Practical quantum receivers for phase-shift keying at the single-photon level*

Increasing channel capacities in optical communications is a constant and non abating technological demand. While multilevel encoding and phase-shift keying schemes have allowed more information storage in optical fibres, the extent of increase to channel capacities ultimately depends on how well receivers decode the stored information through state discrimination. This is of particular importance for the efficient discrimination of weak coherent states. Quantum enhanced receivers are endowed with resources that can 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, our 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.