All-Gaussian State Discrimination Beyond the Coherent Helstrom Bound

TL;DR

This paper demonstrates that using Gaussian states and homodyne detection, it is possible to discriminate BPSK signals with error rates below the Helstrom bound for coherent states, surpassing previous practical limitations.

Contribution

The authors show that Gaussian optics combined with homodyne detection can outperform the Helstrom bound for BPSK signal discrimination, offering a practical approach.

Findings

Error rates below the Helstrom bound for coherent states

Use of only Gaussian states and homodyne detection

Practical implementation of quantum-limited discrimination

Abstract

A core problem in communications is the optimal discrimination of binary-phase-shift-keyed (BPSK) signals. A longstanding goal has been to reach the fundamental quantum limit, known as the Helstrom bound, for BPSK signals encoded in coherent states. However, due to technical constraints, proposals for reaching the bound remain impractical. In this letter we take an alternative approach: using only Gaussian optics - displaced squeezed states and homodyne detection - we achieve discrimination of BPSK signals with error rates below what can be achieved using coherent states and any quantum measurement.