Abstract
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 non-orthogonal 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 non-adaptive detectors at low input powers. Unlike existing approaches, the receiver only exploits linear optical elements and on-off photo-detection. This circumvents the requirement for challenging feed-forward operations that limit communication transmission rates and can be readily implemented with current technology.
Highlights
Quantum mechanics places strict fundamental limits on our ability to discriminate nonorthogonal quantum states [1,2]
The mathematical framework around state discrimination is based on the theory of quantum detection [1], and it has been applied to study the discrimination of various quantum states [10,11,12]
While the following framework can be extended to nonclassical ancillary states, we focus on coherent states given their availability and widespread use in quantum information
Summary
Quantum mechanics places strict fundamental limits on our ability to discriminate nonorthogonal quantum states [1,2]. This work focuses on the discrimination of four weak coherent states with equal amplitude and equidistant phase separations, {|α , |iα , |−α , |−iα }, chosen with equal prior probabilities, where the amplitude α is real valued and positive This ensemble is referred to as quadrature phase-shift keying (QPSK) and is commonplace in fibre networks [19]. It is possible to surpass the heterodyne limit and approach the Helstrom bound using different decoding strategies These schemes generally use a combination of linear optics, photodetection, and globally optimized displacement operations to distinguish coherent states through conditional signal nulling. We show that adaptive feedback is not necessary to beat the conventional heterodyne decoding limit in the fully quantum, weak coherent amplitude regime (α 0.5).
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