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arXiv · 2510.20096

All-Gaussian State Discrimination Beyond the Coherent Helstrom Bound

Angus Walsh·Lorcan Conlon·Biveen Shajilal·Ozlem Erkilic·Jiri Janousek·Syed Assad·Jie Zhao·Ping Koy Lam
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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.

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Angus Walsh, Lorcan Conlon, Biveen Shajilal, Ozlem Erkilic, Jiri Janousek, Syed Assad, Jie Zhao, Ping Koy Lam. 2025-10-23. All-Gaussian State Discrimination Beyond the Coherent Helstrom Bound. https://arxiv.org/abs/2510.20096

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