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Physical Review A

M -ary-state phase-shift-keying discrimination below the homodyne limit

F. E. Becerra, Jingyun Fan, Gerald Baumgartner, Sergey V. Polyakov, J. Goldhar, Jon T. Kosloski, Alan L. Migdall

Published Dec 22, 2011• 66 citations
Publisher

Abstract

We investigate a strategy for $M$-ary discrimination of nonorthogonal phase states with error rates below the homodyne limit. This strategy uses feed forward to update a reference field and signal nulling for the state discrimination. We experimentally analyze the receiver performance using postprocessing and a Bayesian strategy to emulate the feed-forward process. This analysis shows that for a moderate system detection efficiency, it is possible to surpass the homodyne error limit for quadrature phase-shift keying signals using feed forward.

DOI
10.1103/physreva.84.062324

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We investigate a strategy for MMM-ary discrimination of nonorthogonal phase states with error rates below the homodyne limit. This strategy uses feed forward to update a reference field and signal nulling for the state discrimination. We experimentally analyze the receiver performance using postprocessing and a Bayesian strategy to emulate the feed-forward process. This analysis shows that for a moderate system detection efficiency, it is possible to surpass the homodyne error limit for quadrature phase-shift keying signals using feed forward.
We propose a hybrid feed-forward receiver (HFFRE) for the discrimination of binary phase-shift-keyed coherent states based on the appropriate combination of a displacement feed-forward receiver (DFFRE) and a homodynelike setup employing a low-intensity local oscillator and photon-number-resolving detectors. We investigate the performance of the proposed scheme, also addressing realistic scenarios in the presence of nonunit quantum detection efficiency, dark counts, and visibility reduction. The present HFFRE outperforms the DFFRE in all conditions, beating the standard quantum limit in particular regimes.
Optimized state-discrimination receiver strategies for nonorthogonal states can improve the capacity of the communication channels operating with error rates below the ones corresponding to conventional receivers. Coherent signal-nulling receivers use a local oscillator to null the signal state and perform the discrimination of the signal from an alphabet of nonorthogonal states. We describe our study of signal nulling for signals encoded in nonorthogonal phase states. The signal nulling discrimination setup is the first step for the experimental investigation of different discrimination strategies for receivers of coherent multi-phase encoded signals.
Quantum enhanced receivers are endowed with resources to achieve higher sensitivities than conventional technologies. For application in optical communications, they provide improved discriminatory capabilities for multiple nonorthogonal quantum states. In this work, we propose and experimentally demonstrate a new decoding scheme for quadrature phase-shift encoded signals. Our receiver surpasses the standard quantum limit and outperforms all previously known nonadaptive detectors at low input powers. Unlike existing approaches, the receiver only exploits linear optical elements and on-off photodetection. This circumvents the requirement for challenging feed-forward operations that limit communication transmission rates and can be readily implemented with current technology.