The detection of change points is a pivotal task in statistical analysis. In the quantum realm, it is a new primitive where one aims at identifying the point where a source that supposedly prepares a sequence of particles in identical quantum states starts preparing a mutated one. We obtain the optimal procedure to identify the change point with certainty-naturally at the price of having a certain probability of getting an inconclusive answer. We obtain the analytical form of the optimal probability of successful identification for any length of the particle sequence. We show that the conditional success probabilities of identifying each possible change point show an unexpected oscillatory behavior. We also discuss local (online) protocols and compare them with the optimal procedure.

VL - 119 ER - TY - JOUR T1 - Probabilistic metrology or how some measurement outcomes render ultra-precise estimates JF - New Journal of Physics Y1 - 2016 A1 - John Calsamiglia A1 - Gendra, B A1 - Muñoz-Tapia, R A1 - Bagan, E AB -We show on theoretical grounds that, even in the presence of noise, probabilistic measurement strategies (which have a certain probability of failure or abstention) can provide, upon a heralded successful outcome, estimates with a precision that exceeds the deterministic bounds for the average precision. This establishes a new ultimate bound on the phase estimation precision of particular measurement outcomes (or sequence of outcomes). For probe systems subject to local dephasing, we quantify such precision limit as a function of the probability of failure that can be tolerated. Our results show that the possibility of abstaining can set back the detrimental effects of noise.

VL - 18 UR - http://stacks.iop.org/1367-2630/18/i=10/a=103049?key=crossref.fb78efa5084e8d9fdb51f574cf3bfed3http://stacks.iop.org/1367-2630/18/i=10/a=103049/pdfhttp://stacks.iop.org/1367-2630/18/i=10/a=103049?key=crossref.fb78efa5084e8d9fdb51f574cf3bfed3 JO - New J. Phys. ER - TY - JOUR T1 - Quantum Change Point JF - Physical Review Letters Y1 - 2016 A1 - Sentís, Gael A1 - Bagan, Emilio A1 - John Calsamiglia A1 - Chiribella, Giulio A1 - Muñoz-Tapia, Ramon VL - 117 UR - http://link.aps.org/doi/10.1103/PhysRevLett.117.150502http://link.aps.org/article/10.1103/PhysRevLett.117.150502 JO - Phys. Rev. Lett. ER - TY - JOUR T1 - Probabilistic Metrology Attains Macroscopic Cloning of Quantum Clocks JF - Physical Review Letters Y1 - 2014 A1 - Gendra, B. A1 - John Calsamiglia A1 - Muñoz-Tapia, R. A1 - E. Bagan A1 - Chiribella, G. VL - 113 UR - http://link.aps.org/doi/10.1103/PhysRevLett.113.260402 ER - TY - JOUR T1 - Quantum information: The occasional super clock-cloner JF - Nature Physics Y1 - 2014 A1 - John Calsamiglia VL - 10 JO - Nat Phys ER - TY - JOUR T1 - Quantumness of correlations, quantumness of ensembles and quantum data hiding JF - New Journal of Physics Y1 - 2014 A1 - Piani, M A1 - Narasimhachar, V A1 - John Calsamiglia VL - 16 UR - http://stacks.iop.org/1367-2630/16/i=11/a=113001?key=crossref.9f5154f8c85e7f9471e2931d9a02df88 ER - TY - JOUR T1 - Optimal parameter estimation with a fixed rate of abstention JF - Physical Review A Y1 - 2013 A1 - Gendra, B. A1 - Ronco-Bonvehi, E. A1 - Calsamiglia, J. A1 - Muñoz-Tapia, R. A1 - Bagan, E. VL - 88 UR - https://link.aps.org/doi/10.1103/PhysRevA.88.012128http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevA.88.012128/fulltexthttp://link.aps.org/article/10.1103/PhysRevA.88.012128 JO - Phys. Rev. A ER - TY - JOUR T1 - Programmable discrimination with an error margin JF - Physical Review A Y1 - 2013 A1 - Sentís, G A1 - E. Bagan A1 - John Calsamiglia A1 - Muñoz-Tapia, R. VL - 88 IS - 5 JO - Phys. Rev. A ER - TY - JOUR T1 - Quantum Metrology Assisted by Abstention JF - Physical Review Letters Y1 - 2013 A1 - Gendra, B. A1 - Ronco-Bonvehi, E. A1 - John Calsamiglia A1 - Muñoz-Tapia, R. A1 - E. Bagan AB - The main goal of quantum metrology is to obtain accurate values of physical parameters using quantum probes. In this context, we show that abstention, i.e., the possibility of getting an inconclusive answer at readout, can drastically improve the measurement precision and even lead to a change in its asymptotic behavior, from the shot-noise to the Heisenberg scaling. We focus on phase estimation and quantify the required amount of abstention for a given precision. We also develop analytical tools to obtain the asymptotic behavior of the precision and required rate of abstention for arbitrary pure states. VL - 110 UR - http://prl.aps.org/abstract/PRL/v110/i10/e100501 IS - 10 JO - Phys. Rev. Lett. ER - TY - JOUR T1 - Beating noise with abstention in state estimation JF - New Journal of Physics Y1 - 2012 A1 - Gendra, Bernat A1 - Ronco-Bonvehi, Elio A1 - John Calsamiglia A1 - Muñoz-Tapia, Ramon A1 - Bagan, Emilio VL - 14 UR - http://iopscience.iop.org/1367-2630/14/10/105015 ER - TY - THES T1 - Entanglement distribution in quantum complex networks T2 - Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics Y1 - 2012 A1 - Cuquet, Martí AB - This thesis deals with the study of quantum networks with a complex structure, the implications this structure has in the distribution of entanglement and how their functioning can be enhanced by operating in the quantum regime. We first consider a complex network of bipartite states, both pure and mixed, and study the distribution of long-distance entanglement. Then, we move to a network with noisy channels and study the creation and distribution of large, multipartite states. The work contained in this thesis is primarily motivated by the idea that the interplay between quantum information and complex networks may give rise to a new understanding and characterization of natural systems. Complex networks are of particular importance in communication infrastructures, as most present telecommunication networks have a complex structure. In the case of quantum networks, which are the necessary framework for distributed quantum processing and for quantum communication, it is very plausible that in the future they acquire a complex topology resembling that of existing networks, or even that methods will be developed to use current infrastructures in the quantum regime. A central task in quantum networks is to devise strategies to distribute entanglement among its nodes. In the first part of this thesis, we consider the distribution of bipartite entanglement as an entanglement percolation process in a complex network. Within this approach, perfect entanglement is established probabilistically between two arbitrary nodes. We see that for large networks, the probability of doing so is a constant strictly greater than zero (and independent of the size of the network) if the initial amount of entanglement is above a certain critical value. Quantum mechanics offer here the possibility to change the structure of the network without need to establish new, "physical" channels. By a proper local transformation of the network, the critical entanglement can be decreased and the probability increased. We apply this transformation to complex network models with arbitrary degree distribution. In the case of a noisy network of mixed states, we see that for some classes of states, the same approach of entanglement percolation can be used. For general mixed states, we consider a limited-path-length entanglement percolation constrained by the amount of noise in the connections. We see how complex networks still offer a great advantage in the probability of connecting two nodes. In the second part, we move to the multipartite scenario. We study the creation and distribution of graph states with a structure that mimic the underlying communication network. In this case, we use an arbitrary complex network of noisy channels, and consider that operations and measurements are also noisy. We propose an efficient scheme to distribute and purify small subgraphs, which are then merged to reproduce the desired state. We compare this approach with two bipartite protocols that rely on a central station and full knowledge of the network structure. We show that the fidelity of the generated graphs can be written as the partition function of a classical disordered spin system (a spin glass), and its decay rate is the analog of the free energy. Applying the three protocols to a one-dimensional network and to complex networks, we see that they are all comparable, and in some cases the proposed subgraph protocol, which needs only local information of the network, performs even better. JF - Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics PB - Universitat Autònoma de Barcelona CY - Barcelona VL - Doctorat en Física ER - TY - THES T1 - Entanglement percolation with graph-states' T2 - Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics Y1 - 2012 A1 - Franquet, Abert JF - Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics PB - Universitat Autònoma de Barcelona CY - Barcelona VL - Grau en Física ER - TY - JOUR T1 - Growth of graph states in quantum networks JF - Physical Review A Y1 - 2012 A1 - Cuquet, Martí A1 - John Calsamiglia AB - We propose a scheme to distribute graph states over quantum networks in the presence of noise in the channels and in the operations. The protocol can be implemented efficiently for large graph sates of arbitrary (complex) topology. We benchmark our scheme with two protocols where each connected component is prepared in a node belonging to the component and subsequently distributed via quantum repeaters to the remaining connected nodes. We show that the fidelity of the generated graphs can be written as the partition function of a classical Ising-type Hamiltonian. We give exact expressions of the fidelity of the linear cluster and results for its decay rate in random graphs with arbitrary (uncorrelated) degree distributions. VL - 86 UR - http://link.aps.org/doi/10.1103/PhysRevA.86.042304 ER - TY - JOUR T1 - Quantum learning without quantum memory JF - Scientific Reports (Nature Publishing Group) Y1 - 2012 A1 - Sentís, Gael A1 - John Calsamiglia A1 - Muñoz-Tapia, Ramon A1 - Bagan, Emilio AB - A quantum learning machine for binary classification of qubit states that does not require quantum memory is introduced and shown to perform with the minimum error rate allowed by quantum mechanics for any size of the training set. This result is shown to be robust under (an arbitrary amount of) noise and under (statistical) variations in the composition of the training set, provided it is large enough. This machine can be used an arbitrary number of times without retraining. Its required classical memory grows only logarithmically with the number of training qubits, while its excess risk decreases as the inverse of this number, and twice as fast as the excess risk of an “estimate-and-discriminate” machine, which estimates the states of the training qubits and classifies the data qubit with a discrimination protocol tailored to the obtained estimates. PB - Nature Publishing Group VL - 2 UR - http://www.nature.com/srep/2012/121005/srep00708/full/srep00708.html ER - TY - JOUR T1 - All Nonclassical Correlations Can Be Activated into Distillable Entanglement JF - Phys. Rev. Lett. Y1 - 2011 A1 - Piani, Marco A1 - Gharibian, Sevag A1 - Adesso, Gerardo A1 - John Calsamiglia A1 - Horodecki, Paweł A1 - Winter, Andreas PB - American Physical Society VL - 106 UR - http://prl.aps.org/abstract/PRL/v106/i22/e220403 ER - TY - JOUR T1 - Characterizing quantumness via entanglement creation JF - International Journal of Quantum Information Y1 - 2011 A1 - Gharibian, Sevag A1 - Piani, Marco A1 - Adesso, Gerardo A1 - John Calsamiglia A1 - Pawel Horodecki VL - 9 UR - http://arxiv.org/abs/1105.3419 ER - TY - JOUR T1 - Limited-path-length entanglement percolation in quantum complex networks JF - Physical Review A Y1 - 2011 A1 - Cuquet, Martí A1 - John Calsamiglia AB - We study entanglement distribution in quantum complex networks where nodes are connected by bipartite entangled states. These networks are characterized by a complex structure, which dramatically affects how information is transmitted through them. For pure quantum state links, quantum networks exhibit a remarkable feature absent in classical networks: it is possible to effectively rewire the network by performing local operations on the nodes. We propose a family of such quantum operations that decrease the entanglement percolation threshold of the network and increase the size of the giant connected component. We provide analytic results for complex networks with an arbitrary (uncorrelated) degree distribution. These results are in good agreement with numerical simulations, which also show enhancement in correlated and real-world networks. The proposed quantum preprocessing strategies are not robust in the presence of noise. However, even when the links consist of (noisy) mixed-state links, one can send quantum information through a connecting path with a fidelity that decreases with the path length. In this noisy scenario, complex networks offer a clear advantage over regular lattices, namely, the fact that two arbitrary nodes can be connected through a relatively small number of steps, known as the small-world effect. We calculate the probability that two arbitrary nodes in the network can successfully communicate with a fidelity above a given threshold. This amounts to working out the classical problem of percolation with a limited path length. We find that this probability can be significant even for paths limited to few connections and that the results for standard (unlimited) percolation are soon recovered if the path length exceeds by a finite amount the average path length, which in complex networks generally scales logarithmically with the size of the network. VL - 83 UR - http://pra.aps.org/abstract/PRA/v83/i3/e032319 ER - TY - THES T1 - Quantum Accuracy Limits and Benchmarks in Continuos Variable Systems T2 - Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics Y1 - 2011 A1 - Bracons, Mariona (formerly Mariona Aspachs) JF - Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics PB - Universitat Autònoma de Barcelona CY - Barcelona VL - Doctorat en Física ER - TY - JOUR T1 - Scavenging quantum information: Multiple observations of quantum systems JF - Physical Review A Y1 - 2011 A1 - Rapčan, P. A1 - Calsamiglia, J. A1 - Muñoz-Tapia, R. A1 - Bagan, E. A1 - Bužek, V. VL - 84 UR - https://link.aps.org/doi/10.1103/PhysRevA.84.032326http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevA.84.032326/fulltexthttp://link.aps.org/article/10.1103/PhysRevA.84.032326 JO - Phys. Rev. A ER - TY - JOUR T1 - Adiabatic Markovian Dynamics JF - Phys. Rev. Lett. Y1 - 2010 A1 - Oreshkov, Ognyan A1 - John Calsamiglia KW - Quantum Physics AB - We propose a theory of adiabaticity in quantum Markovian dynamics based on a structural decomposition of the Hilbert space induced by the asymptotic behavior of the Lindblad semigroup. A central idea of our approach is that the natural generalization of the concept of eigenspace of the Hamiltonian in the case of Markovian dynamics is a noiseless subsystem with a minimal noisy cofactor. Unlike previous attempts to define adiabaticity for open systems, our approach deals exclusively with physical entities and provides a simple, intuitive picture at the underlying Hilbert-space level, linking the notion of adiabaticity to the theory of noiseless subsystems. As an application of our theory, we propose a framework for decoherence-assisted computation in noiseless codes under general Markovian noise. We also formulate a dissipation-driven approach to holonomic computation based on adiabatic dragging of subsystems that is generally not achievable by non-dissipative means. VL - 105 UR - http://arxiv.org/abs/1002.2219 ER - TY - JOUR T1 - Estimation of quantum finite mixtures JF - Physical Review A Y1 - 2010 A1 - de Vicente, J. I. A1 - John Calsamiglia A1 - Muñoz-Tapia, Ramon A1 - Bagan, Emilio AB - We consider the problem of determining the weights of a quantum ensemble. That is to say, given a quantum system that is in a set of possible known states according to an unknown probability law, we give strategies to estimate the individual probabilities, weights, or mixing proportions. Such strategies can be used to estimate the frequencies at which different independent signals are emitted by a source. They can also be used to estimate the weights of particular terms in a canonical decomposition of a quantum channel. The quality of these strategies is quantified by a covariance-type error matrix. According with this cost function, we give optimal strategies in both the single-shot and multiple-copy scenarios. The latter is also analyzed in the asymptotic limit of large number of copies. We give closed expressions of the error matrix for two-component quantum mixtures of qubit systems. The Fisher information plays an unusual role in the problem at hand, providing exact expressions of the minimum covariance matrix for any number of copies. VL - 81 UR - http://link.aps.org/doi/10.1103/PhysRevA.81.012332 ER - TY - JOUR T1 - Local Discrimination of Mixed States JF - Phys. Rev. Lett. Y1 - 2010 A1 - John Calsamiglia A1 - de Vicente, J. I. A1 - Muñoz-Tapia, R. A1 - E. Bagan PB - American Physical Society VL - 105 ER - TY - THES T1 - Localitat i separabilitat en discriminació d'hipòtesis quàntiques T2 - Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics Y1 - 2010 A1 - Caminal Armadans, Roger AB - En aquest treball s'estudiarà la probabilitat òptima d'obtenir l'estat correcte en una mesura sobre un sistema de N còpies de tres estats quàntics amb probabilitats a priori idèntiques i separats 120º. S'estudiaran diferents estratègies, posant especial èmfasi en la localitat i l'entrellaçament de les mesures. Finalment es farà una generalització d'aquestes estratègies per a estats mescla, estudiant la probabilitat de discriminar-los en funció de la seva puresa i l'estratègia portada a terme. JF - Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics PB - Universitat Autònoma de Barcelona CY - Barcelona VL - Grau en Física ER - TY - THES T1 - Localització en xarxes complexes T2 - Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics Y1 - 2010 A1 - Perarnau Llobet, Martí AB - En els anys 1950s, P.W. Anderson va suggerir la possibilitat d'un nou fenomen en física de l'estat sòlid: la localització. Aquest fenomen consisteix en que la funció d'ona dels electrons es queda totalment localitzada en l'espai provocant que el material passi de ser un conductor a un aïllant. Ha estat extensament estudiat en xarxes ordenades i és conseqüència de les impureses del material. En aquest treball, pretenem estudiar la localització en xarxes amb naturalesa aleatòria, en particular, en xarxes complexes. La idea bàsica és, tenint en compte que l'aleatorietat de les impureses del material provoca localització, veure si l'aleatorietat en l'estructura del material produeix el mateix efecte. JF - Departament de Físca, Grup de Física Teòria: Informació i fenòmens quàntics PB - Universitat Autònoma de Barcelona CY - Barcelona VL - Grau en Física ER - TY - JOUR T1 - Multicopy programmable discrimination of general qubit states JF - Physical Review A Y1 - 2010 A1 - Sentís, Gael A1 - Bagan, Emili A1 - John Calsamiglia A1 - Muñoz-Tapia, Ramon AB - Quantum state discrimination is a fundamental primitive in quantum statistics where one has to correctly identify the state of a system that is in one of two possible known states. A programmable discrimination machine performs this task when the pair of possible states is not a priori known but instead the two possible states are provided through two respective program ports. We study optimal programmable discrimination machines for general qubit states when several copies of states are available in the data or program ports. Two scenarios are considered: One in which the purity of the possible states is a priori known, and the fully universal one where the machine operates over generic mixed states of unknown purity. We find analytical results for both the unambiguous and minimum error discrimination strategies. This allows us to calculate the asymptotic performance of programmable discrimination machines when a large number of copies are provided and to recover the standard state discrimination and state comparison values as different limiting cases. VL - 82 UR - http://link.aps.org/doi/10.1103/PhysRevA.82.042312 ER - TY - JOUR T1 - Recycling of qubits JF - Physica Scripta Y1 - 2010 A1 - P Rapcan A1 - John Calsamiglia A1 - Muñoz-Tapia, Ramon A1 - Bagan, Emilio A1 - V Buzek AB - We consider a finite number, N, of qubits that encode a pure single qubit state SU(2) covariantly. Given the N-qubit state has already been measured optimally to estimate the single-qubit state, we analyse the maximum information obtainable by a second, and subsequent observers ignorant of important details of the previous measurements. We quantify the information acquired by each observer as a function of N and of the number of independent observers that in succession have independently measured the same ensemble of qubits before him. VL - T140 UR - http://iopscience.iop.org/1402-4896/2010/T140/014059?fromSearchPage=true ER - TY - JOUR T1 - Comment on “Nongeometric Conditional Phase Shift via Adiabatic Evolution of Dark Eigenstates: A New Approach to Quantum Computation” JF - Physical Review Letters Y1 - 2009 A1 - Oreshkov, Ognyan A1 - John Calsamiglia VL - 103 UR - http://prl.aps.org/abstract/PRL/v103/i18/e188901 ER - TY - JOUR T1 - Distinguishability measures between ensembles of quantum states JF - Physical Review A (Atomic, Molecular, and Optical Physics) Y1 - 2009 A1 - Oreshkov, Ognyan A1 - John Calsamiglia VL - 79 UR - http://link.aps.org/doi/10.1103/PhysRevA.79.032336 ER - TY - JOUR T1 - Entanglement Percolation in Quantum Complex Networks JF - Physical Review Letters Y1 - 2009 A1 - Cuquet, Martí A1 - John Calsamiglia KW - complex networks KW - entanglement distribution KW - entanglement percolation KW - erdos-renyi KW - generating function KW - quantum complex networks KW - quantum networks KW - scale-free KW - small-world AB - Quantum networks are essential to quantum information distributed applications, and communicatingover them is a key challenge. Complex networks have richand intriguing properties, which are as yet unexplored in thequantum setting. Here, we study the effect of entanglement percolationas a means to establish long-distance entanglement between arbitrary nodesof quantum complex networks. We develop a theory to analyticallystudy random graphs with arbitrary degree distribution and give exactresults for some models. Our findings are in good agreementwith numerical simulations and show that the proposed quantum strategiesenhance the percolation threshold substantially. Simulations also show a clearenhancement in small-world and other real-world networks. PB - APS VL - 103 UR - http://link.aps.org/abstract/PRL/v103/e240503 ER - TY - JOUR T1 - Phase estimation for thermal Gaussian states JF - Physical Review A (Atomic, Molecular, and Optical Physics) Y1 - 2009 A1 - Aspachs, Mariona A1 - John Calsamiglia A1 - Muñoz-Tapia, Ramon A1 - Bagan, Emili KW - gaussian noise KW - gaussian states KW - optical losses KW - optical squeezing KW - phase estimation KW - quantum noise KW - thermal noise VL - 79 UR - http://link.aps.org/abstract/PRA/v79/e033834 ER - TY - JOUR T1 - Phase-covariant quantum benchmarks JF - Physical Review A (Atomic, Molecular, and Optical Physics) Y1 - 2009 A1 - John Calsamiglia A1 - Aspachs, Mariona A1 - Muñoz-Tapia, Ramon A1 - Bagan, Emili KW - quantum benchmarks VL - 79 UR - http://link.aps.org/doi/10.1103/PhysRevA.79.050301 ER - TY - JOUR T1 - Quantum Chernoff bound as a measure of distinguishability between density matrices: Application to qubit and Gaussian states JF - Physical Review A (Atomic, Molecular, and Optical Physics) Y1 - 2008 A1 - John Calsamiglia A1 - Muñoz-Tapia, Ramon A1 - Masanes, Lluís A1 - Acín, Antonio A1 - Bagan, Emili KW - Bayes methods KW - error statistics KW - quantum chernoff bound KW - quantum statistical mechanics KW - quantum theory VL - 77 UR - http://link.aps.org/abstract/PRA/v77/e032311 ER - TY - JOUR T1 - Discriminating States: The Quantum Chernoff Bound JF - Physical Review Letters Y1 - 2007 A1 - K. M. R. Audenaert A1 - John Calsamiglia A1 - Muñoz-Tapia, Ramon A1 - Bagan, Emili A1 - Masanes, Lluís A1 - Acín, Antonio A1 - Verstraete, Frank KW - quantum chernoff bound KW - state discrimination VL - 98 UR - http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000098000016160501000001&idtype=cvips&gifs=yes ER - TY - JOUR T1 - Discriminating States: The Quantum Chernoff Bound JF - Physical Review Letters Y1 - 2007 A1 - K. M. R. Audenaert A1 - John Calsamiglia A1 - R. {Munoz-Tapia} A1 - E. Bagan A1 - Ll. Masanes A1 - A. Acin A1 - F. Verstraete VL - 98 UR - http://link.aps.org/abstract/PRL/v98/e160501 ER - TY - JOUR T1 - How to hide a secret direction JF - New Journal of Physics Y1 - 2007 A1 - Bagan, Emili A1 - John Calsamiglia A1 - Demkowicz-Dobrzanski, R. A1 - Muñoz-Tapia, Ramon AB - We present a procedure to share a secret spatial direction in the absence of a common reference frame using a multipartite quantum state. The procedure guarantees that the parties can determine the direction if they perform joint measurements on the state, but fail to do so if they restrict themselves to local operations and classical communication (LOCC). We calculate the fidelity for joint measurements, give bounds on the fidelity achievable by LOCC, and prove that there is a non-vanishing gap between the two of them, even in the limit of infinitely many copies. The robustness of the procedure under particle loss is also studied. As a by-product we find bounds on the probability of discriminating by LOCC between the invariant subspaces of total angular momentum N/2 and N/2-1 in a system of N elementary spins. VL - 9 UR - http://iopscience.iop.org/1367-2630/9/8/244/pdf/1367-2630\_9\_8\_244.pdf ER - TY - JOUR T1 - Recycling of quantum information: Multiple observations of quantum systems JF - arXiv Y1 - 2007 A1 - Rapcan, Peter A1 - John Calsamiglia A1 - Muñoz-Tapia, Ramon A1 - Bagan, Emili A1 - Bużek, Vladimir AB - Given a finite number of copies of an unknown qubit state that have already been measured optimally, can one still extract any information about the original unknown state? We give a positive answer to this question and quantify the information obtainable by a given observer as a function of the number of copies in the ensemble, and of the number of independent observers that, one after the other, have independently measured the same ensemble of qubits before him. The optimality of the protocol is proven and extensions to other states and encodings are also studied. According to the general lore, the state after a measurement has no information about the state before the measurement. Our results manifestly show that this statement has to be taken with a grain of salt, specially in situations where the quantum states encode confidential information. UR - http://arxiv.org/abs/0708.1086 ER - TY - JOUR T1 - Weighted graph states and applications to spin chains, lattices and gases JF - Journal of Physics B: Atomic, Molecular and Optical Physics Y1 - 2007 A1 - Hartmann, L. A1 - John Calsamiglia A1 - Dür, Wolfgang A1 - Briegel, Hans J. VL - 40 UR - http://www.iop.org/EJ/abstract/0953-4075/40/9/S01 ER - TY - JOUR T1 - Spin Gases: Quantum Entanglement Driven by Classical Kinematics JF - Physical Review Letters Y1 - 2005 A1 - John Calsamiglia A1 - Hartmann, L. A1 - Dür, Wolfgang A1 - Briegel, Hans J. KW - entanglement distribution KW - spin gas AB - A spin gas is a natural extension of a classical gas. It consists of a large number of particles whose (random) motion is described classically, but, in addition, have internal (quantum mechanical) degrees of freedom that interact during collisions. For specific types of quantum interactions we determine the entanglement that occurs naturally in such systems. We analyze how the evolution of the quantum state is determined by the underlying classical kinematics of the gas. For the Boltzmann gas, we calculate the rate at which entanglement is produced and characterize the entanglement properties of the equilibrium state. VL - 95 UR - http://link.aps.org/abstract/PRL/v95/e180502 ER -