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{\textemdash}a prerequisite for any system to function as a clock{\textemdash}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{\textquoteright}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.

}, doi = {10.1103/PhysRevX.7.031022}, url = {https://link.aps.org/doi/10.1103/PhysRevX.7.031022}, author = {Erker, Paul and Mitchison, Mark T. and Silva, Ralph and Woods, Mischa P. and Brunner, Nicolas and Huber, Marcus} } @article {nakata_decoupling_2017, title = {Decoupling with random diagonal unitaries}, journal = {Quantum}, volume = {1}, year = {2017}, month = {jul}, pages = {18}, abstract = {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{\textellipsis}}, doi = {10.22331/q-2017-07-21-18}, url = {https://quantum-journal.org/papers/q-2017-07-21-18/}, author = {Nakata, Yoshifumi and Hirche, Christoph and Morgan, Ciara and Winter, Andreas} } @article {correa_enhancement_2017, title = {Enhancement of low-temperature thermometry by strong coupling}, journal = {PHYSICAL REVIEW A}, volume = {96}, number = {6}, year = {2017}, abstract = {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.}, issn = {2469-9926}, doi = {10.1103/PhysRevA.96.062103}, author = {Correa, Luis A. and Perarnau-Llobet, Marti and Hovhannisyan, Karen V. and Hernandez-Santana, Senaida and Mehboudi, Mohammad and Sanpera, Anna} } @article {garcia-pintos_equilibration_2017, title = {Equilibration Time Scales of Physically Relevant Observables}, journal = {PHYSICAL REVIEW X}, volume = {7}, number = {3}, year = {2017}, abstract = {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.

}, issn = {2160-3308}, doi = {10.1103/PhysRevX.7.031027}, author = {Garcia-Pintos, Luis Pedro and Linden, Noah and Malabarba, Artur S. L. and Short, Anthony J. and Winter, Andreas} } @article {891, title = {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.

}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.119.140506}, author = {Sent{\'\i}s, Gael and Calsamiglia, John and Mu{\~n}oz-Tapia, Ramon} } @article {clivaz_genuine-multipartite_2017, title = {Genuine-multipartite entanglement criteria based on positive maps}, journal = {JOURNAL OF MATHEMATICAL PHYSICS}, volume = {58}, number = {8}, year = {2017}, month = {aug}, abstract = {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.}, issn = {0022-2488}, doi = {10.1063/1.4998433}, author = {Clivaz, Fabien and Huber, Marcus and Lami, Ludovico and Murta, Glaucia} } @article {campbell_global_2017, title = {Global and local thermometry schemes in coupled quantum systems}, journal = {NEW JOURNAL OF PHYSICS}, volume = {19}, year = {2017}, month = {oct}, abstract = {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.}, issn = {1367-2630}, doi = {10.1088/1367-2630/aa7fac}, author = {Campbell, Steve and Mehboudi, Mohammad and De Chiara, Gabriele and Paternostro, Mauro} } @article {munoz-tapia_heating_2017, title = {Heating without heat: Thermodynamics of passive energy filters between finite systems}, journal = {PHYSICAL REVIEW E}, volume = {96}, number = {3}, year = {2017}, abstract = {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.

}, issn = {2470-0045}, doi = {10.1103/PhysRevE.96.030103}, author = {Mu{\~n}oz-Tapia, R. and Brito, R. and Parrondo, J. M. R.} } @article {mcalpine_magnetic_2017, title = {Magnetic phases of spin-1 lattice gases with random interactions}, journal = {PHYSICAL REVIEW B}, volume = {95}, number = {23}, year = {2017}, month = {jun}, abstract = {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.}, issn = {2469-9950}, doi = {10.1103/PhysRevB.95.235128}, author = {McAlpine, Kenneth D. and Paganelli, Simone and Ciuchi, Sergio and Sanpera, Anna and De Chiara, Gabriele} } @article {ben_dana_resource_2017, title = {Resource theory of coherence: Beyon} states}, journal = {PHYSICAL REVIEW A}, volume = {95}, number = {6}, year = {2017}, abstract = {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 ({\textquotedblright}incoherent{\textquotedblright}) 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 {\textquotedblleft}strictly incoherent{\textquotedblright} 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.

}, issn = {2469-9926}, doi = {10.1103/PhysRevA.95.062327}, author = {Ben Dana, Khaled and Garcia Diaz, Maria and Mejatty, Mohamed and Winter, Andreas} } @article {nakata_unitary_2017, title = {Unitary 2-designs from random X- and Z-diagonal unitaries}, journal = {JOURNAL OF MATHEMATICAL PHYSICS}, volume = {58}, number = {5}, year = {2017}, abstract = {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.

}, issn = {0022-2488}, doi = {10.1063/1.4983266}, author = {Nakata, Yoshifumi and Hirche, Christoph and Morgan, Ciara and Winter, Andreas} } @article {780, title = {Achieving sub-shot-noise sensing at finite temperatures}, journal = {Physical Review A}, volume = {94}, year = {2016}, month = {10/2016}, issn = {2469-9934}, doi = {10.1103/PhysRevA.94.042121}, author = {Mehboudi, Mohammad and Correa, Luis A. and Sanpera, Anna} } @article {karumanchi_classical_2016, title = {Classical capacities of quantum channels with environment assistance}, journal = {Problems of Information Transmission}, volume = {52}, number = {3}, year = {2016}, month = {jul}, pages = {214{\textendash}238}, abstract = {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{\textquoteright}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.}, issn = {0032-9460, 1608-3253}, doi = {10.1134/S0032946016030029}, url = {https://link.springer.com/article/10.1134/S0032946016030029}, author = {Karumanchi, S. and Mancini, S. and Winter, A. and Yang, D.} } @article {738, title = {Multi-photon entanglement in high dimensions}, journal = {Nature Photonics}, volume = {10}, year = {2016}, month = {04/2016}, pages = {248 - 252}, publisher = {Nature Publishing Group}, isbn = {1749-4885}, url = {http://dx.doi.org/10.1038/nphoton.2016.12}, author = {Malik, Mehul and Erhard, Manuel and Huber, Marcus and Krenn, Mario and Robert Fickler and Anton Zeilinger} } @article {710, title = {Polar Codes in Network Quantum Information Theory}, journal = {IEEE Transactions on Information Theory}, volume = {62}, year = {2016}, month = {02/2016}, pages = {915 - 924}, issn = {1557-9654}, doi = {10.1109/TIT.2016.2514319}, author = {Christoph Hirche and Ciara Morgan and Wilde, Mark M.} } @article {787, title = {Probabilistic metrology or how some measurement outcomes render ultra-precise estimates}, journal = {New Journal of Physics}, volume = {18}, year = {2016}, month = {Jan-10-2016}, pages = {103049}, abstract = {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.\

}, doi = {10.1088/1367-2630/18/10/103049}, url = {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}, author = {Calsamiglia, J and Gendra, B and Mu{\~n}oz-Tapia, R and Bagan, E} } @article {784, title = {Quantum Change Point}, journal = {Physical Review Letters}, volume = {117}, year = {2016}, month = {Jan-10-2016}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.117.150502}, url = {http://link.aps.org/doi/10.1103/PhysRevLett.117.150502http://link.aps.org/article/10.1103/PhysRevLett.117.150502}, author = {Sent{\'\i}s, Gael and Bagan, Emilio and Calsamiglia, John and Chiribella, Giulio and Mu{\~n}oz-Tapia, Ramon} } @article {685, title = {Quantum Channel Capacities With Passive Environment Assistance}, journal = {IEEE Transactions on Information Theory}, volume = {62}, number = {4}, year = {2016}, month = {4/2016}, pages = {1733-1747}, issn = {1557-9654}, doi = {10.1109/TIT.2016.2522192}, author = {Karumanchi, Siddharth and Mancini, Stefano and Winter, Andreas and Dong Yang} } @article {696, title = {Quantum learning of classical stochastic processes: The completely positive realization problem}, journal = {Journal of Mathematical Physics}, volume = {57}, number = {1}, year = {2016}, month = {01/2016}, pages = {015219}, issn = {1089-7658}, doi = {10.1063/1.4936935}, author = {Monr{\`a}s, Alex and Winter, Andreas} } @article {mitchison_realising_2016, title = {Realising a quantum absorption refrigerator with an atom-cavity system}, journal = {Quantum Science and Technology}, volume = {1}, number = {1}, year = {2016}, pages = {015001}, abstract = {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.}, issn = {2058-9565}, doi = {10.1088/2058-9565/1/1/015001}, url = {http://stacks.iop.org/2058-9565/1/i=1/a=015001}, author = {Mitchison, Mark T. and Huber, Marcus and Prior, Javier and Woods, Mischa P. and Plenio, Martin B.} } @article {moroder_steering_2016, title = {Steering Maps and Their Application to Dimension-Bounded Steering}, journal = {Physical Review Letters}, volume = {116}, number = {9}, year = {2016}, issn = {0031-9007, 1079-7114}, doi = {10.1103/PhysRevLett.116.090403}, url = {http://link.aps.org/doi/10.1103/PhysRevLett.116.090403}, author = {Moroder, Tobias and Gittsovich, Oleg and Huber, Marcus and Uola, Roope and G{\"u}hne, Otfried} } @article {707, title = {Testing a Quantum Heat Pump with a Two-Level Spin}, journal = {Entropy}, volume = {18}, year = {2016}, month = {4/2016}, pages = {141}, doi = {10.3390/e18040141}, url = {http://www.mdpi.com/1099-4300/18/4/141/htm}, author = {Correa, Luis and Mehboudi, Mohammad} } @article {mitchison_coherence-assisted_2015, title = {Coherence-assisted single-shot cooling by quantum absorption refrigerators}, journal = {New Journal of Physics}, volume = {17}, number = {11}, year = {2015}, pages = {115013}, issn = {1367-2630}, doi = {10.1088/1367-2630/17/11/115013}, url = {http://stacks.iop.org/1367-2630/17/i=11/a=115013?key=crossref.3c2d7f1bc5de4bbea1cba38fd2236293}, author = {Mitchison, Mark T and Woods, Mischa P and Prior, Javier and Huber, Marcus} } @article {700, title = {Decoupling with random diagonal-unitary matrices}, year = {2015}, url = {http://arxiv.org/abs/1509.05155}, author = {Yoshifumi Nakata and Christoph Hirche and Ciara Morgan and Winter, Andreas} } @article {702, title = {Implementing unitary 2-designs using random diagonal-unitary matrices}, year = {2015}, url = {http://arxiv.org/abs/1502.07514}, author = {Yoshifumi Nakata and Christoph Hirche and Ciara Morgan and Winter, Andreas} } @article {708, title = {Individual Quantum Probes for Optimal Thermometry}, journal = {Physical Review Letters}, volume = {114}, year = {2015}, month = {6/2015}, issn = {1079-7114}, doi = {10.1103/PhysRevLett.114.220405}, author = {Correa, Luis A. and Mehboudi, Mohammad and Adesso, Gerardo and Sanpera, Anna} } @article {709, title = {Thermometry precision in strongly correlated ultracold lattice gases}, journal = {New Journal of Physics}, volume = {17}, year = {2015}, month = {05/2015}, pages = {055020}, doi = {10.1088/1367-2630/17/5/055020}, author = {Mehboudi, M and Moreno-Cardoner, M. and Chiara, G De and Sanpera, A.} } @article {510, title = {Bounds on entanglement assisted source-channel coding via the Lovasz theta number and its variants}, journal = {IEEE Transactions on Information Theory}, volume = {60}, number = {11}, year = {2014}, month = {11/2014}, pages = {7330-7344}, doi = {10.1109/TIT.2014.2349502}, author = {Toby S. Cubitt and Laura Mancinska and David Roberson and Stahlke, Daniel and Winter, Andreas} } @article {577, title = {Case study of the uniaxial anisotropic spin-1 bilinear-biquadratic Heisenberg model on a triangular lattice}, journal = {Physical Review B}, volume = {90}, year = {2014}, month = {10/2014}, abstract = {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.}, issn = {1550-235X}, doi = {10.1103/PhysRevB.90.144409}, author = {Moreno-Cardoner, M. and Perrin, H. and Paganelli, S. and De Chiara, G. and Sanpera, A.} } @article {580, title = {Entanglement properties of spin models in triangular lattices}, journal = {Journal of Statistical Mechanics: Theory and Experiment}, volume = {2014}, year = {2014}, month = {10/2014}, pages = {P10008}, abstract = {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.}, doi = {10.1088/1742-5468/2014/10/P10008}, author = {Moreno-Cardoner, M. and Paganelli, S. and De Chiara, G. and Sanpera, A.} } @article {687, title = {Everything You Always Wanted to Know About LOCC (But Were Afraid to Ask)}, journal = {Communications in Mathematical Physics}, volume = {328}, number = {1}, year = {2014}, month = {5/2014}, pages = {303-326}, issn = {1432-0916}, doi = {10.1007/s00220-014-1953-9}, author = {Eric Chitambar and Debbie Leung and Man{\v c}inska, Laura and Maris Ozols and Winter, Andreas} } @article {684, title = {Full Security of Quantum Key Distribution From No-Signaling Constraints}, journal = {IEEE Transactions on Information Theory}, volume = {60}, number = {8}, year = {2014}, month = {8/2014}, pages = {4973-4986}, issn = {1557-9654}, doi = {10.1109/TIT.2014.2329417}, author = {Masanes, Lluis and Renner, Renato and Christandl, Matthias and Winter, Andreas and Barrett, Jonathan} } @article {504, title = {{\textquoteleft}Pretty strong{\textquoteright} converse for the quantum capacity of degradable channels}, journal = {IEEE Transactions on Information Theory}, volume = {60}, number = {1}, year = {2014}, month = {01/2014}, pages = {317-333}, doi = {10.1109/TIT.2013.2288971}, author = {Ciara Morgan and Winter, Andreas} } @article {gendra_probabilistic_2014, title = {Probabilistic Metrology Attains Macroscopic Cloning of Quantum Clocks}, journal = {Physical Review Letters}, volume = {113}, number = {26}, year = {2014}, issn = {0031-9007, 1079-7114}, doi = {10.1103/PhysRevLett.113.260402}, url = {http://link.aps.org/doi/10.1103/PhysRevLett.113.260402}, author = {Gendra, B. and Calsamiglia, John and Mu{\~n}oz-Tapia, R. and E. Bagan and Chiribella, G.} } @article {563, title = {Steering Bound Entangled States: A Counterexample to the Stronger Peres Conjecture}, journal = {Physical Review Letters}, volume = {113}, year = {2014}, month = {8/2014}, abstract = {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.}, issn = {1079-7114}, doi = {10.1103/PhysRevLett.113.050404}, author = {Moroder, Tobias and Gittsovich, Oleg and Huber, Marcus and G{\"u}hne, Otfried} } @article {429, title = {Bose-Glass Phases of Ultracold Atoms due to Cavity Backaction}, journal = {Physical Review Letters}, volume = {110}, year = {2013}, month = {2/2013}, abstract = {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.}, issn = {1079-7114}, doi = {10.1103/PhysRevLett.110.075304}, url = {http://prl.aps.org/abstract/PRL/v110/i7/e075304}, author = {Hessam Habibian and Andr{\'e} Winter and Paganelli, Simone and Heiko Rieger and Giovanna Morigi} } @article {499, title = {Programmable discrimination with an error margin}, journal = {Physical Review A}, volume = {88}, year = {2013}, month = {11/2013}, pages = {052304}, issn = {1094-1622}, doi = {10.1103/PhysRevA.88.052304}, author = {Sent{\'\i}s, G and E. Bagan and Calsamiglia, John and Mu{\~n}oz-Tapia, R.} } @proceedings {505, title = {The Quantum Entropy Cone of Stabiliser States}, volume = {22}, number = {1302.5453}, year = {2013}, month = {02/2013}, pages = {270-284}, address = {Guelph, ON}, doi = {10.4230/LIPIcs.TQC.2013.270}, author = {Noah Linden and Frantisek Matus and Mary Beth Ruskai and Winter, Andreas}, editor = {Severini, Simone and Fernando Brandao} } @article {517, title = {Quantum hypergraph states}, journal = {New Journal of Physics}, volume = {15}, year = {2013}, month = {11/2013}, pages = {113022}, doi = {10.1088/1367-2630/15/11/113022}, author = {Rossi, M and Huber, M and Bru{\ss}, D. and Macchiavello, C.} } @article {436, title = {Quantum Metrology Assisted by Abstention}, journal = {Physical Review Letters}, volume = {110}, year = {2013}, month = {3/2013}, pages = {100501}, abstract = {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.}, issn = {1079-7114}, doi = {10.1103/PhysRevLett.110.100501}, url = {http://prl.aps.org/abstract/PRL/v110/i10/e100501}, author = {Gendra, B. and Ronco-Bonvehi, E. and Calsamiglia, John and Mu{\~n}oz-Tapia, R. and E. Bagan} } @article {471, title = {Quantum phases of incommensurate optical lattices due to cavity backaction}, journal = {Phyiscal Review A}, volume = {88}, year = {2013}, pages = {043618}, author = {Hessam Habibian and Andr{\'e} Winter and Paganelli, Simone and Heiko Rieger and Giovanna Morigi} } @article {497, title = {The structure of Renyi entropic inequalities}, journal = {Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences}, volume = {469}, year = {2013}, month = {10/2013}, pages = {20120737 }, issn = {0018-9448}, doi = {10.1098/rspa.2012.0737}, author = {Linden, N. and Mosonyi, M. and Winter, A.} } @article {gendra_beating_2012, title = {Beating noise with abstention in state estimation}, journal = {New Journal of Physics}, volume = {14}, number = {10}, year = {2012}, pages = {105015}, issn = {1367-2630}, doi = {10.1088/1367-2630/14/10/105015}, url = {http://iopscience.iop.org/1367-2630/14/10/105015}, author = {Gendra, Bernat and Ronco-Bonvehi, Elio and Calsamiglia, John and Mu{\~n}oz-Tapia, Ramon and Bagan, Emilio} } @article {Stasinska2011, title = {Glass to superfluid transition in dirty bosons on a lattice}, journal = {New J. Phys.}, volume = {14}, year = {2012}, pages = {043043}, abstract = {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.}, doi = {10.1088/1367-2630/14/4/043043}, url = {http://arxiv.org/abs/1111.3494}, author = {Julia Stasi{\'n}ska and Massignan, Pietro and Bishop, Michael and Wehr, Jan and Sanpera, Anna and Lewenstein, Maciej} } @article {Sentis:2012uq, title = {Quantum learning without quantum memory}, journal = {Scientific Reports (Nature Publishing Group)}, volume = {2}, year = {2012}, month = {10/2012}, pages = {708}, publisher = {Nature Publishing Group}, abstract = {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 {\textquotedblleft}estimate-and-discriminate{\textquotedblright} machine, which estimates the states of the training qubits and classifies the data qubit with a discrimination protocol tailored to the obtained estimates.}, doi = {10.1038/srep00708}, url = {http://www.nature.com/srep/2012/121005/srep00708/full/srep00708.html}, author = {Sent{\'\i}s, Gael and Calsamiglia, John and Mu{\~n}oz-Tapia, Ramon and Bagan, Emilio} } @article {444, title = {Quantum Memory Assisted Probing of Dynamical Spin Correlations}, journal = {Physical Review Letters}, volume = {108}, year = {2012}, month = {2/2012}, abstract = {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.}, issn = {1079-7114}, doi = {10.1103/PhysRevLett.108.065302}, author = {O. Romero-Isart and Rizzi, M. and Muschik, C. A. and Polzik, E. S. and Lewenstein, M. and Sanpera, A.} } @article {383, title = {Spin-driven spatial symmetry breaking of spinor condensates in a double well}, journal = {Physical Review A}, volume = {86}, year = {2012}, month = {2012/11/26/}, pages = {053626 - }, publisher = {American Physical Society}, url = {http://link.aps.org/doi/10.1103/PhysRevA.86.053626}, author = {Mel{\'e}-Messeguer, M. and Paganelli, Simone and Juli{\'a}-D{\'\i}az, B. and Ann Sanpera and Polls, A.} } @book {360, title = {Ultracold atoms in optical lattices: simulating Quantum Many-Body systems}, year = {2012}, publisher = {Oxford University Press}, organization = {Oxford University Press}, isbn = {978-0199573127}, issn = {0199573123}, author = {Lewenstein Maciej and Sanpera Anna and Ahufinger Ver{\`o}nica} } @article {431, title = {Entangling two distant oscillators with a quantum reservoir}, journal = {EPL (Europhysics Letters)}, volume = {95}, year = {2011}, month = {09/2011}, pages = {60008}, abstract = {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{\textquoteright}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.}, issn = {1286-4854}, doi = {10.1209/0295-5075/95/60008}, url = {http://epljournal.edpsciences.org/index.php?option=com_article\&access=standard\&Itemid=129\&url=/articles/epl/abs/2011/18/epl13816/epl13816.html}, author = {Wolf, A and De Chiara, G. and Kajari, E and Lutz, E and G. Morigi} } @article {432, title = {Quantifying, characterizing, and controlling information flow in ultracold atomic gases}, journal = {Physical Review A}, volume = {84}, year = {2011}, month = {9/2011}, abstract = {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).}, issn = {1094-1622}, doi = {10.1103/PhysRevA.84.031602}, url = {http://pra.aps.org/abstract/PRA/v84/i3/e031602}, author = {Haikka, P. and McEndoo, S. and De Chiara, G. and G. M. Palma and Maniscalco, S.} } @mastersthesis {392, title = {Quantum Accuracy Limits and Benchmarks in Continuos Variable Systems}, volume = {Doctorat en F{\'\i}sica}, year = {2011}, month = {10/2011}, school = {Universitat Aut{\`o}noma de Barcelona}, type = {PhD}, address = {Barcelona}, author = {Bracons, Mariona (formerly Mariona Aspachs)} } @article {PhysRevA.81.031607, title = {Creating p-wave superfluids and topological excitations in optical lattices}, journal = {Phys. Rev. A}, volume = {81}, number = {3}, year = {2010}, month = {Mar/2010}, pages = {031607}, publisher = {American Physical Society}, abstract = {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.}, doi = {10.1103/PhysRevA.81.031607}, url = {http://link.aps.org/doi/10.1103/PhysRevA.81.031607}, author = {Massignan, P. and Sanpera, A. and Lewenstein, M.} } @article {PhysRevLett.105.020403, title = {Decay of Polarons and Molecules in a Strongly Polarized Fermi Gas}, journal = {Phys. Rev. Lett.}, volume = {105}, number = {2}, year = {2010}, month = {Jul}, pages = {020403}, publisher = {American Physical Society}, abstract = {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.}, doi = {10.1103/PhysRevLett.105.020403}, url = {http://link.aps.org/doi/10.1103/PhysRevLett.105.020403}, author = {Bruun, G. M. and Massignan, P.} } @mastersthesis {295, title = {Discriminaci{\'o} qu{\`a}ntica}, volume = {Grau en F{\'\i}sica}, year = {2010}, month = {07/2010}, school = {Universitat Aut{\`o}noma de Barcelona}, type = {BSc Thesis}, address = {Barcelona}, abstract = {S{\textquoteright}estudia la discriminaci{\'o} de diferents sistemes qu{\`a}ntics. S{\textquoteright}analitza el proc{\'e}s de discriminaci{\'o} entre q{\'u}bits, i es compara amb la discriminaci{\'o} de mesures projectives. Seguidament, es busquen les condicions en les quals la discriminaci{\'o} entre mesures projectives {\'e}s perfecta, i es realitza el mateix per a la discriminaci{\'o} de transformacions unit{\`a}ries, per tal de comparar ambd{\'o}s resultats.}, author = {Arnau Pujol} } @article {de_vicente_estimation_2010, title = {Estimation of quantum finite mixtures}, journal = {Physical Review A}, volume = {81}, number = {1}, year = {2010}, month = {1/2010}, pages = {012332}, abstract = {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.}, doi = {10.1103/PhysRevA.81.012332}, url = {http://link.aps.org/doi/10.1103/PhysRevA.81.012332}, author = {de Vicente, J. I. and Calsamiglia, John and Mu{\~n}oz-Tapia, Ramon and Bagan, Emilio} } @article {PhysRevLett.105.080504, title = {Local Discrimination of Mixed States}, journal = {Phys. Rev. Lett.}, volume = {105}, number = {8}, year = {2010}, month = {Aug}, pages = {080504}, publisher = {American Physical Society}, doi = {10.1103/PhysRevLett.105.080504}, author = {Calsamiglia, John and de Vicente, J. I. and Mu{\~n}oz-Tapia, R. and E. Bagan} } @article {sentis_multicopy_2010, title = {Multicopy programmable discrimination of general qubit states}, journal = {Physical Review A}, volume = {82}, number = {4}, year = {2010}, month = {10/2010}, pages = {042312}, abstract = {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.}, doi = {10.1103/PhysRevA.82.042312}, url = {http://link.aps.org/doi/10.1103/PhysRevA.82.042312}, author = {Sent{\'\i}s, Gael and Bagan, Emili and Calsamiglia, John and Mu{\~n}oz-Tapia, Ramon} } @article {218, title = {The quantum ground state of self-organized atomic crystals in optical resonators}, journal = {Phys. Rev. A}, volume = {81}, year = {2010}, month = {04/2010}, pages = {043407}, abstract = {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.}, doi = {10.1103/PhysRevA.81.043407}, url = {http://link.aps.org/doi/10.1103/PhysRevA.81.043407}, author = {S. Fern{\'a}ndez-Vidal and De Chiara, Gabriele and J. Larson and G. Morigi} } @article {rapcan_recycling_2010, title = {Recycling of qubits}, journal = {Physica Scripta}, volume = {T140}, year = {2010}, pages = {014059}, abstract = {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.}, issn = {0031-8949}, doi = {10.1088/0031-8949/2010/T140/014059}, url = {http://iopscience.iop.org/1402-4896/2010/T140/014059?fromSearchPage=true}, author = {P Rapcan and Calsamiglia, John and Mu{\~n}oz-Tapia, Ramon and Bagan, Emilio and V Buzek} } @article {288, title = {Spontaneous nucleation of structural defects in inhomogeneous ion chains}, journal = {New. J. Phys.}, volume = {12}, year = {2010}, month = {12/2010}, pages = {115003}, abstract = {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. }, doi = {10.1088/1367-2630/12/11/115003}, url = {http://iopscience.iop.org/1367-2630/12/11/115003?fromSearchPage=true}, author = {De Chiara, Gabriele and del Campo, Adolfo and G. Morigi and M. B. Plenio and A. Retzker} } @article {220, title = {Structural defects in ion crystals by quenching the external potential: the inhomogeneous Kibble-Zurek mechanism}, journal = {Phys. Rev. Lett.}, volume = {105}, year = {2010}, month = {08/2010}, pages = {075701}, doi = {10.1103/PhysRevLett.105.075701}, url = {http://link.aps.org/doi/10.1103/PhysRevLett.105.075701}, author = {del Campo, Adolfo and De Chiara, Gabriele and G. Morigi and M. B. Plenio and A. Retzker} } @article {302, title = {Topological superfluids on a lattice with non-Abelian gauge fields}, journal = {Europhysics Letters}, volume = {92}, number = {4}, year = {2010}, month = {12/2010}, pages = {46004}, doi = {10.1209/0295-5075/92/46004}, url = {http://arxiv.org/abs/1007.4827}, author = {A. Kubasiak and Massignan, P. and Lewenstein, M.} } @article {Aspachs2009, title = {Phase estimation for thermal Gaussian states}, journal = {Physical Review A (Atomic, Molecular, and Optical Physics)}, volume = {79}, number = {3}, year = {2009}, month = {03/2009}, pages = {033834{\textendash}12}, keywords = {gaussian noise, gaussian states, optical losses, optical squeezing, phase estimation, quantum noise, thermal noise}, doi = {10.1103/PhysRevA.79.033834}, url = {http://link.aps.org/abstract/PRA/v79/e033834}, author = {Aspachs, Mariona and Calsamiglia, John and Mu{\~n}oz-Tapia, Ramon and Bagan, Emili} } @article {Calsamiglia2009, title = {Phase-covariant quantum benchmarks}, journal = {Physical Review A (Atomic, Molecular, and Optical Physics)}, volume = {79}, number = {5}, year = {2009}, pages = {050301{\textendash}4}, keywords = {quantum benchmarks}, issn = {1050-2947}, doi = {10.1103/PhysRevA.79.050301}, url = {http://link.aps.org/doi/10.1103/PhysRevA.79.050301}, author = {Calsamiglia, John and Aspachs, Mariona and Mu{\~n}oz-Tapia, Ramon and Bagan, Emili} } @article {Ahufinger2008, title = {Double-barrier potentials for matter-wave gap solitons}, journal = {Physical Review A (Atomic, Molecular, and Optical Physics)}, volume = {78}, number = {1}, year = {2008}, month = {07/2008}, pages = {013608{\textendash}5}, keywords = {Bose-Einstein condensation, Fabry-Perot interferometers, matter waves, optical solitons}, doi = {10.1103/PhysRevA.78.013608}, url = {http://link.aps.org/abstract/PRA/v78/e013608 http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal\&id=PLRAAN000078000001013608000001\&idtype=cvips\&gifs=yes}, author = {Ahufinger, Veronica and Malomed, B. A. and Birkl, G. and Corbal{\'a}n, Ramon and Sanpera, Anna} } @article {Boixo2008a, title = {Operational Interpretation for Global Multipartite Entanglement}, journal = {Physical Review Letters}, volume = {100}, number = {10}, year = {2008}, month = {03/2008}, pages = {100503{\textendash}4}, keywords = {multipartite entanglement}, doi = {10.1103/PhysRevLett.100.100503}, url = {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://}, author = {Boixo, Sergio and Monras, Alex} } @article {Bagan2008, title = {Phase variance of squeezed vacuum states}, journal = {Physical Review A (Atomic, Molecular, and Optical Physics)}, volume = {78}, number = {4}, year = {2008}, month = {10/2008}, pages = {043829{\textendash}7}, keywords = {optical squeezing}, doi = {10.1103/PhysRevA.78.043829}, url = {http://link.aps.org/abstract/PRA/v78/e043829}, author = {Bagan, Emili and Monras, Alex and Mu{\~n}oz-Tapia, Ramon} } @article {Calsamiglia2008, title = {Quantum Chernoff bound as a measure of distinguishability between density matrices: Application to qubit and Gaussian states}, journal = {Physical Review A (Atomic, Molecular, and Optical Physics)}, volume = {77}, number = {3}, year = {2008}, month = {03/2008}, pages = {032311{\textendash}15}, keywords = {Bayes methods, error statistics, quantum chernoff bound, quantum statistical mechanics, quantum theory}, doi = {10.1103/PhysRevA.77.032311}, url = {http://link.aps.org/abstract/PRA/v77/e032311}, author = {Calsamiglia, John and Mu{\~n}oz-Tapia, Ramon and Masanes, Llu{\'\i}s and Ac{\'\i}n, Antonio and Bagan, Emili} } @article {Monras2008, title = {Quantum information processing in spin chains via quantum Zeno dynamics}, journal = {arXiv}, year = {2008}, month = {01/2008}, abstract = {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.}, keywords = {spin chain}, url = {http://arxiv.org/abs/0801.1959}, author = {Monras, Alex and Romero-Isart, Oriol} } @article {audenaert_discriminating_2007, title = {Discriminating States: The Quantum Chernoff Bound}, journal = {Physical Review Letters}, volume = {98}, number = {16}, year = {2007}, pages = {160501{\textendash}4}, doi = {10.1103/PhysRevLett.98.160501}, url = {http://link.aps.org/abstract/PRL/v98/e160501}, author = {K. M. R. Audenaert and J. Calsamiglia and R. {Munoz-Tapia} and E. Bagan and Ll. Masanes and A. Acin and F. Verstraete} } @article {Audenaert2007, title = {Discriminating States: The Quantum Chernoff Bound}, journal = {Physical Review Letters}, volume = {98}, number = {16}, year = {2007}, month = {04/2007}, pages = {160501{\textendash}4}, keywords = {quantum chernoff bound, state discrimination}, doi = {10.1103/PhysRevLett.98.160501}, url = {http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal\&id=PRLTAO000098000016160501000001\&idtype=cvips\&gifs=yes}, author = {K. M. R. Audenaert and Calsamiglia, John and Mu{\~n}oz-Tapia, Ramon and Bagan, Emili and Masanes, Llu{\'\i}s and Ac{\'\i}n, Antonio and Verstraete, Frank} } @article {Bagan2007, title = {How to hide a secret direction}, journal = {New Journal of Physics}, volume = {9}, year = {2007}, pages = {244}, abstract = {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.}, doi = {doi:10.1088/1367-2630/9/8/244}, url = {http://iopscience.iop.org/1367-2630/9/8/244/pdf/1367-2630\_9\_8\_244.pdf}, author = {Bagan, Emili and Calsamiglia, John and Demkowicz-Dobrzanski, R. and Mu{\~n}oz-Tapia, Ramon} } @article {Monras2007, title = {Optimal Quantum Estimation of Loss in Bosonic Channels}, journal = {Physical Review Letters}, volume = {98}, number = {16}, year = {2007}, month = {04/2007}, pages = {160401{\textendash}4}, doi = {10.1103/PhysRevLett.98.160401}, url = {http://link.aps.org/abstract/PRL/v98/e160401}, author = {Monras, Alex and Paris, Matteo G. A.} } @article {Moreno-Cardoner2007, title = {Predicting Spinor Condensate Dynamics from Simple Principles}, journal = {Physical Review Letters}, volume = {99}, number = {2}, year = {2007}, month = {07/2007}, pages = {020404{\textendash}4}, keywords = {Bose-Einstein condensation, entropy}, doi = {10.1103/PhysRevLett.99.020404}, url = {http://link.aps.org/abstract/PRL/v99/e020404}, author = {Moreno-Cardoner, M. and Mur-Petit, J. and Guilleumas, M. and Polls, Artur and Sanpera, Anna and Lewenstein, Maciej} } @article {Ahufinger2007, title = {Quantum switches and quantum memories for matter-wave lattice solitons}, journal = {New Journal of Physics}, volume = {9}, number = {1}, year = {2007}, pages = {4}, url = {http://iopscience.iop.org/1367-2630/9/1/004/pdf/1367-2630\_9\_1\_004.pdf}, author = {Ahufinger, Veronica and Mebrahtu, A. and Corbal{\'a}n, Ramon and Sanpera, Anna} } @article {Rapcan2007, title = {Recycling of quantum information: Multiple observations of quantum systems}, journal = {arXiv}, year = {2007}, month = {08/2007}, abstract = {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.}, url = {http://arxiv.org/abs/0708.1086}, author = {Rapcan, Peter and Calsamiglia, John and Mu{\~n}oz-Tapia, Ramon and Bagan, Emili and Bu{\.z}ek, Vladimir} } @article {Bagan2006c, title = {Aligning spatial frames through quantum channels}, journal = {International Journal of Quantum Information}, volume = {4}, year = {2006}, pages = {5}, author = {Bagan, Emili and Mu{\~n}oz-Tapia, Ramon} } @article {Mur-Petit2006, title = {Dynamics of F = 1 [sup 87]Rb condensates at finite temperatures}, journal = {Physical Review A (Atomic, Molecular, and Optical Physics)}, volume = {73}, number = {1}, year = {2006}, month = {01/2006}, pages = {013629{\textendash}6}, keywords = {atom optics, atom-photon collisions, Bose-Einstein condensation, radiation pressure, rubidium, spin dynamics, vectors}, doi = {10.1103/PhysRevA.73.013629}, url = {http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal\&id=PLRAAN000073000001013629000001\&idtype=cvips\&gifs=yes}, author = {Mur-Petit, J. and Guilleumas, M. and Polls, Artur and Sanpera, Anna and Lewenstein, Maciej and Bongs, K. and Sengstock, K.} } @article {Bagan2006, title = {Optimal full estimation of qubit mixed states}, journal = {Physical Review A (Atomic, Molecular, and Optical Physics)}, volume = {73}, number = {3}, year = {2006}, month = {03/2006}, pages = {032301{\textendash}18}, keywords = {mixed states, probability, state estimation}, doi = {10.1103/PhysRevA.73.032301}, url = {http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal\&id=PLRAAN000073000003032301000001\&idtype=cvips\&gifs=yes}, author = {Bagan, Emili and Ballester, M. A. and Gill, R. D. and Monras, Alex and Mu{\~n}oz-Tapia, Ramon} } @article {Monras2006, title = {Optimal phase measurements with pure Gaussian states}, journal = {Physical Review A (Atomic, Molecular, and Optical Physics)}, volume = {73}, number = {3}, year = {2006}, month = {03/2006}, pages = {033821{\textendash}8}, keywords = {gaussian states, optical squeezing, optical tomography, quantum theory}, doi = {10.1103/PhysRevA.73.033821}, url = {http://scitation.aip.org/vsearch/servlet/VerityServlet?KEY=ALL\&possible1=Monras\%2C+Alex\&possible1zone=author\&maxdisp=25\&smode=strresults\&aqs=true}, author = {Monras, Alex} } @article {Bagan2006b, title = {Relative states, quantum axes, and quantum references}, journal = {Physical Review A (Atomic, Molecular, and Optical Physics)}, volume = {73}, number = {2}, year = {2006}, month = {02/2006}, pages = {022341{\textendash}6}, keywords = {quantum theory}, doi = {10.1103/PhysRevA.73.022341}, url = {http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal\&id=PLRAAN000073000002022341000001\&idtype=cvips\&gifs=yes}, author = {Bagan, Emili and Iblisdir, Sofyan and Mu{\~n}oz-Tapia, Ramon} } @article {Acin2006, title = {Secrecy properties of quantum channels}, journal = {Physical Review A (Atomic, Molecular, and Optical Physics)}, volume = {73}, number = {1}, year = {2006}, month = {01/2006}, pages = {012327{\textendash}5}, keywords = {information theory, protocols, quantum cryptography}, doi = {10.1103/PhysRevA.73.012327}, url = {http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal\&id=PLRAAN000073000001012327000001\&idtype=cvips\&gifs=yes}, author = {Ac{\'\i}n, Antonio and Bae, J. and Bagan, Emili and Baig, Mari{\`a} and Masanes, Llu{\'\i}s and Mu{\~n}oz-Tapia, Ramon} } @article {Bagan2006a, title = {Separable Measurement Estimation of Density Matrices and its Fidelity Gap with Collective Protocols}, journal = {Physical Review Letters}, volume = {97}, number = {13}, year = {2006}, pages = {130501{\textendash}4}, keywords = {mixed states, state estimation}, doi = {10.1103/PhysRevLett.97.130501}, url = {http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal\&id=PRLTAO000097000013130501000001\&idtype=cvips\&gifs=yes}, author = {Bagan, Emili and Ballester, M. A. and Gill, R. D. and Mu{\~n}oz-Tapia, Ramon and Romero-Isart, Oriol} } @article {Mebrahtu2006, title = {Splitting and merging an elongated Bose-Einstein condensate at finite temperature}, journal = {Physical Review A (Atomic, Molecular, and Optical Physics)}, volume = {73}, number = {3}, year = {2006}, month = {03/2006}, pages = {033601{\textendash}7}, keywords = {Bose-Einstein condensation, critical points, fluctuations}, doi = {10.1103/PhysRevA.73.033601}, url = {http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal\&id=PLRAAN000073000003033601000001\&idtype=cvips\&gifs=yes}, author = {Mebrahtu, A. and Sanpera, Anna and Lewenstein, Maciej} } @article {Bagan2005a, title = {Comprehensive analysis of quantum pure-state estimation for two-level systems}, journal = {Physical Review A (Atomic, Molecular, and Optical Physics)}, volume = {71}, number = {6}, year = {2005}, month = {06/2005}, pages = {062318}, publisher = {American Physical Society}, abstract = {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) Cram{\'e}r-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.}, issn = {1050-2947}, doi = {10.1103/PhysRevA.71.062318}, url = {http://link.aps.org/doi/10.1103/PhysRevA.71.062318}, author = {Bagan, Emili and Monras, Alex and Mu{\~n}oz-Tapia, Ramon} } @article {Acin2005, title = {Multiple-copy two-state discrimination with individual measurements}, journal = {Physical Review A (Atomic, Molecular, and Optical Physics)}, volume = {71}, number = {3}, year = {2005}, pages = {032338}, keywords = {state discrimination}, doi = {10.1103/PhysRevA.71.032338}, url = {http://link.aps.org/abstract/PRA/v71/e032338}, author = {Ac{\'\i}n, Antonio and Bagan, Emili and Baig, Mari{\`a} and Masanes, Llu{\'\i}s and Mu{\~n}oz-Tapia, Ramon} } @article {Bagan2005, title = {Purity Estimation with Separable Measurements}, journal = {Physical Review Letters}, volume = {95}, number = {11}, year = {2005}, month = {09/2005}, pages = {110504}, publisher = {American Physical Society}, abstract = {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 Cram{\'e}r-Rao bound is discussed.}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.95.110504}, url = {http://link.aps.org/doi/10.1103/PhysRevLett.95.110504}, author = {Bagan, Emili and Ballester, M. A. and Mu{\~n}oz-Tapia, Ramon and Romero-Isart, Oriol} } @article {Bagan2004, title = {Collective versus local measurements in a qubit mixed-state estimation}, journal = {Physical Review A (Atomic, Molecular, and Optical Physics)}, volume = {69}, number = {1}, year = {2004}, month = {01/2004}, pages = {010304}, publisher = {American Physical Society}, abstract = {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.}, issn = {1050-2947}, doi = {10.1103/PhysRevA.69.010304}, url = {http://link.aps.org/doi/10.1103/PhysRevA.69.010304}, author = {Bagan, Emili and Baig, Mari{\`a} and Mu{\~n}oz-Tapia, Ramon and Rodriguez, A.} } @article {Bagan2004a, title = {Entanglement-assisted alignment of reference frames using a dense covariant coding}, journal = {Physical Review A (Atomic, Molecular, and Optical Physics)}, volume = {69}, number = {5}, year = {2004}, month = {05/2004}, pages = {050303}, publisher = {American Physical Society}, abstract = {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.}, issn = {1050-2947}, doi = {10.1103/PhysRevA.69.050303}, url = {http://link.aps.org/doi/10.1103/PhysRevA.69.050303}, author = {Bagan, Emili and Baig, Mari{\`a} and Mu{\~n}oz-Tapia, Ramon} } @inbook {Bramon2004d, title = {Popper{\textquoteright}s test of Quantum Mechanics}, booktitle = {Fundamental Physics Meeting "Alberto Galindo"}, year = {2004}, pages = {201{\textendash}207}, publisher = {Aula Documental de Investigacion}, organization = {Aula Documental de Investigacion}, address = {Madrid}, url = {http://arxiv.org/abs/quant-ph/0501134}, author = {Bramon, Albert and Escribano, Rafel}, editor = {Alvarez-Estrada, R. F. and Dobado, A. and Fernandez, L. A. and Mart\'{\i}n-Delgado, M. A. and {Munoz Sudupe}, A.} } @article {Bagan2004b, title = {Quantum reverse engineering and reference-frame alignment without nonlocal correlations}, journal = {Physical Review A (Atomic, Molecular, and Optical Physics)}, volume = {70}, number = {3}, year = {2004}, month = {09/2004}, pages = {030301}, publisher = {American Physical Society}, abstract = {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.}, issn = {1050-2947}, doi = {10.1103/PhysRevA.70.030301}, url = {http://link.aps.org/doi/10.1103/PhysRevA.70.030301}, author = {Bagan, Emili and Baig, Mari{\`a} and Mu{\~n}oz-Tapia, Ramon} } @inbook {Eckert2003, title = {Entanglement properties of composite quantum systems}, booktitle = {Quantum Information Processing}, year = {2003}, pages = {87{\textendash}95}, publisher = {Wilew-VCH}, organization = {Wilew-VCH}, address = {Berlin}, isbn = {352740371X}, author = {Eckert, Kai and Guhne, O. and Hulpke, Florian and Hyllus, Philipp and Korbicz, J. and Mompart, Jordi and Bruss, Dagmar and Lewenstein, Maciej and Sanpera, Anna}, editor = {Leuchs, G. and Beth, T.} } @article {Guhne2003, title = {Experimental detection of entanglement via witness operators and local measurements}, journal = {Journal of Modern Optics}, volume = {50}, number = {6-7}, year = {2003}, pages = {1079{\textendash}1102}, doi = {10.1080/09500340308234554}, url = {http://www.informaworld.com/smpp/36433790-99697256/content\~{}db=all\~{}content=a713826204}, author = {G{\"u}hne, Otfried and Hyllus, Philipp and Bruss, Dagmar and Ekert, Artur K. and Lewenstein, Maciej and Macchiavello, Chiara and Sanpera, Anna} } @article {Bagan2003, title = {Minimal measurements of the gate fidelity of a qudit map}, journal = {Physical Review A}, volume = {67}, number = {1}, year = {2003}, issn = {1050-2947}, doi = {10.1103/PhysRevA.67.014303}, url = {http://pra.aps.org/abstract/PRA/v67/i1/e014303}, author = {E. Bagan and Baig, M. and Mu{\~n}oz-Tapia, R.} } @conference {Bagan2003a, title = {Optimal reconstruction of a pure qubit state with local measurements}, booktitle = {Proceedings of the 8th ICSSUR}, year = {2003}, publisher = {Rinton Press (2003)}, organization = {Rinton Press (2003)}, author = {Bagan, Emili and Baig, Mari{\`a} and Monras, Alex and Mu{\~n}oz-Tapia, Ramon} } @article {Guhne2002, title = {Detection of entanglement with few local measurements}, journal = {Physical Review A}, volume = {66}, number = {6}, year = {2002}, issn = {1050-2947}, doi = {10.1103/PhysRevA.66.062305}, url = {http://pra.aps.org/abstract/PRA/v66/i6/e062305}, author = {G{\"u}hne, O. and Hyllus, P. and Bru{\ss}, D. and Ekert, A. and Lewenstein, M. and Macchiavello, C. and Sanpera, A.} } @article {Bagan2002, title = {Optimal Scheme for Estimating a Pure Qubit State via Local Measurements}, journal = {Physical Review Letters}, volume = {89}, number = {27}, year = {2002}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.89.277904}, url = {http://prl.aps.org/abstract/PRL/v89/i27/e277904}, author = {E. Bagan and Baig, M. and Mu{\~n}oz-Tapia, R.} } @article {Bagan2001b, title = {Aligning Reference Frames with Quantum States}, journal = {arXiv}, year = {2001}, url = {http://arxiv.org/abs/quant-ph/0106155}, author = {Bagan, Emili and Baig, Mari{\`a} and Mu{\~n}oz-Tapia, Ramon} } @article {Bagan2001a, title = {Aligning Reference Frames with Quantum States}, journal = {Physical Review Letters}, volume = {87}, number = {25}, year = {2001}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.87.257903}, url = {http://prl.aps.org/abstract/PRL/v87/i25/e257903}, author = {E. Bagan and Baig, M. and Mu{\~n}oz-Tapia, R.} } @article {Bagan2001, title = {Communication of spin directions with product states and finite measurements}, journal = {Physical Review A}, volume = {64}, number = {2}, year = {2001}, issn = {1050-2947}, doi = {10.1103/PhysRevA.64.022305}, url = {http://pra.aps.org/abstract/PRA/v64/i2/e022305}, author = {E. Bagan and Baig, M. and Mu{\~n}oz-Tapia, R.} } @article {Salieres2001, title = {Feynman{\textquoteright}s path-integral approach for intense-laser-atom interactions}, journal = {Science (New York, N.Y.)}, volume = {292}, number = {5518}, year = {2001}, pages = {902{\textendash}5}, abstract = {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.}, issn = {0036-8075}, doi = {10.1126/science.108836}, url = {http://www.sciencemag.org/cgi/content/abstract/292/5518/902}, author = {Salieres, P. and Carre, B. and Le Deroff, L. and Grasbon, F. and Paulus, G. G. and Walther, H. and Kopold, R. and Becker, W. and Milosevic, D. B. and Sanpera, A. and Lewenstein, M.} } @article {Bagan2001c, title = {Optimal encoding and decoding of a spin direction}, journal = {Physical Review A}, volume = {63}, number = {5}, year = {2001}, issn = {1050-2947}, doi = {10.1103/PhysRevA.63.052309}, url = {http://pra.aps.org/abstract/PRA/v63/i5/e052309}, author = {E. Bagan and Baig, M. and Brey, A. and Mu{\~n}oz-Tapia, R. and Tarrach, R.} } @article {798, title = {Optimal Strategies for Sending Information through A Quantum Channel}, journal = {Physical Review Letters}, volume = {85}, year = {2000}, month = {Jan-12-2000}, pages = {5230 - 5233}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.85.5230}, url = {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}, author = {Bagan, E. and Baig, M. and Brey, A. and Mu{\~n}oz-Tapia, R. and Tarrach, R.} }