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.

1 aSutter, David1 aScholz, Volkher, B.1 aWinter, Andreas1 aRenner, Renato uhttps://grupsderecerca.uab.cat/giq/node/83900594nas a2200145 4500008003900000022001400039245006500053210006500118260001600183490000700199100001900206700001700225700001800242856018800260 2017 d a2469-992600aEntanglement and nonlocality in diagonal symmetric states of0 aEntanglement and nonlocality in diagonal symmetric states of cJan-04-20170 v951 aQuesada, Ruben1 aRana, Swapan1 aSanpera, Anna uhttp://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.95.04212800641nas a2200145 4500008003900000245007100039210006900110260001600179490000600195100002500201700001700226700002300243700002000266856020900286 2017 d00aGeneralized laws of thermodynamics in the presence of correlations0 aGeneralized laws of thermodynamics in the presence of correlatio cJan-12-20170 v81 aBera, Manabendra, N.1 aRiera, Arnau1 aLewenstein, Maciej1 aWinter, Andreas uhttp://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-x01253nas a2200157 4500008003900000022001400039245007700053210006900130490000700199520076000206100001700966700002100983700002001004700002301024856004801047 2017 d a2469-992600aLogarithmic coherence: Operational interpretation of l(1)-norm coherence0 aLogarithmic coherence Operational interpretation of l1norm coher0 v963 aWe 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.

1 aRana, Swapan1 aParashar, Preeti1 aWinter, Andreas1 aLewenstein, Maciej uhttps://grupsderecerca.uab.cat/giq/node/84000724nas a2200181 4500008003900000022001400039245005600053210005600109260001600165490000800181100001800189700001800207700001300225700001300238700001400251700001800265856025900283 2016 d a0031-900700aEntanglement and Coherence in Quantum State Merging0 aEntanglement and Coherence in Quantum State Merging cJan-06-20160 v1161 aStreltsov, A.1 aChitambar, E.1 aRana, S.1 aBera, N.1 aWinter, A1 aLewenstein, M uhttp://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.116.24040500583nas a2200169 4500008004100000022002500041245007300066210006900139300001100208490000700219100001600226700001800242700001700260700001600277700002100293856009900314 2016 eng d a1751-8113, 1751-812100aThe role of quantum information in thermodynamics—a topical review0 arole of quantum information in thermodynamics—a topical review a1430010 v491 aGoold, John1 aHuber, Marcus1 aRiera, Arnau1 aRio, Lídia1 aSkrzypczyk, Paul uhttp://stacks.iop.org/1751-8121/49/i=14/a=143001?key=crossref.ce80cac2171964cc6d6c51ed12d1e01401842nas a2200205 4500008004100000245009600041210006900137260001300206300001400219490000600233520118400239653002201423653001901445100002301464700002001487700001801507700001801525700002101543856007201564 2015 eng d00aExperimental access to higher-dimensional entangled quantum systems using integrated optics0 aExperimental access to higherdimensional entangled quantum syste bOSAcJun a523–5290 v23 aIntegrated 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.10aIntegrated optics10aquantum optics1 aSchaeff, Christoph1 aPolster, Robert1 aHuber, Marcus1 aRamelow, Sven1 aZeilinger, Anton uhttp://www.osapublishing.org/optica/abstract.cfm?URI=optica-2-6-52300655nas a2200169 4500008003900000245010300039210006900142260001200211300001400223490000700237100002000244700002100264700002000285700002000305700002000325856014000345 2014 d00aBounds on entanglement assisted source-channel coding via the Lovasz theta number and its variants0 aBounds on entanglement assisted sourcechannel coding via the Lov c11/2014 a7330-73440 v601 aCubitt, Toby, S1 aMancinska, Laura1 aRoberson, David1 aStahlke, Daniel1 aWinter, Andreas uhttps://grupsderecerca.uab.cat/giq/publications/bounds-entanglement-assisted-source-channel-coding-lovasz-theta-number-and-its-variants00633nas a2200181 4500008003900000022001400039245007600053210006900129260001100198300001400209490000700223100001900230700001900249700002500268700002000293700002200313856011600335 2014 d a1557-965400aFull Security of Quantum Key Distribution From No-Signaling Constraints0 aFull Security of Quantum Key Distribution From NoSignaling Const c8/2014 a4973-49860 v601 aMasanes, Lluis1 aRenner, Renato1 aChristandl, Matthias1 aWinter, Andreas1 aBarrett, Jonathan uhttps://grupsderecerca.uab.cat/giq/publications/full-security-quantum-key-distribution-no-signaling-constraints01511nas a2200205 4500008003900000022001400039245008600053210006900139260001200208300001600220490000800236520084300244100001301087700001301100700001501113700001801128700001501146700001701161856012701178 2014 d a1091-649000aGeneration and confirmation of a (100 x 100)-dimensional entangled quantum system0 aGeneration and confirmation of a 100 x 100dimensional entangled c04/2014 a6243 - 62470 v1113 aEntangled 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.1 aKrenn, M1 aHuber, M1 aFickler, R1 aLapkiewicz, R1 aRamelow, S1 aZeilinger, A uhttps://grupsderecerca.uab.cat/giq/publications/generation-and-confirmation-100-x-100-dimensional-entangled-quantum-system01541nas a2200181 4500008003900000022001400039245008900053210006900142260001100211490000700222520091500229100001801144700001401162700001901176700001701195700001501212856013201227 2014 d a1094-162200aLong-range multipartite entanglement close to a first-order quantum phase transition0 aLongrange multipartite entanglement close to a firstorder quantu c3/20140 v893 aWe 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.1 aStasińska, J1 aRogers, B1 aPaternostro, M1 aDe Chiara, G1 aSanpera, A uhttps://grupsderecerca.uab.cat/giq/publications/long-range-multipartite-entanglement-close-first-order-quantum-phase-transition01659nas a2200181 4500008003900000022001400039245006600053210006500119260001100184490000800195520112100203100002101324700001901345700002201364700001801386700002101404856005201425 2013 d a1079-711400aBose-Glass Phases of Ultracold Atoms due to Cavity Backaction0 aBoseGlass Phases of Ultracold Atoms due to Cavity Backaction c2/20130 v1103 aWe 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.1 aHabibian, Hessam1 aWinter, André1 aPaganelli, Simone1 aRieger, Heiko1 aMorigi, Giovanna uhttp://prl.aps.org/abstract/PRL/v110/i7/e07530400601nas a2200193 4500008004100000022002500041245007000066210006600136260000800202490000700210100001700217700002000234700001800254700002200272700001900294700001800313700002100331856005500352 2013 eng d a1050-2947, 1094-162200aEntangled singularity patterns of photons in {Ince}-{Gauss} modes0 aEntangled singularity patterns of photons in Ince Gauss modes cjan0 v871 aKrenn, Mario1 aFickler, Robert1 aHuber, Marcus1 aLapkiewicz, Radek1 aPlick, William1 aRamelow, Sven1 aZeilinger, Anton uhttp://link.aps.org/doi/10.1103/PhysRevA.87.01232600574nas a2200181 4500008003900000245005000039210004600089260002400135300001200159490000700171100001700178700002100195700002200216700002000238700002100258700002200279856009100301 2013 d00aThe Quantum Entropy Cone of Stabiliser States0 aQuantum Entropy Cone of Stabiliser States aGuelph, ONc02/2013 a270-2840 v221 aLinden, Noah1 aMatus, Frantisek1 aRuskai, Mary Beth1 aWinter, Andreas1 aSeverini, Simone1 aBrandao, Fernando uhttps://grupsderecerca.uab.cat/giq/publications/quantum-entropy-cone-stabiliser-states00424nas a2200157 4500008003900000245003000039210003000069260001200099300001100111490000700122100001300129700001300142700001300155700002000168856007800188 2013 d00aQuantum hypergraph states0 aQuantum hypergraph states c11/2013 a1130220 v151 aRossi, M1 aHuber, M1 aBruß, D1 aMacchiavello, C uhttps://grupsderecerca.uab.cat/giq/publications/quantum-hypergraph-states01126nas a2200193 4500008003900000022001400039245004500053210004500098260001100143300001100154490000800165520061600173100001400789700002100803700002200824700002000846700001300866856005300879 2013 d a1079-711400aQuantum Metrology Assisted by Abstention0 aQuantum Metrology Assisted by Abstention c3/2013 a1005010 v1103 aThe 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.1 aGendra, B1 aRonco-Bonvehi, E1 aCalsamiglia, John1 aMuñoz-Tapia, R1 aBagan, E uhttp://prl.aps.org/abstract/PRL/v110/i10/e10050100585nas a2200157 4500008003900000245007900039210006900118300001100187490000700198100002100205700001900226700002200245700001800267700002100285856012100306 2013 d00aQuantum phases of incommensurate optical lattices due to cavity backaction0 aQuantum phases of incommensurate optical lattices due to cavity a0436180 v881 aHabibian, Hessam1 aWinter, André1 aPaganelli, Simone1 aRieger, Heiko1 aMorigi, Giovanna uhttps://grupsderecerca.uab.cat/giq/publications/quantum-phases-incommensurate-optical-lattices-due-cavity-backaction00509nas a2200169 4500008003900000022001400039245005400053210005400107300001100161490000700172100001900179700002400198700002200222700002400244700001800268856005300286 2012 d a1367-263000aBeating noise with abstention in state estimation0 aBeating noise with abstention in state estimation a1050150 v141 aGendra, Bernat1 aRonco-Bonvehi, Elio1 aCalsamiglia, John1 aMuñoz-Tapia, Ramon1 aBagan, Emilio uhttp://iopscience.iop.org/1367-2630/14/10/10501501302nas a2200193 4500008003900000022001400039245006700053210006700120260001100187490000800198520069100206100002000897700001300917700001700930700001600947700001800963700001500981856011200996 2012 d a1079-711400aQuantum Memory Assisted Probing of Dynamical Spin Correlations0 aQuantum Memory Assisted Probing of Dynamical Spin Correlations c2/20120 v1083 aWe 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.1 aRomero-Isart, O1 aRizzi, M1 aMuschik, C A1 aPolzik, E S1 aLewenstein, M1 aSanpera, A uhttps://grupsderecerca.uab.cat/giq/publications/quantum-memory-assisted-probing-dynamical-spin-correlations01249nas a2200133 4500008004100000245007300041210006900114520082900183100001801012700002201030700001301052700001501065856003501080 2011 eng d00aA continuous-variable formalism for the Faraday atom-light interface0 acontinuousvariable formalism for the Faraday atomlight interface3 aQuantum 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.1 aStasińska, J1 aPaganelli, Simone1 aRodó, C1 aSanpera, A uhttp://arxiv.org/abs/1007.040301162nas a2200145 4500008003900000245005400039210005400093300001100147490000700158520074100165100002400906700002000930700001500950856005100965 2011 d00aProbing magnetic order in ultracold lattice gases0 aProbing magnetic order in ultracold lattice gases a0216040 v833 aA 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.1 aDe Chiara, Gabriele1 aRomero-Isart, O1 aSanpera, A uhttp://pra.aps.org/abstract/PRA/v83/i2/e02160404248nas a2200133 4500008003900000245005700039210005700096260005900153490000800212520375800220100001803978700001803996856010004014 2010 d00aQuantum Information with continuous variable systems0 aQuantum Information with continuous variable systems aBarcelonabUniversitat Autònoma de Barcelonac04/20100 vPhD3 aThis 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.1 aSanpera, Anna1 aRodó, Carles uhttps://grupsderecerca.uab.cat/giq/publications/quantum-information-continuous-variable-systems01017nas a2200181 4500008003900000022001400039245002400053210002400077300001100101490000900112520054600121100001400667700002200681700002400703700001800727700001300745856007700758 2010 d a0031-894900aRecycling of qubits0 aRecycling of qubits a0140590 vT1403 aWe 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.1 aRapcan, P1 aCalsamiglia, John1 aMuñoz-Tapia, Ramon1 aBagan, Emilio1 aBuzek, V uhttp://iopscience.iop.org/1402-4896/2010/T140/014059?fromSearchPage=true02221nas a2200181 4500008003900000245007700039210006900116260001200185300001100197490000700208520166400215100002401879700001801903700001401921700001601935700001501951856007301966 2010 d00aSpontaneous nucleation of structural defects in inhomogeneous ion chains0 aSpontaneous nucleation of structural defects in inhomogeneous io c12/2010 a1150030 v123 aStructural 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. 1 aDe Chiara, Gabriele1 aCampo, Adolfo1 aMorigi, G1 aPlenio, M B1 aRetzker, A uhttp://iopscience.iop.org/1367-2630/12/11/115003?fromSearchPage=true00572nas a2200169 4500008003900000245011700039210006900156260001200225300001100237490000800248100001800256700002400274700001400298700001600312700001500328856005900343 2010 d00aStructural defects in ion crystals by quenching the external potential: the inhomogeneous Kibble-Zurek mechanism0 aStructural defects in ion crystals by quenching the external pot c08/2010 a0757010 v1051 aCampo, Adolfo1 aDe Chiara, Gabriele1 aMorigi, G1 aPlenio, M B1 aRetzker, A uhttp://link.aps.org/doi/10.1103/PhysRevLett.105.07570101755nas a2200265 4500008004100000245008100041210006900122260001700191300001500208490000700223520094900230653002701179653001601206653002301222653002801245653003001273653002501303653001901328100002201347700001801369700002201387700001801409700001301427856004901440 2009 eng d00aManipulating mesoscopic multipartite entanglement with atom-light interfaces0 aManipulating mesoscopic multipartite entanglement with atomlight bAPSc12/2009 a062304–80 v803 aEntanglementbetween 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 Society10aatom-photon collisions10aEPR paradox10ameasurement theory10amesoscopic entanglement10amultipartite entanglement10aquantum entanglement10aquantum optics1 aStasińska, Julia1 aRodó, Carles1 aPaganelli, Simone1 aSanpera, Anna1 aBirkl, G uhttp://link.aps.org/abstract/PRA/v80/e06230400960nas a2200241 4500008004100000245007800041210006900119260001200188300001600200490000700216653002700223653001800250653002400268653001900292653001800311100001700329700001900346700002400365700002900389700001800418700002200436856026000458 2009 eng d00aPreparation of decoherence-free cluster states with optical superlattices0 aPreparation of decoherencefree cluster states with optical super c02/2009 a022309–160 v7910aatom-photon collisions10aground states10aoptical multilayers10aquantum optics10asuperlattices1 aJiang, Liang1 aRey, Ana Maria1 aRomero-Isart, Oriol1 aGarcia-Ripoll, Juan Jose1 aSanpera, Anna1 aLukin, Mikhail, D uhttp://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=PLRAAN00007900000200737nas a2200205 4500008004100000245008200041210006900123260001200192300001600204490000700220653002300227653002300250653002500273653002500298100002200323700001800345700002000363700001800383856013000401 2008 eng d00aMultipartite continuous-variable solution for the Byzantine agreement problem0 aMultipartite continuousvariable solution for the Byzantine agree c06/2008 a062307–110 v7710aGaussian processes10ahomodyne detection10aquantum cryptography10aquantum entanglement1 aNeigovzen, Rodion1 aRodó, Carles1 aAdesso, Gerardo1 aSanpera, Anna uhttp://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PLRAAN000077000006062307000001&idtype=cvips&prog=normal00461nas a2200145 4500008004100000245006700041210006600108260001200174300001500186490000800201100001800209700002000227700001800247856005000265 2008 eng d00aOperational Quantification of Continuous-Variable Correlations0 aOperational Quantification of ContinuousVariable Correlations c03/2008 a110505–40 v1001 aRodó, Carles1 aAdesso, Gerardo1 aSanpera, Anna uhttp://link.aps.org/abstract/PRL/v100/e11050501327nas a2200133 4500008004100000245007600041210006900117260001200186520090400198653001501102100001701117700002401134856003501158 2008 eng d00aQuantum information processing in spin chains via quantum Zeno dynamics0 aQuantum information processing in spin chains via quantum Zeno d c01/20083 aWe 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.10aspin chain1 aMonras, Alex1 aRomero-Isart, Oriol uhttp://arxiv.org/abs/0801.195900603nas a2200193 4500008004100000022001400041245006800055210006700123260001200190300001200202490000600214100001600220700002400236700002100260700002300281700002200304700001800326856006500344 2008 eng d a1745-247300aQuantum non-demolition detection of strongly correlated systems0 aQuantum nondemolition detection of strongly correlated systems c01/2008 a50–540 v41 aEckert, Kai1 aRomero-Isart, Oriol1 aRodriguez, Mirta1 aLewenstein, Maciej1 aPolzik, Eugene, S1 aSanpera, Anna uhttp://www.nature.com/nphys/journal/v4/n1/full/nphys776.html01568nas a2200205 4500008004100000245008400041210006900125260001200194300001200206490000700218520093900225653002001164653002501184653002901209100001801238700002401256700001601280700001801296856004801314 2007 eng d00aEfficiency in Quantum Key Distribution Protocols with Entangled Gaussian States0 aEfficiency in Quantum Key Distribution Protocols with Entangled c03/2007 a69–800 v143 aAbstract 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.10agaussian states10aquantum cryptography10aquantum key distribution1 aRodó, Carles1 aRomero-Isart, Oriol1 aEckert, Kai1 aSanpera, Anna uhttp://dx.doi.org/10.1007/s11080-007-9030-x00525nas a2200157 4500008004100000245007400041210006900115300000800184490000600192653001500198653001900213100001600232700002400248700001800272856007700290 2007 eng d00aEfficient quantum-state transfer in spin chains via adiabatic passage0 aEfficient quantumstate transfer in spin chains via adiabatic pas a1550 v910aspin chain10astate transfer1 aEckert, Kai1 aRomero-Isart, Oriol1 aSanpera, Anna uhttp://iopscience.iop.org/1367-2630/9/5/155/pdf/1367-2630\_9\_5\_155.pdf00707nas a2200193 4500008004100000245007400041210006900115260001200184300001500196490000700211653002700218653002300245653002500268653001900293653001800312100002400330700002900354856013000383 2007 eng d00aQuantum ratchets for quantum communication with optical superlattices0 aQuantum ratchets for quantum communication with optical superlat c11/2007 a052304–80 v7610aatom-photon collisions10aoptical modulation10aquantum entanglement10aquantum optics10asuperlattices1 aRomero-Isart, Oriol1 aGarcia-Ripoll, Juan Jose uhttp://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PLRAAN000076000005052304000001&idtype=cvips&prog=normal00679nas a2200241 4500008004100000245004400041210004300085260001200128300001500140490000700155653002800162653001800190653002100208653002500229653002500254653001500279653001700294653001900311100002400330700001600354700001800370856004900388 2007 eng d00aQuantum state transfer in spin-1 chains0 aQuantum state transfer in spin1 chains c05/2007 a050303–40 v7510aferromagnetic materials10aground states10aHeisenberg model10amagnetic transitions10aquantum entanglement10aspin chain10aspin systems10astate transfer1 aRomero-Isart, Oriol1 aEckert, Kai1 aSanpera, Anna uhttp://link.aps.org/abstract/PRA/v75/e05030301361nas a2200157 4500008004100000245007900041210006900120260001200189520086500201100001801066700002201084700002401106700001701130700002101147856003501168 2007 eng d00aRecycling of quantum information: Multiple observations of quantum systems0 aRecycling of quantum information Multiple observations of quantu c08/20073 aGiven 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.1 aRapcan, Peter1 aCalsamiglia, John1 aMuñoz-Tapia, Ramon1 aBagan, Emili1 aBużek, Vladimir uhttp://arxiv.org/abs/0708.108600491nas a2200145 4500008004100000245006800041210006700109300001600176490000700192100002400199700001600223700001800239700001800257856007000275 2007 eng d00aTransport and entanglement generation in the Bose-Hubbard model0 aTransport and entanglement generation in the BoseHubbard model a8019–80320 v401 aRomero-Isart, Oriol1 aEckert, Kai1 aRodó, Carles1 aSanpera, Anna uhttp://www.iop.org/EJ/article/1751-8121/40/28/S11/a7\_28\_s11.pdf00680nas a2200181 4500008004100000245010400041210006900145300001500214490000700229653001700236653002100253100001700274700001900291700001400310700002400324700002400348856012600372 2006 eng d00aSeparable Measurement Estimation of Density Matrices and its Fidelity Gap with Collective Protocols0 aSeparable Measurement Estimation of Density Matrices and its Fid a130501–40 v9710amixed states10astate estimation1 aBagan, Emili1 aBallester, M A1 aGill, R D1 aMuñoz-Tapia, Ramon1 aRomero-Isart, Oriol uhttp://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000097000013130501000001&idtype=cvips&gifs=yes00625nas a2200205 4500008004100000245006500041210006500106260003200171300001400203100001900217700001600236700002300252700002400275700001700299700001800316700001500334700001600349700001200365856004200377 2005 eng d00aDisordered complex systems using cold gases and trapped ions0 aDisordered complex systems using cold gases and trapped ions aSingaporebWorld Scientific a158–1661 aSen(De), Aditi1 aSen, Ujjwal1 aLewenstein, Maciej1 aAhufinger, Veronica1 aPons, Marisa1 aSanpera, Anna1 aHinds, E A1 aFerguson, A1 aRiis, E uhttp://arxiv.org/abs/quant-ph/050801801086nas a2200181 4500008004100000022001400041245005000055210005000105260003900155300001100194490000700205520055000212100001700762700001900779700002400798700002400822856005800846 2005 eng d a0031-900700aPurity Estimation with Separable Measurements0 aPurity Estimation with Separable Measurements bAmerican Physical Societyc09/2005 a1105040 v953 aGiven 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.1 aBagan, Emili1 aBallester, M A1 aMuñoz-Tapia, Ramon1 aRomero-Isart, Oriol uhttp://link.aps.org/doi/10.1103/PhysRevLett.95.11050401077nas a2200181 4500008004100000022001400041245007500055210006900130260003900199300001100238490000700249520050900256100001700765700001700782700002400799700001700823856005500840 2004 eng d a1050-294700aCollective versus local measurements in a qubit mixed-state estimation0 aCollective versus local measurements in a qubit mixedstate estim bAmerican Physical Societyc01/2004 a0103040 v693 aWe 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/N3/4.1 aBagan, Emili1 aBaig, Marià1 aMuñoz-Tapia, Ramon1 aRodriguez, A uhttp://link.aps.org/doi/10.1103/PhysRevA.69.01030401570nas a2200169 4500008003700000022001600037245007300053210006900126260001300195490000800208520105100216100002501267700001701292700002301309700002001332856004801352 0 d a{2041-1723}00a{Generalized laws of thermodynamics in the presence of correlations}0 aGeneralized laws of thermodynamics in the presence of correlatio c{DEC 19}0 v{8}3 a{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.}1 aBera, Manabendra, N.1 aRiera, Arnau1 aLewenstein, Maciej1 aWinter, Andreas uhttps://grupsderecerca.uab.cat/giq/node/926