - 2003 A1 - Gühne, Otfried A1 - Hyllus, Philipp A1 - Bruss, Dagmar A1 - Ekert, Artur K. A1 - Lewenstein, Maciej A1 - Macchiavello, Chiara A1 - Sanpera, Anna VL - 50 UR - http://www.informaworld.com/smpp/36433790-99697256/content\~{}db=all\~{}content=a713826204 ER - TY - JOUR T1 - Minimal measurements of the gate fidelity of a qudit map JF - Physical Review A Y1 - 2003 A1 - E. Bagan A1 - Baig, M. A1 - Muñoz-Tapia, R. VL - 67 UR - http://pra.aps.org/abstract/PRA/v67/i1/e014303 ER - TY - CONF T1 - Optimal reconstruction of a pure qubit state with local measurements T2 - Proceedings of the 8th ICSSUR Y1 - 2003 A1 - Bagan, Emili A1 - Baig, Marià A1 - Monras, Alex A1 - Muñoz-Tapia, Ramon JF - Proceedings of the 8th ICSSUR PB - Rinton Press (2003) ER - TY - JOUR T1 - Detection of entanglement with few local measurements JF - Physical Review A Y1 - 2002 A1 - Gühne, O. A1 - Hyllus, P. A1 - Bruß, D. A1 - Ekert, A. A1 - Lewenstein, M. A1 - Macchiavello, C. A1 - Sanpera, A. VL - 66 UR - http://pra.aps.org/abstract/PRA/v66/i6/e062305 ER - TY - JOUR T1 - Optimal Scheme for Estimating a Pure Qubit State via Local Measurements JF - Physical Review Letters Y1 - 2002 A1 - E. Bagan A1 - Baig, M. A1 - Muñoz-Tapia, R. VL - 89 UR - http://prl.aps.org/abstract/PRL/v89/i27/e277904 ER - TY - JOUR T1 - Aligning Reference Frames with Quantum States JF - arXiv Y1 - 2001 A1 - Bagan, Emili A1 - Baig, Marià A1 - Muñoz-Tapia, Ramon UR - http://arxiv.org/abs/quant-ph/0106155 ER - TY - JOUR T1 - Aligning Reference Frames with Quantum States JF - Physical Review Letters Y1 - 2001 A1 - E. Bagan A1 - Baig, M. A1 - Muñoz-Tapia, R. VL - 87 UR - http://prl.aps.org/abstract/PRL/v87/i25/e257903 ER - TY - JOUR T1 - Communication of spin directions with product states and finite measurements JF - Physical Review A Y1 - 2001 A1 - E. Bagan A1 - Baig, M. A1 - Muñoz-Tapia, R. VL - 64 UR - http://pra.aps.org/abstract/PRA/v64/i2/e022305 ER - TY - JOUR T1 - Feynman's path-integral approach for intense-laser-atom interactions JF - Science (New York, N.Y.) Y1 - 2001 A1 - Salieres, P. A1 - Carre, B. A1 - Le Deroff, L. A1 - Grasbon, F. A1 - Paulus, G. G. A1 - Walther, H. A1 - Kopold, R. A1 - Becker, W. A1 - Milosevic, D. B. A1 - Sanpera, A. A1 - Lewenstein, M. AB - 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. VL - 292 UR - http://www.sciencemag.org/cgi/content/abstract/292/5518/902 ER - TY - JOUR T1 - Optimal encoding and decoding of a spin direction JF - Physical Review A Y1 - 2001 A1 - E. Bagan A1 - Baig, M. A1 - Brey, A. A1 - Muñoz-Tapia, R. A1 - Tarrach, R. VL - 63 UR - http://pra.aps.org/abstract/PRA/v63/i5/e052309 ER - TY - JOUR T1 - Optimal Strategies for Sending Information through A Quantum Channel JF - Physical Review Letters Y1 - 2000 A1 - Bagan, E. A1 - Baig, M. A1 - Brey, A. A1 - Muñoz-Tapia, R. A1 - Tarrach, R. VL - 85 UR - 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 JO - Phys. Rev. Lett. ER -