TY - JOUR
T1 - Multicopy programmable discrimination of general qubit states
JF - Physical Review A
Y1 - 2010
A1 - Sentís, Gael
A1 - Bagan, Emili
A1 - Calsamiglia, John
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 - Phase estimation for thermal Gaussian states
JF - Physical Review A (Atomic, Molecular, and Optical Physics)
Y1 - 2009
A1 - Aspachs, Mariona
A1 - Calsamiglia, John
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 - Calsamiglia, John
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 - Phase variance of squeezed vacuum states
JF - Physical Review A (Atomic, Molecular, and Optical Physics)
Y1 - 2008
A1 - Bagan, Emili
A1 - Monras, Alex
A1 - Muñoz-Tapia, Ramon
KW - optical squeezing
VL - 78
UR - http://link.aps.org/abstract/PRA/v78/e043829
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 - Calsamiglia, John
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 - Quantum-limited metrology with product states
JF - Physical Review A (Atomic, Molecular, and Optical Physics)
Y1 - 2008
A1 - Boixo, Sergio
A1 - Datta, Animesh
A1 - Flammia, Steven T.
A1 - Shaji, Anil
A1 - Bagan, Emili
A1 - Caves, Carlton M.
KW - angular momentum theory
KW - Bose-Einstein condensation
KW - bound states
KW - protocols
KW - quantum entanglement
VL - 77
UR - http://link.aps.org/abstract/PRA/v77/e012317
ER -
TY - JOUR
T1 - Discriminating States: The Quantum Chernoff Bound
JF - Physical Review Letters
Y1 - 2007
A1 - K. M. R. Audenaert
A1 - Calsamiglia, John
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 - How to hide a secret direction
JF - New Journal of Physics
Y1 - 2007
A1 - Bagan, Emili
A1 - Calsamiglia, John
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 - Calsamiglia, John
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 - Aligning spatial frames through quantum channels
JF - International Journal of Quantum Information
Y1 - 2006
A1 - Bagan, Emili
A1 - Muñoz-Tapia, Ramon
VL - 4
ER -
TY - JOUR
T1 - Optimal full estimation of qubit mixed states
JF - Physical Review A (Atomic, Molecular, and Optical Physics)
Y1 - 2006
A1 - Bagan, Emili
A1 - Ballester, M. A.
A1 - Gill, R. D.
A1 - Monras, Alex
A1 - Muñoz-Tapia, Ramon
KW - mixed states
KW - probability
KW - state estimation
VL - 73
UR - http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PLRAAN000073000003032301000001&idtype=cvips&gifs=yes
ER -
TY - JOUR
T1 - Quantum field theory of photons with orbital angular momentum
JF - Physical Review A (Atomic, Molecular, and Optical Physics)
Y1 - 2006
A1 - Calvo, Gabriel F.
A1 - Picon, Antonio
A1 - Bagan, Emili
KW - information theory
KW - orbital angular momentum
KW - quantum field theory
KW - quantum optics
VL - 73
UR - http://link.aps.org/abstract/PRA/v73/e013805
ER -
TY - JOUR
T1 - Relative states, quantum axes, and quantum references
JF - Physical Review A (Atomic, Molecular, and Optical Physics)
Y1 - 2006
A1 - Bagan, Emili
A1 - Iblisdir, Sofyan
A1 - Muñoz-Tapia, Ramon
KW - quantum theory
VL - 73
UR - http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PLRAAN000073000002022341000001&idtype=cvips&gifs=yes
ER -
TY - JOUR
T1 - Secrecy properties of quantum channels
JF - Physical Review A (Atomic, Molecular, and Optical Physics)
Y1 - 2006
A1 - Acín, Antonio
A1 - Bae, J.
A1 - Bagan, Emili
A1 - Baig, Marià
A1 - Masanes, Lluís
A1 - Muñoz-Tapia, Ramon
KW - information theory
KW - protocols
KW - quantum cryptography
VL - 73
UR - http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PLRAAN000073000001012327000001&idtype=cvips&gifs=yes
ER -
TY - JOUR
T1 - Separable Measurement Estimation of Density Matrices and its Fidelity Gap with Collective Protocols
JF - Physical Review Letters
Y1 - 2006
A1 - Bagan, Emili
A1 - Ballester, M. A.
A1 - Gill, R. D.
A1 - Muñoz-Tapia, Ramon
A1 - Romero-Isart, Oriol
KW - mixed states
KW - state estimation
VL - 97
UR - http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000097000013130501000001&idtype=cvips&gifs=yes
ER -
TY - JOUR
T1 - Comprehensive analysis of quantum pure-state estimation for two-level systems
JF - Physical Review A (Atomic, Molecular, and Optical Physics)
Y1 - 2005
A1 - Bagan, Emili
A1 - Monras, Alex
A1 - Muñoz-Tapia, Ramon
AB - 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.
PB - American Physical Society
VL - 71
UR - http://link.aps.org/doi/10.1103/PhysRevA.71.062318
ER -
TY - JOUR
T1 - Multiple-copy two-state discrimination with individual measurements
JF - Physical Review A (Atomic, Molecular, and Optical Physics)
Y1 - 2005
A1 - Acín, Antonio
A1 - Bagan, Emili
A1 - Baig, Marià
A1 - Masanes, Lluís
A1 - Muñoz-Tapia, Ramon
KW - state discrimination
VL - 71
UR - http://link.aps.org/abstract/PRA/v71/e032338
ER -
TY - JOUR
T1 - Purity Estimation with Separable Measurements
JF - Physical Review Letters
Y1 - 2005
A1 - Bagan, Emili
A1 - Ballester, M. A.
A1 - Muñoz-Tapia, Ramon
A1 - Romero-Isart, Oriol
AB - 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.
PB - American Physical Society
VL - 95
UR - http://link.aps.org/doi/10.1103/PhysRevLett.95.110504
ER -
TY - JOUR
T1 - Collective versus local measurements in a qubit mixed-state estimation
JF - Physical Review A (Atomic, Molecular, and Optical Physics)
Y1 - 2004
A1 - Bagan, Emili
A1 - Baig, Marià
A1 - Muñoz-Tapia, Ramon
A1 - Rodriguez, A.
AB - 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.
PB - American Physical Society
VL - 69
UR - http://link.aps.org/doi/10.1103/PhysRevA.69.010304
ER -
TY - JOUR
T1 - Entanglement-assisted alignment of reference frames using a dense covariant coding
JF - Physical Review A (Atomic, Molecular, and Optical Physics)
Y1 - 2004
A1 - Bagan, Emili
A1 - Baig, Marià
A1 - Muñoz-Tapia, Ramon
AB - 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.
PB - American Physical Society
VL - 69
UR - http://link.aps.org/doi/10.1103/PhysRevA.69.050303
ER -
TY - JOUR
T1 - Quantum reverse engineering and reference-frame alignment without nonlocal correlations
JF - Physical Review A (Atomic, Molecular, and Optical Physics)
Y1 - 2004
A1 - Bagan, Emili
A1 - Baig, Marià
A1 - Muñoz-Tapia, Ramon
AB - 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.
PB - American Physical Society
VL - 70
UR - http://link.aps.org/doi/10.1103/PhysRevA.70.030301
ER -
TY - CONF
T1 - Optimal reconstruction of a pure qubit state with local measurements
T2 - Proceedings of the 8th ICSSUR
Y1 - 2003
A1 - Bagan, Emili
A1 - Baig, Marià
A1 - Monras, Alex
A1 - Muñoz-Tapia, Ramon
JF - Proceedings of the 8th ICSSUR
PB - Rinton Press (2003)
ER -
TY - JOUR
T1 - Aligning Reference Frames with Quantum States
JF - arXiv
Y1 - 2001
A1 - Bagan, Emili
A1 - Baig, Marià
A1 - Muñoz-Tapia, Ramon
UR - http://arxiv.org/abs/quant-ph/0106155
ER -