Abstract
Generalized quantum measurements identifying non-orthogonal states without ambiguity often play an indispensable role in various quantum applications. For such unambiguous state discrimination scenario, we have a finite probability of obtaining inconclusive results and minimizing the probability of inconclusive results is of particular importance. In this paper, we experimentally demonstrate an adaptive generalized measurement that can unambiguously discriminate the quaternary phase-shift-keying coherent states with a near-optimal performance. Our scheme is composed of displacement operations, single photon detections and adaptive control of the displacements dependent on a history of photon detection outcomes. Our experimental results show a clear improvement of both a probability of conclusive results and a ratio of erroneous decision caused by unavoidable experimental imperfections over conventional static generalized measurements.
Highlights
Quantum measurements are ubiquitous in quantuminformation science
By including feedback control of the displacement operation, adapted by the detection events, it is possible to reach optimal minimum error discrimination (MED) performance for binary phaseshift-keying (BPSK) [21] and nearoptimal performance for quaternary phaseshift-keying (QPSK) signals [27,28,29] assuming that the feedback control is infinitely fast
We propose and experimentally realize an adaptive generalized quantum measurement that unambiguously discriminates quaternary phase-shift-keying coherent states with a near-optimal performance
Summary
Quantum measurements are ubiquitous in quantuminformation science. Designing measurements to appropriately discriminate quantum states and efficiently read out the information encoded in them is of general importance for quantum computation [1,2], sensing [3,4], and communication [5,6,7]. By including feedback control of the displacement operation, adapted by the detection events, it is possible to reach optimal MED performance for BPSK [21] and nearoptimal performance for QPSK signals [27,28,29] assuming that the feedback control is infinitely fast Such an adaptive measurement scheme is technically challenging, its clear advantage has been experimentally observed in several experiments [30,31,32,33,34,35]. As for the USD strategy, generalized measurements enabling USD for M -ary PSK coherent states are realizable with M displacement operations and photon detectors [16] This simple strategy without the complicated adaptive displacements is known to accomplish the optimal USD performance for BPSK coherent states: It maximizes the probability of identifying the states; in other words, minimizes the probability of inconclusive results [36,37].
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