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21 p, 1.4 MB |
Probabilistic metrology or how some measurement outcomes render ultra-precise estimates
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Calsamiglia Costa, John (Universitat Autònoma de Barcelona. Departament de Física) ;
Gendra Casalí, Bernat (Universitat Autònoma de Barcelona. Departament de Física) ;
Muñoz Tapia, Ramon (Universitat Autònoma de Barcelona. Departament de Física) ;
Bagán Capella, Emili (Universitat Autònoma de Barcelona. Departament de Física)
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. [...]
2016 - 10.1088/1367-2630/18/10/103049
New journal of physics, Vol. 18 (October 2016) , art. 103049
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22 p, 562.0 KB |
Unsupervised classification of quantum data
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Sentís Herrera, Gael (Universität Siegen. Naturwissenschaftlich-Technische Fakultät) ;
Monras Blasi, Àlex (Universitat Autònoma de Barcelona. Departament de Física) ;
Muñoz Tapia, Ramon (Universitat Autònoma de Barcelona. Departament de Física) ;
Calsamiglia Costa, John (Universitat Autònoma de Barcelona. Departament de Física) ;
Bagán Capella, Emili (Universitat Autònoma de Barcelona. Departament de Física)
We introduce the problem of unsupervised classification of quantum data, namely, of systems whose quantum states are unknown. We derive the optimal single-shot protocol for the binary case, where the states in a disordered input array are of two types. [...]
2019 - 10.1103/PhysRevX.9.041029
Physical Review, Vol. 9 Núm. art. 041029 (2019)
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17 p, 934.7 KB |
Beating noise with abstention in state estimation
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Gendra Casalí, Bernat (Universitat Autònoma de Barcelona. Departament de Física) ;
Ronco Bonvehi, Elio (Universitat Autònoma de Barcelona. Departament de Física) ;
Calsamiglia Costa, John (Universitat Autònoma de Barcelona. Departament de Física) ;
Muñoz Tapia, Ramon (Universitat Autònoma de Barcelona. Departament de Física) ;
Bagán Capella, Emili (Universitat Autònoma de Barcelona. Departament de Física)
We address the problem of estimating pure qubit states with nonideal (noisy) measurements in the multiple-copy scenario, where the data consist of a number N of identically prepared qubits. We show that the average fidelity of the estimates can increase significantly if the estimation protocol allows for inconclusive answers, or abstentions. [...]
2012 - 10.1088/1367-2630/14/10/105015
New journal of physics, Vol. 14 (October 2012) , art. 105015
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4 p, 72.1 KB |
Communication of spin directions with product states and finite measurements
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Bagán Capella, Emili (Universitat Autònoma de Barcelona. Departament de Física) ;
Baig i Aleu, Marià, 1955- (Universitat Autònoma de Barcelona. Departament de Física) ;
Muñoz Tapia, Ramon (Universitat Autònoma de Barcelona. Departament de Física) ;
American Physical Society
Total spin eigenstates can be used to intrinsically encode a direction, which can later be decoded by means of a quantum measurement. We study the optimal strategy that can be adopted if, as is likely in practical applications, only product states of N spins are available. [...]
2001 - 10.1103/PhysRevA.64.022305
Physical review. A, Vol. 64, Issue 2 (July 2001) , p. 022305-1-022305-4
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5.
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11 p, 136.5 KB |
Optimal encoding and decoding of a spin direction
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Bagán Capella, Emili (Universitat Autònoma de Barcelona. Departament de Física) ;
Baig i Aleu, Marià, 1955- (Universitat Autònoma de Barcelona. Departament de Física) ;
Brey, Antoni, 1967- (Institut de Física d'Altes Energies) ;
Muñoz Tapia, Ramon (Universitat Autònoma de Barcelona. Departament de Física) ;
Tarrach Siegel, Rolf (Universitat de Barcelona. Departament d'Estructura i Constituents de la Matèria) ;
American Physical Society
For a system of N spins there are quantum states that can encode a direction in an intrinsic way. Information on this direction can later be decoded by means of a quantum measurement. We present here the optimal encoding and decoding procedure using the fidelity as a figure of merit. [...]
2001 - 10.1103/PhysRevA.63.052309
Physical review. A, Vol. 63, Issue 5 (April 2001) , p. 052309-1-52309-11
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4 p, 62.4 KB |
Collective versus local measurements in a qubit mixed-state estimation
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Bagán Capella, Emili (Universitat Autònoma de Barcelona. Departament de Física) ;
Baig i Aleu, Marià, 1955- (Universitat Autònoma de Barcelona. Departament de Física) ;
Muñoz Tapia, Ramon (Universitat Autònoma de Barcelona. Departament de Física) ;
Rodríguez González, Alberto (Universidad de Salamanca. Area de Física Teórica) ;
American Physical Society
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. [...]
2004 - 10.1103/PhysRevA.69.010304
Physical review. A, Vol. 69, Issue 1 (January 2004) , p. 010304-1-010304-4
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5 p, 72.8 KB |
Secrecy properties of quantum channels
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Acín, Antonio (Institut de Ciències Fotòniques) ;
Bae, J. (Institut de Ciències Fotòniques) ;
Bagán Capella, Emili (Universitat Autònoma de Barcelona. Departament de Física) ;
Baig i Aleu, Marià, 1955- (Universitat Autònoma de Barcelona. Departament de Física) ;
Masanes, Ll. (Universitat de Barcelona. Departament d'Estructura i Constituents de la Matèria) ;
Muñoz Tapia, Ramon (Universitat Autònoma de Barcelona. Departament de Física) ;
American Physical Society
We identify those properties of a quantum channel that are relevant for cryptography. We focus on general key distribution protocols that use prepare and measure schemes and the existing classical reconciliation techniques, as these are the protocols feasible with current technology. [...]
2006 - 10.1103/PhysRevA.73.012327
Physical review. A, Vol. 73, Issue 1 (January 2006) , p. 012327-1-012327-5
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