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

%B IEEE TRANSACTIONS ON INFORMATION THEORY %V 63 %P 7832–7844 %8 dec %R 10.1109/TIT.2017.2754268 %0 Journal Article %J Physical Review A %D 2017 %T Entanglement and nonlocality in diagonal symmetric states of %A Quesada, Ruben %A Rana, Swapan %A Sanpera, Anna %B Physical Review A %V 95 %8 Jan-04-2017 %U http://link.aps.org/doi/10.1103/PhysRevA.95.042128http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevA.95.042128/fulltexthttp://link.aps.org/article/10.1103/PhysRevA.95.042128 %! Phys. Rev. A %R 10.1103/PhysRevA.95.042128 %0 Journal Article %J Nature Communications %D 2017 %T Generalized laws of thermodynamics in the presence of correlations %A Bera, Manabendra N. %A Riera, Arnau %A Lewenstein, Maciej %A Winter, Andreas %B Nature Communications %V 8 %8 Jan-12-2017 %U http://www.nature.com/articles/s41467-017-02370-xhttp://www.nature.com/articles/s41467-017-02370-x.pdfhttp://www.nature.com/articles/s41467-017-02370-x.pdfhttp://www.nature.com/articles/s41467-017-02370-x %! Nat Commun %R 10.1038/s41467-017-02370-x %0 Journal Article %J PHYSICAL REVIEW A %D 2017 %T Logarithmic coherence: Operational interpretation of l(1)-norm coherence %A Rana, Swapan %A Parashar, Preeti %A Winter, Andreas %A Lewenstein, Maciej %XWe show that the distillable coherence-which is equal to the relative entropy of coherence-is, up to a constant factor, always bounded by the l(1)-norm measure of coherence (defined as the sum of absolute values of off diagonals). Thus the latter plays a similar role as logarithmic negativity plays in entanglement theory and this is the best operational interpretation from a resource-theoretic viewpoint. Consequently the two measures are intimately connected to another operational measure, the robustness of coherence. We find also relationships between these measures, which are tight for general states, and the tightest possible for pure and qubit states. For a given robustness, we construct a state having minimum distillable coherence.

%B PHYSICAL REVIEW A %V 96 %R 10.1103/PhysRevA.96.052336 %0 Journal Article %J Physical Review Letters %D 2016 %T Entanglement and Coherence in Quantum State Merging %A Streltsov, A. %A Chitambar, E. %A Rana, S. %A Bera, N. %A Winter, A. %A Lewenstein, M. %B Physical Review Letters %V 116 %8 Jan-06-2016 %U http://link.aps.org/doi/10.1103/PhysRevLett.116.240405http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevLett.116.240405/fulltexthttp://link.aps.org/accepted/10.1103/PhysRevLett.116.240405http://link.aps.org/article/10.1103/PhysRevLett.116.240405 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.116.240405 %0 Journal Article %J Journal of Physics A: Mathematical and Theoretical %D 2016 %T The role of quantum information in thermodynamics—a topical review %A Goold, John %A Huber, Marcus %A Riera, Arnau %A Rio, Lídia del %A Paul Skrzypczyk %B Journal of Physics A: Mathematical and Theoretical %V 49 %P 143001 %G eng %U http://stacks.iop.org/1751-8121/49/i=14/a=143001?key=crossref.ce80cac2171964cc6d6c51ed12d1e014 %R 10.1088/1751-8113/49/14/143001 %0 Journal Article %J Optica %D 2015 %T Experimental access to higher-dimensional entangled quantum systems using integrated optics %A Christoph Schaeff %A Robert Polster %A Huber, Marcus %A Ramelow, Sven %A Anton Zeilinger %K Integrated optics %K quantum optics %X Integrated optics allows for the generation and control of increasingly complex photonic states on chip-based architectures. Here, we implement two entangled qutrits&\#x2014;a nine-dimensional quantum system&\#x2014;and demonstrate an exceptionally high degree of experimental control. The approach, which is conceptually different to common bulk optical implementations, is heavily based on methods of integrated in-fiber and on-chip technologies and further motivated by methods commonly used in today&\#x2019;s telecommunications industry. The system is composed of an in-fiber source creating entangled qutrit states of any amplitude and phase, and an on-chip integrated general Multiport enabling the realization of any desired local unitary transformation within the two qutrit nine-dimensional Hilbert space. The complete design is readily extendible toward higher dimensions with moderate increase in complexity. Ultimately, our scheme allows for complete on-chip integration. We demonstrate the flexibility and generality of our system by realizing a complete characterization of the two-qutrit space of higher-order Einstein&\#x2013;Podolsky&\#x2013;Rosen correlations. %B Optica %I OSA %V 2 %P 523–529 %8 Jun %G eng %U http://www.osapublishing.org/optica/abstract.cfm?URI=optica-2-6-523 %R 10.1364/OPTICA.2.000523 %0 Journal Article %J IEEE Transactions on Information Theory %D 2014 %T Bounds on entanglement assisted source-channel coding via the Lovasz theta number and its variants %A Toby S. Cubitt %A Laura Mancinska %A David Roberson %A Stahlke, Daniel %A Winter, Andreas %B IEEE Transactions on Information Theory %V 60 %P 7330-7344 %8 11/2014 %R 10.1109/TIT.2014.2349502 %0 Journal Article %J IEEE Transactions on Information Theory %D 2014 %T Full Security of Quantum Key Distribution From No-Signaling Constraints %A Masanes, Lluis %A Renner, Renato %A Christandl, Matthias %A Winter, Andreas %A Barrett, Jonathan %B IEEE Transactions on Information Theory %V 60 %P 4973-4986 %8 8/2014 %N 8 %! IEEE Trans. Inform. Theory %R 10.1109/TIT.2014.2329417 %0 Journal Article %J Proceedings of the National Academy of Sciences %D 2014 %T Generation and confirmation of a (100 x 100)-dimensional entangled quantum system %A Krenn, M. %A Huber, M %A Fickler, R. %A Lapkiewicz, R. %A Ramelow, S. %A Zeilinger, A. %X Entangled quantum systems have properties that have fundamentally overthrown the classical worldview. Increasing the complexity of entangled states by expanding their dimensionality allows the implementation of novel fundamental tests of nature, and moreover also enables genuinely new protocols for quantum information processing. Here we present the creation of a (100x100)-dimensional entangled quantum system, using spatial modes of photons. For its verification we develop a novel nonlinear criterion which infers entanglement dimensionality of a global state by using only information about its subspace correlations. This allows very practical experimental implementation as well as highly efficient extraction of entanglement dimensionality information. Applications in quantum cryptography and other protocols are very promising. %B Proceedings of the National Academy of Sciences %V 111 %P 6243 - 6247 %8 04/2014 %N 17 %! Proceedings of the National Academy of Sciences %R 10.1073/pnas.1402365111 %0 Journal Article %J Physical Review A %D 2014 %T Long-range multipartite entanglement close to a first-order quantum phase transition %A Stasińska, J. %A Rogers, B. %A M. Paternostro %A De Chiara, G. %A Sanpera, A. %X We provide insight into the quantum correlations structure present in strongly correlated systems beyond the standard framework of bipartite entanglement. To this aim we first exploit rotationally invariant states as a test bed to detect genuine tripartite entanglement beyond the nearest neighbor in spin-1/2 models. Then we construct in a closed analytical form a family of entanglement witnesses which provides a sufficient condition to determine if a state of a many-body system formed by an arbitrary number of spin-1/2 particles possesses genuine tripartite entanglement, independently of the details of the model. We illustrate our method by analyzing in detail the anisotropic XXZ spin chain close to its phase transitions, where we demonstrate the presence of long-range multipartite entanglement near the critical point and the breaking of the symmetries associated with the quantum phase transition. %B Physical Review A %V 89 %8 3/2014 %N 3 %! Phys. Rev. A %R 10.1103/PhysRevA.89.032330 %0 Journal Article %J Physical Review Letters %D 2013 %T Bose-Glass Phases of Ultracold Atoms due to Cavity Backaction %A Hessam Habibian %A André Winter %A Paganelli, Simone %A Heiko Rieger %A Giovanna Morigi %X We determine the quantum ground-state properties of ultracold bosonic atoms interacting with the mode of a high-finesse resonator. The atoms are confined by an external optical lattice, whose period is incommensurate with the cavity mode wavelength, and are driven by a transverse laser, which is resonant with the cavity mode. While for pointlike atoms photon scattering into the cavity is suppressed, for sufficiently strong lasers quantum fluctuations can support the buildup of an intracavity field, which in turn amplifies quantum fluctuations. The dynamics is described by a Bose-Hubbard model where the coefficients due to the cavity field depend on the atomic density at all lattice sites. Quantum Monte Carlo simulations and mean-field calculations show that, for large parameter regions, cavity backaction forces the atoms into clusters with a checkerboard density distribution. Here, the ground state lacks superfluidity and possesses finite compressibility, typical of a Bose glass. This system constitutes a novel setting where quantum fluctuations give rise to effects usually associated with disorder. %B Physical Review Letters %V 110 %8 2/2013 %U http://prl.aps.org/abstract/PRL/v110/i7/e075304 %N 7 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.110.075304 %0 Journal Article %J Physical Review A %D 2013 %T Entangled singularity patterns of photons in {Ince}-{Gauss} modes %A Krenn, Mario %A Robert Fickler %A Huber, Marcus %A Radek Lapkiewicz %A Plick, William %A Ramelow, Sven %A Anton Zeilinger %B Physical Review A %V 87 %8 jan %G eng %U http://link.aps.org/doi/10.1103/PhysRevA.87.012326 %R 10.1103/PhysRevA.87.012326 %0 Conference Proceedings %B 8th Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2013) %D 2013 %T The Quantum Entropy Cone of Stabiliser States %A Noah Linden %A Frantisek Matus %A Mary Beth Ruskai %A Winter, Andreas %E Severini, Simone %E Fernando Brandao %B 8th Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2013) %S LIPICS %C Guelph, ON %V 22 %P 270-284 %8 02/2013 %R 10.4230/LIPIcs.TQC.2013.270 %0 Journal Article %J New Journal of Physics %D 2013 %T Quantum hypergraph states %A Rossi, M %A Huber, M %A Bruß, D. %A Macchiavello, C. %B New Journal of Physics %V 15 %P 113022 %8 11/2013 %N 11 %! New J. Phys. %R 10.1088/1367-2630/15/11/113022 %0 Journal Article %J Physical Review Letters %D 2013 %T Quantum Metrology Assisted by Abstention %A Gendra, B. %A Ronco-Bonvehi, E. %A Calsamiglia, John %A Muñoz-Tapia, R. %A E. Bagan %X The main goal of quantum metrology is to obtain accurate values of physical parameters using quantum probes. In this context, we show that abstention, i.e., the possibility of getting an inconclusive answer at readout, can drastically improve the measurement precision and even lead to a change in its asymptotic behavior, from the shot-noise to the Heisenberg scaling. We focus on phase estimation and quantify the required amount of abstention for a given precision. We also develop analytical tools to obtain the asymptotic behavior of the precision and required rate of abstention for arbitrary pure states. %B Physical Review Letters %V 110 %P 100501 %8 3/2013 %U http://prl.aps.org/abstract/PRL/v110/i10/e100501 %N 10 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.110.100501 %0 Journal Article %J Phyiscal Review A %D 2013 %T Quantum phases of incommensurate optical lattices due to cavity backaction %A Hessam Habibian %A André Winter %A Paganelli, Simone %A Heiko Rieger %A Giovanna Morigi %B Phyiscal Review A %V 88 %P 043618 %0 Journal Article %J New Journal of Physics %D 2012 %T Beating noise with abstention in state estimation %A Gendra, Bernat %A Ronco-Bonvehi, Elio %A Calsamiglia, John %A Muñoz-Tapia, Ramon %A Bagan, Emilio %B New Journal of Physics %V 14 %P 105015 %U http://iopscience.iop.org/1367-2630/14/10/105015 %R 10.1088/1367-2630/14/10/105015 %0 Journal Article %J Physical Review Letters %D 2012 %T Quantum Memory Assisted Probing of Dynamical Spin Correlations %A O. Romero-Isart %A Rizzi, M. %A Muschik, C. A. %A Polzik, E. S. %A Lewenstein, M. %A Sanpera, A. %X We propose a method to probe time-dependent correlations of nontrivial observables in many-body ultracold lattice gases. The scheme uses a quantum nondemolition matter-light interface, first to map the observable of interest on the many-body system into the light and then to store coherently such information into an external system acting as a quantum memory. Correlations of the observable at two (or more) instances of time are retrieved with a single final measurement that includes the readout of the quantum memory. Such a method brings to reach the study of dynamics of many-body systems in and out of equilibrium by means of quantum memories in the field of quantum simulators. %B Physical Review Letters %V 108 %8 2/2012 %N 6 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.108.065302 %0 Journal Article %J ArXiv e-prints %D 2011 %T A continuous-variable formalism for the Faraday atom-light interface %A Stasińska, J. %A Paganelli, Simone %A Rodó, C. %A Sanpera, A. %X Quantum interfaces between polarized atomic ensembles and coherent states of light, applied recently to manipulate bipartite and multipartite entanglement, are revisited by means of a continuous-variable formalism. The explicit use of the continuous-variable formalism facilitates significantly the analysis of entanglement between different modes, reducing it to the study of the properties of a final covariance matrix which can be found analytically. Furthermore, it allows to study matter-light interfaces for mixed states, adapting the formalism to the experimental situations in which the initial prepared Gaussian states are, unavoidably, affected by a certain amount of noise. A multipartite scenario, leading to the generation of macroscopic cluster states is presented and analyzed in detail within this formalism. %B ArXiv e-prints %G eng %U http://arxiv.org/abs/1007.0403 %0 Journal Article %J Phys. Rev. A %D 2011 %T Probing magnetic order in ultracold lattice gases %A De Chiara, Gabriele %A O. Romero-Isart %A Sanpera, A. %X A forthcoming challenge in ultracold lattice gases is the simulation of quantum magnetism. That involves both the preparation of the lattice atomic gas in the desired spin state and the probing of the state. Here we demonstrate how a probing scheme based on atom-light interfaces gives access to the order parameters of nontrivial quantum magnetic phases, allowing us to characterize univocally strongly correlated magnetic systems produced in ultracold gases. This method, which is also nondemolishing, yields spatially resolved spin correlations and can be applied to bosons or fermions. As a proof of principle, we apply this method to detect the complete phase diagram displayed by a chain of (rotationally invariant) spin-1 bosons. %B Phys. Rev. A %V 83 %P 021604 %U http://pra.aps.org/abstract/PRA/v83/i2/e021604 %R 10.1103/PhysRevA.83.021604 %0 Thesis %B Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics %D 2010 %T Quantum Information with continuous variable systems %A Rodó, Carles %Y Sanpera, Anna %X This thesis deals with the study of quantum communication protocols with Continuous Variable (CV) systems. CV systems are those described by canonical conjugated coordinates $x$ and $p$ endowed with infinite dimensional Hilbert spaces, thus involving a complex mathematical structure. A special class of CV states, are the so-called Gaussian states. We present a protocol that permits to extract quantum keys from entangled Gaussian states. Differently from discrete systems, Gaussian entangled states cannot be distilled with Gaussian operations only. However it was already shown, that it is still possible to extract perfectly correlated classical bits to establish secret random keys. We properly modify the protocol using bipartite Gaussian entanglement to perform quantum key distribution in an efficient and realistic way. We describe and demonstrate security in front of different possible attacks on the communication, detailing the resources demanded. We also consider a simple 3-partite protocol known as Byzantine Agreement. It is an old classical communication problem in which parties (with possible traitors among them) can only communicate pairwise, while trying to reach a common decision. Classically, there is a bound in the maximal number of possible traitors that can be involved in the game. Nevertheless, a quantum solution exist. We show that solution within CV using multipartite entangled Gaussian states and Gaussian operations. Furthermore, we show under which premises concerning entanglement content of the state, noise, inefficient homodyne detectors, our protocol is efficient and applicable with present technology. It is known that in spite of their exceptional role within the space of all CV states, in fact, Gaussian states are not always the best candidates to perform quantum information tasks. Thus, we tackle the problem of quantification of correlations (quantum and/or classical) between two CV modes (Gaussian and non-Gaussian). We propose to define correlations between the two modes as the maximal number of correlated bits extracted via local quadrature measurements on each mode. On Gaussian states, where entanglement is accessible via their covariance matrix our quantification majorizes entanglement, reducing to an entanglement monotone for pure states. For non-Gaussian states, such as photonic Bell states, photon subtracted states and mixtures of Gaussian states, the bit quadrature correlations are shown to be also a monotonic function of the negativity. This quantification yields a feasible, operational way to measure non-Gaussian entanglement in current experiments by means of direct homodyne detection, without needing a complete state tomography with the same complexity as if dealing with Gaussian states. Finally we focus to atomic ensembles described as CV. Measurement induced entanglement between two macroscopical atomic samples was reported experimentally in 2001. There, the interaction between a single laser pulse propagating through two spatially separated atomic samples combined with a final projective measurement on the light led to the creation of pure EPR entanglement between the two samples. We show how to generate, manipulate and detect mesoscopic entanglement between an arbitrary number of atomic samples through a quantum non-demolition matter-light interface. Our proposal extends in a non-trivial way for multipartite entanglement (GHZ and cluster-like) without needing local magnetic fields. Moreover, we show quite surprisingly that given the irreversible character of a measurement, the interaction of the atomic sample with a second pulse light can modify and even reverse the entangling action of the first one leaving the samples in a separable state. %B Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics %I Universitat Autònoma de Barcelona %C Barcelona %V PhD %8 04/2010 %9 PhD Thesis %0 Journal Article %J Physica Scripta %D 2010 %T Recycling of qubits %A P Rapcan %A Calsamiglia, John %A Muñoz-Tapia, Ramon %A Bagan, Emilio %A V Buzek %X We consider a finite number, N, of qubits that encode a pure single qubit state SU(2) covariantly. Given the N-qubit state has already been measured optimally to estimate the single-qubit state, we analyse the maximum information obtainable by a second, and subsequent observers ignorant of important details of the previous measurements. We quantify the information acquired by each observer as a function of N and of the number of independent observers that in succession have independently measured the same ensemble of qubits before him. %B Physica Scripta %V T140 %P 014059 %U http://iopscience.iop.org/1402-4896/2010/T140/014059?fromSearchPage=true %R 10.1088/0031-8949/2010/T140/014059 %0 Journal Article %J New. J. Phys. %D 2010 %T Spontaneous nucleation of structural defects in inhomogeneous ion chains %A De Chiara, Gabriele %A del Campo, Adolfo %A G. Morigi %A M. B. Plenio %A A. Retzker %X Structural defects in ion crystals can be formed during a linear quench of the transverse trapping frequency across the mechanical instability from a linear chain to the zigzag structure. The density of defects after the sweep can be conveniently described by the Kibble-Zurek mechanism. In particular, the number of kinks in the zigzag ordering can be derived from a time-dependent Ginzburg-Landau equation for the order parameter, here the zigzag transverse size, under the assumption that the ions are continuously laser cooled. In a linear Paul trap the transition becomes inhomogeneous, being the charge density larger in the center and more rarefied at the edges. During the linear quench the mechanical instability is first crossed in the center of the chain, and a front, at which the mechanical instability is crossed during the quench, is identified which propagates along the chain from the center to the edges. If the velocity of this front is smaller than the sound velocity, the dynamics becomes adiabatic even in the thermodynamic limit and no defect is produced. Otherwise, the nucleation of kinks is reduced with respect to the case in which the charges are homogeneously distributed, leading to a new scaling of the density of kinks with the quenching rate. The analytical predictions are verified numerically by integrating the Langevin equations of motion of the ions, in presence of a time-dependent transverse confinement. We argue that the non-equilibrium dynamics of an ion chain in a Paul trap constitutes an ideal scenario to test the inhomogeneous extension of the Kibble-Zurek mechanism, which lacks experimental evidence to date. %B New. J. Phys. %V 12 %P 115003 %8 12/2010 %U http://iopscience.iop.org/1367-2630/12/11/115003?fromSearchPage=true %R 10.1088/1367-2630/12/11/115003 %0 Journal Article %J Phys. Rev. Lett. %D 2010 %T Structural defects in ion crystals by quenching the external potential: the inhomogeneous Kibble-Zurek mechanism %A del Campo, Adolfo %A De Chiara, Gabriele %A G. Morigi %A M. B. Plenio %A A. Retzker %B Phys. Rev. Lett. %V 105 %P 075701 %8 08/2010 %U http://link.aps.org/doi/10.1103/PhysRevLett.105.075701 %R 10.1103/PhysRevLett.105.075701 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2009 %T Manipulating mesoscopic multipartite entanglement with atom-light interfaces %A Julia Stasińska %A Rodó, Carles %A Paganelli, Simone %A Sanpera, Anna %A Birkl, G. %K atom-photon collisions %K EPR paradox %K measurement theory %K mesoscopic entanglement %K multipartite entanglement %K quantum entanglement %K quantum optics %X Entanglementbetween two macroscopic atomic ensembles induced by measurement on anancillary light system has proven to be a powerful methodfor engineering quantum memories and quantum state transfer. Here weinvestigate the feasibility of such methods for generation, manipulation, anddetection of genuine multipartite entanglement (Greenberger-Horne-Zeilinger and clusterlike states) betweenmesoscopic atomic ensembles without the need of individual addressing ofthe samples. Our results extend in a nontrivial way theEinstein-Podolsky-Rosen entanglement between two macroscopic gas samples reported experimentally in[B. Julsgaard, A. Kozhekin, and E. Polzik, Nature (London) 413,400 (2001)]. We find that under realistic conditions, a secondorthogonal light pulse interacting with the atomic samples, can modifyand even reverse the entangling action of the first oneleaving the samples in a separable state. ©2009 The American Physical Society %B Physical Review A (Atomic, Molecular, and Optical Physics) %I APS %V 80 %P 062304–8 %8 12/2009 %G eng %U http://link.aps.org/abstract/PRA/v80/e062304 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2009 %T Preparation of decoherence-free cluster states with optical superlattices %A Jiang, Liang %A Rey, Ana Maria %A Romero-Isart, Oriol %A Garcia-Ripoll, Juan Jose %A Sanpera, Anna %A Lukin, Mikhail D. %K atom-photon collisions %K ground states %K optical multilayers %K quantum optics %K superlattices %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 79 %P 022309–16 %8 02/2009 %G eng %U http://link.aps.org/abstract/PRA/v79/e022309 http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PLRAAN000079000002022309000001&idtype=cvips&gifs=yes http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PLRAAN000079000002 %R 10.1103/PhysRevA.79.022309 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2008 %T Multipartite continuous-variable solution for the Byzantine agreement problem %A Neigovzen, Rodion %A Rodó, Carles %A Adesso, Gerardo %A Sanpera, Anna %K Gaussian processes %K homodyne detection %K quantum cryptography %K quantum entanglement %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 77 %P 062307–11 %8 06/2008 %G eng %U http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PLRAAN000077000006062307000001&idtype=cvips&prog=normal %R 10.1103/PhysRevA.77.062307 %0 Journal Article %J Physical Review Letters %D 2008 %T Operational Quantification of Continuous-Variable Correlations %A Rodó, Carles %A Adesso, Gerardo %A Sanpera, Anna %B Physical Review Letters %V 100 %P 110505–4 %8 03/2008 %G eng %U http://link.aps.org/abstract/PRL/v100/e110505 %R 10.1103/PhysRevLett.100.110505 %0 Journal Article %J arXiv %D 2008 %T Quantum information processing in spin chains via quantum Zeno dynamics %A Monras, Alex %A Romero-Isart, Oriol %K spin chain %X We show how the quantum Zeno effect can be exploited to control quantum information in a spin chain in a flexible way. In particular, we consider a one dimensional array of three level systems interacting via a swap operator, an interaction found in a generalized Heisenberg Hamiltonian. By encoding the qubit into two levels and using simple projective frequent measurements, the dynamics of the chain is guided to achieve basic quantum information manipulation tools, $\backslash$emph\{i.e.\}, quantum registers, single qubit operations and quantum state transfer on demand, while local addressability is not required. State transfer is perfect, robust against errors and delivers qubits at a constant rate, unconditional of the length of the transfer. Moreover, two dimensional lattices with tunable global interactions in different directions offer the possibility of performing two-qubit gates. %B arXiv %8 01/2008 %G eng %U http://arxiv.org/abs/0801.1959 %0 Journal Article %J Nature Physics %D 2008 %T Quantum non-demolition detection of strongly correlated systems %A Eckert, Kai %A Romero-Isart, Oriol %A Rodriguez, Mirta %A Lewenstein, Maciej %A Polzik, Eugene S. %A Sanpera, Anna %B Nature Physics %V 4 %P 50–54 %8 01/2008 %G eng %U http://www.nature.com/nphys/journal/v4/n1/full/nphys776.html %R 10.1038/nphys776 %0 Journal Article %J Open Systems & Information Dynamics %D 2007 %T Efficiency in Quantum Key Distribution Protocols with Entangled Gaussian States %A Rodó, Carles %A Romero-Isart, Oriol %A Eckert, Kai %A Sanpera, Anna %K gaussian states %K quantum cryptography %K quantum key distribution %X Abstract Quantum key distribution (QKD) refers to specific quantum strategies which permit the secure distribution of a secret key between two parties that wish to communicate secretly. Quantum cryptography has proven unconditionally secure in ideal scenarios and has been successfully implemented using quantum states with finite (discrete) as well as infinite (continuous) degrees of freedom. Here, we analyze the efficiency of QKD protocols that use as a resource entangled gaussian states and gaussian operations only. In this framework, it has already been shown that QKD is possible [1] but the issue of its efficiency has not been considered. We propose a figure of merit (the efficiency E) to quantify the number of classical correlated bits that can be used to distill a key from a sample of N entangled states. We relate the efficiency of the protocol to the entanglement and purity of the states shared between the parties. %B Open Systems & Information Dynamics %V 14 %P 69–80 %8 03/2007 %G eng %U http://dx.doi.org/10.1007/s11080-007-9030-x %R 10.1007/s11080-007-9030-x %0 Journal Article %J New Journal of Physics %D 2007 %T Efficient quantum-state transfer in spin chains via adiabatic passage %A Eckert, Kai %A Romero-Isart, Oriol %A Sanpera, Anna %K spin chain %K state transfer %B New Journal of Physics %V 9 %P 155 %G eng %U http://iopscience.iop.org/1367-2630/9/5/155/pdf/1367-2630\_9\_5\_155.pdf %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2007 %T Quantum ratchets for quantum communication with optical superlattices %A Romero-Isart, Oriol %A Garcia-Ripoll, Juan Jose %K atom-photon collisions %K optical modulation %K quantum entanglement %K quantum optics %K superlattices %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 76 %P 052304–8 %8 11/2007 %G eng %U http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PLRAAN000076000005052304000001&idtype=cvips&prog=normal %R 10.1103/PhysRevA.76.052304 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2007 %T Quantum state transfer in spin-1 chains %A Romero-Isart, Oriol %A Eckert, Kai %A Sanpera, Anna %K ferromagnetic materials %K ground states %K Heisenberg model %K magnetic transitions %K quantum entanglement %K spin chain %K spin systems %K state transfer %B Physical Review A (Atomic, Molecular, and Optical Physics) %V 75 %P 050303–4 %8 05/2007 %G eng %U http://link.aps.org/abstract/PRA/v75/e050303 %R 10.1103/PhysRevA.75.050303 %0 Journal Article %J arXiv %D 2007 %T Recycling of quantum information: Multiple observations of quantum systems %A Rapcan, Peter %A Calsamiglia, John %A Muñoz-Tapia, Ramon %A Bagan, Emili %A Bużek, Vladimir %X Given a finite number of copies of an unknown qubit state that have already been measured optimally, can one still extract any information about the original unknown state? We give a positive answer to this question and quantify the information obtainable by a given observer as a function of the number of copies in the ensemble, and of the number of independent observers that, one after the other, have independently measured the same ensemble of qubits before him. The optimality of the protocol is proven and extensions to other states and encodings are also studied. According to the general lore, the state after a measurement has no information about the state before the measurement. Our results manifestly show that this statement has to be taken with a grain of salt, specially in situations where the quantum states encode confidential information. %B arXiv %8 08/2007 %G eng %U http://arxiv.org/abs/0708.1086 %0 Journal Article %J Journal of Physics A: Mathematical and General %D 2007 %T Transport and entanglement generation in the Bose-Hubbard model %A Romero-Isart, Oriol %A Eckert, Kai %A Rodó, Carles %A Sanpera, Anna %B Journal of Physics A: Mathematical and General %V 40 %P 8019–8032 %G eng %U http://www.iop.org/EJ/article/1751-8121/40/28/S11/a7\_28\_s11.pdf %0 Journal Article %J Physical Review Letters %D 2006 %T Separable Measurement Estimation of Density Matrices and its Fidelity Gap with Collective Protocols %A Bagan, Emili %A Ballester, M. A. %A Gill, R. D. %A Muñoz-Tapia, Ramon %A Romero-Isart, Oriol %K mixed states %K state estimation %B Physical Review Letters %V 97 %P 130501–4 %G eng %U http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000097000013130501000001&idtype=cvips&gifs=yes %R 10.1103/PhysRevLett.97.130501 %0 Conference Paper %B Laser Spectroscopy: Proceedings of the XVII International Conference, Aviemore, Scotland UK, 19-24 June 2005 %D 2005 %T Disordered complex systems using cold gases and trapped ions %A Sen(De), Aditi %A Sen, Ujjwal %A Lewenstein, Maciej %A Ahufinger, Veronica %A Pons, Marisa %A Sanpera, Anna %E Hinds, E. A. %E Ferguson, A. %E Riis, E. %B Laser Spectroscopy: Proceedings of the XVII International Conference, Aviemore, Scotland UK, 19-24 June 2005 %I World Scientific %C Singapore %P 158–166 %G eng %U http://arxiv.org/abs/quant-ph/0508018 %0 Journal Article %J Physical Review Letters %D 2005 %T Purity Estimation with Separable Measurements %A Bagan, Emili %A Ballester, M. A. %A Muñoz-Tapia, Ramon %A Romero-Isart, Oriol %X Given a large number N of copies of a qubit state of which we wish to estimate its purity, we prove that separable-measurement protocols can be as efficient as the optimal joint-measurement one if classical communication is used. This shows that the optimal estimation of the entanglement of a two-qubit state can also be achieved asymptotically with fully separable measurements. Thus, quantum memories provide no advantage in this situation. The relationship between our global Bayesian approach and the quantum Cramér-Rao bound is discussed. %B Physical Review Letters %I American Physical Society %V 95 %P 110504 %8 09/2005 %G eng %U http://link.aps.org/doi/10.1103/PhysRevLett.95.110504 %R 10.1103/PhysRevLett.95.110504 %0 Journal Article %J Physical Review A (Atomic, Molecular, and Optical Physics) %D 2004 %T Collective versus local measurements in a qubit mixed-state estimation %A Bagan, Emili %A Baig, Marià %A Muñoz-Tapia, Ramon %A Rodriguez, A. %X We discuss the problem of estimating a qubit mixed state. We give the optimal estimation that can be inferred from any given set of measurements. For collective measurements and for a large number N of copies, we show that the error in the estimation varies as 1/N. For local measurements, we focus on the simpler case of states lying on the equatorial plane of the Bloch sphere. We show that the error using plain tomography varies as 1/N1/4, while our approach leads to an error proportional to 1/N3/4. %B Physical Review A (Atomic, Molecular, and Optical Physics) %I American Physical Society %V 69 %P 010304 %8 01/2004 %G eng %U http://link.aps.org/doi/10.1103/PhysRevA.69.010304 %R 10.1103/PhysRevA.69.010304 %0 Journal Article %J {NATURE COMMUNICATIONS} %D 0 %T {Generalized laws of thermodynamics in the presence of correlations} %A Bera, Manabendra N. %A Riera, Arnau %A Lewenstein, Maciej %A Winter, Andreas %X {The laws of thermodynamics, despite their wide range of applicability, are known to break down when systems are correlated with their environments. Here we generalize thermodynamics to physical scenarios which allow presence of correlations, including those where strong correlations are present. We exploit the connection between information and physics, and introduce a consistent redefinition of heat dissipation by systematically accounting for the information flow from system to bath in terms of the conditional entropy. As a consequence, the formula for the Helmholtz free energy is accordingly modified. Such a remedy not only fixes the apparent violations of Landauer's erasure principle and the second law due to anomalous heat flows, but also leads to a generally valid reformulation of the laws of thermodynamics. In this information-theoretic approach, correlations between system and environment store work potential. Thus, in this view, the apparent anomalous heat flows are the refrigeration processes driven by such potentials.} %B {NATURE COMMUNICATIONS} %V {8} %8 {DEC 19} %R {10.1038/s41467-017-02370-x}