%0 Journal Article
%J Physical Review A
%D 2010
%T Multicopy programmable discrimination of general qubit states
%A Sentís, Gael
%A Bagan, Emili
%A Calsamiglia, John
%A Muñoz-Tapia, Ramon
%X 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.
%B Physical Review A
%V 82
%P 042312
%8 10/2010
%U http://link.aps.org/doi/10.1103/PhysRevA.82.042312
%R 10.1103/PhysRevA.82.042312
%0 Journal Article
%J Physical Review A (Atomic, Molecular, and Optical Physics)
%D 2009
%T Phase estimation for thermal Gaussian states
%A Aspachs, Mariona
%A Calsamiglia, John
%A Muñoz-Tapia, Ramon
%A Bagan, Emili
%K gaussian noise
%K gaussian states
%K optical losses
%K optical squeezing
%K phase estimation
%K quantum noise
%K thermal noise
%B Physical Review A (Atomic, Molecular, and Optical Physics)
%V 79
%P 033834–12
%8 03/2009
%G eng
%U http://link.aps.org/abstract/PRA/v79/e033834
%R 10.1103/PhysRevA.79.033834
%0 Journal Article
%J Physical Review A (Atomic, Molecular, and Optical Physics)
%D 2009
%T Phase-covariant quantum benchmarks
%A Calsamiglia, John
%A Aspachs, Mariona
%A Muñoz-Tapia, Ramon
%A Bagan, Emili
%K quantum benchmarks
%B Physical Review A (Atomic, Molecular, and Optical Physics)
%V 79
%P 050301–4
%G eng
%U http://link.aps.org/doi/10.1103/PhysRevA.79.050301
%R 10.1103/PhysRevA.79.050301
%0 Journal Article
%J Physical Review A (Atomic, Molecular, and Optical Physics)
%D 2008
%T Phase variance of squeezed vacuum states
%A Bagan, Emili
%A Monras, Alex
%A Muñoz-Tapia, Ramon
%K optical squeezing
%B Physical Review A (Atomic, Molecular, and Optical Physics)
%V 78
%P 043829–7
%8 10/2008
%G eng
%U http://link.aps.org/abstract/PRA/v78/e043829
%R 10.1103/PhysRevA.78.043829
%0 Journal Article
%J Physical Review A (Atomic, Molecular, and Optical Physics)
%D 2008
%T Quantum Chernoff bound as a measure of distinguishability between density matrices: Application to qubit and Gaussian states
%A Calsamiglia, John
%A Muñoz-Tapia, Ramon
%A Masanes, Lluís
%A Acín, Antonio
%A Bagan, Emili
%K Bayes methods
%K error statistics
%K quantum chernoff bound
%K quantum statistical mechanics
%K quantum theory
%B Physical Review A (Atomic, Molecular, and Optical Physics)
%V 77
%P 032311–15
%8 03/2008
%G eng
%U http://link.aps.org/abstract/PRA/v77/e032311
%R 10.1103/PhysRevA.77.032311
%0 Journal Article
%J Physical Review A (Atomic, Molecular, and Optical Physics)
%D 2008
%T Quantum-limited metrology with product states
%A Boixo, Sergio
%A Datta, Animesh
%A Flammia, Steven T.
%A Shaji, Anil
%A Bagan, Emili
%A Caves, Carlton M.
%K angular momentum theory
%K Bose-Einstein condensation
%K bound states
%K protocols
%K quantum entanglement
%B Physical Review A (Atomic, Molecular, and Optical Physics)
%V 77
%P 012317–15
%8 01/2008
%G eng
%U http://link.aps.org/abstract/PRA/v77/e012317
%R 10.1103/PhysRevA.77.012317
%0 Journal Article
%J Physical Review Letters
%D 2007
%T Discriminating States: The Quantum Chernoff Bound
%A K. M. R. Audenaert
%A Calsamiglia, John
%A Muñoz-Tapia, Ramon
%A Bagan, Emili
%A Masanes, Lluís
%A Acín, Antonio
%A Verstraete, Frank
%K quantum chernoff bound
%K state discrimination
%B Physical Review Letters
%V 98
%P 160501–4
%8 04/2007
%G eng
%U http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000098000016160501000001&idtype=cvips&gifs=yes
%R 10.1103/PhysRevLett.98.160501
%0 Journal Article
%J New Journal of Physics
%D 2007
%T How to hide a secret direction
%A Bagan, Emili
%A Calsamiglia, John
%A Demkowicz-Dobrzanski, R.
%A Muñoz-Tapia, Ramon
%X 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.
%B New Journal of Physics
%V 9
%P 244
%G eng
%U http://iopscience.iop.org/1367-2630/9/8/244/pdf/1367-2630\_9\_8\_244.pdf
%R doi:10.1088/1367-2630/9/8/244
%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 International Journal of Quantum Information
%D 2006
%T Aligning spatial frames through quantum channels
%A Bagan, Emili
%A Muñoz-Tapia, Ramon
%B International Journal of Quantum Information
%V 4
%P 5
%G eng
%0 Journal Article
%J Physical Review A (Atomic, Molecular, and Optical Physics)
%D 2006
%T Optimal full estimation of qubit mixed states
%A Bagan, Emili
%A Ballester, M. A.
%A Gill, R. D.
%A Monras, Alex
%A Muñoz-Tapia, Ramon
%K mixed states
%K probability
%K state estimation
%B Physical Review A (Atomic, Molecular, and Optical Physics)
%V 73
%P 032301–18
%8 03/2006
%G eng
%U http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PLRAAN000073000003032301000001&idtype=cvips&gifs=yes
%R 10.1103/PhysRevA.73.032301
%0 Journal Article
%J Physical Review A (Atomic, Molecular, and Optical Physics)
%D 2006
%T Quantum field theory of photons with orbital angular momentum
%A Calvo, Gabriel F.
%A Picon, Antonio
%A Bagan, Emili
%K information theory
%K orbital angular momentum
%K quantum field theory
%K quantum optics
%B Physical Review A (Atomic, Molecular, and Optical Physics)
%V 73
%P 013805–10
%8 01/2006
%G eng
%U http://link.aps.org/abstract/PRA/v73/e013805
%R 10.1103/PhysRevA.73.013805
%0 Journal Article
%J Physical Review A (Atomic, Molecular, and Optical Physics)
%D 2006
%T Relative states, quantum axes, and quantum references
%A Bagan, Emili
%A Iblisdir, Sofyan
%A Muñoz-Tapia, Ramon
%K quantum theory
%B Physical Review A (Atomic, Molecular, and Optical Physics)
%V 73
%P 022341–6
%8 02/2006
%G eng
%U http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PLRAAN000073000002022341000001&idtype=cvips&gifs=yes
%R 10.1103/PhysRevA.73.022341
%0 Journal Article
%J Physical Review A (Atomic, Molecular, and Optical Physics)
%D 2006
%T Secrecy properties of quantum channels
%A Acín, Antonio
%A Bae, J.
%A Bagan, Emili
%A Baig, Marià
%A Masanes, Lluís
%A Muñoz-Tapia, Ramon
%K information theory
%K protocols
%K quantum cryptography
%B Physical Review A (Atomic, Molecular, and Optical Physics)
%V 73
%P 012327–5
%8 01/2006
%G eng
%U http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PLRAAN000073000001012327000001&idtype=cvips&gifs=yes
%R 10.1103/PhysRevA.73.012327
%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 Journal Article
%J Physical Review A (Atomic, Molecular, and Optical Physics)
%D 2005
%T Comprehensive analysis of quantum pure-state estimation for two-level systems
%A Bagan, Emili
%A Monras, Alex
%A Muñoz-Tapia, Ramon
%X Given N identical copies of the state of a quantum two-level system, we analyze its optimal estimation. We consider two situations: general pure states and (pure) states restricted to lie on the equator of the Bloch sphere. We perform a complete and comprehensive analysis of the optimal schemes based on local measurements, and give results (optimal measurements, maximum fidelity, etc.) for arbitrary N, not necessarily large, within the Bayesian framework. We also make a comparative analysis of the asymptotic limit of these results with those derived from a (pointwise) Cramér-Rao type of approach. We give explicit schemes based on local measurements and no classical communication that saturate the fidelity bounds of the most general collective schemes.
%B Physical Review A (Atomic, Molecular, and Optical Physics)
%I American Physical Society
%V 71
%P 062318
%8 06/2005
%G eng
%U http://link.aps.org/doi/10.1103/PhysRevA.71.062318
%R 10.1103/PhysRevA.71.062318
%0 Journal Article
%J Physical Review A (Atomic, Molecular, and Optical Physics)
%D 2005
%T Multiple-copy two-state discrimination with individual measurements
%A Acín, Antonio
%A Bagan, Emili
%A Baig, Marià
%A Masanes, Lluís
%A Muñoz-Tapia, Ramon
%K state discrimination
%B Physical Review A (Atomic, Molecular, and Optical Physics)
%V 71
%P 032338
%G eng
%U http://link.aps.org/abstract/PRA/v71/e032338
%R 10.1103/PhysRevA.71.032338
%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 Physical Review A (Atomic, Molecular, and Optical Physics)
%D 2004
%T Entanglement-assisted alignment of reference frames using a dense covariant coding
%A Bagan, Emili
%A Baig, Marià
%A Muñoz-Tapia, Ramon
%X We present a procedure inspired by dense coding, which enables a highly efficient transmission of information of a continuous nature. The procedure requires the sender and the recipient to share a maximally entangled state. We deal with the concrete problem of aligning reference frames or trihedra by means of a quantum system. We find the optimal covariant measurement and compute the corresponding average error, which has a remarkably simple close form. The connection of this procedure with that of estimating unitary transformations on qubits is briefly discussed.
%B Physical Review A (Atomic, Molecular, and Optical Physics)
%I American Physical Society
%V 69
%P 050303
%8 05/2004
%G eng
%U http://link.aps.org/doi/10.1103/PhysRevA.69.050303
%R 10.1103/PhysRevA.69.050303
%0 Journal Article
%J Physical Review A (Atomic, Molecular, and Optical Physics)
%D 2004
%T Quantum reverse engineering and reference-frame alignment without nonlocal correlations
%A Bagan, Emili
%A Baig, Marià
%A Muñoz-Tapia, Ramon
%X The estimation of unknown qubit elementary gates and the alignment of reference frames are formally the same problem. Using quantum states made out of N qubits, we show that the theoretical precision limit for both problems, which behaves as 1∕N2, can be asymptotically attained with a covariant protocol that exploits the quantum correlation of internal degrees of freedom instead of the more fragile entanglement between distant parties. This cuts by half the number of qubits needed to achieve the precision of the dense covariant coding protocol.
%B Physical Review A (Atomic, Molecular, and Optical Physics)
%I American Physical Society
%V 70
%P 030301
%8 09/2004
%G eng
%U http://link.aps.org/doi/10.1103/PhysRevA.70.030301
%R 10.1103/PhysRevA.70.030301
%0 Conference Paper
%B Proceedings of the 8th ICSSUR
%D 2003
%T Optimal reconstruction of a pure qubit state with local measurements
%A Bagan, Emili
%A Baig, Marià
%A Monras, Alex
%A Muñoz-Tapia, Ramon
%B Proceedings of the 8th ICSSUR
%I Rinton Press (2003)
%G eng
%0 Journal Article
%J arXiv
%D 2001
%T Aligning Reference Frames with Quantum States
%A Bagan, Emili
%A Baig, Marià
%A Muñoz-Tapia, Ramon
%B arXiv
%G eng
%U http://arxiv.org/abs/quant-ph/0106155