The free space optical interference channel

TL;DR

This paper analyzes the ultimate limits of quantum optical interference channels, comparing various detection strategies and establishing capacity regions, especially in low photon-number regimes with strong interference.

Contribution

It provides a quantum framework for multi-user free-space optical channels, evaluates detection strategies, and explores capacity regions with a focus on joint detection and recent quantum decoders.

Findings

Joint detection surpasses homodyne and heterodyne in very strong interference.

Capacity regions are characterized for homodyne and heterodyne under strong interference.

Conjectures existence of joint detection strategies outperforming traditional methods.

Abstract

Semiclassical models for multiple-user optical communication cannot assess the ultimate limits on reliable communication as permitted by the laws of physics. In all optical communications settings that have been analyzed within a quantum framework so far, the gaps between the quantum limit to the capacity and the Shannon limit for structured receivers become most significant in the low photon-number regime. Here, we present a quantum treatment of a multiple-transmitter multiple-receiver multi-spatial-mode free-space interference channel with diffraction-limited loss and a thermal background. We consider the performance of a laser-light (coherent state) encoding in conjunction with various detection strategies such as homodyne, heterodyne, and joint detection. Joint detection outperforms both homodyne and heterodyne detection whenever the channel exhibits "very strong" interference. We determine the capacity region for homodyne or heterodyne detection when the channel has "strong" interference, and we conjecture the existence of a joint detection strategy that outperforms the former two strategies in this case. Finally, we determine the Han-Kobayashi achievable rate regions for both homodyne and heterodyne detection and compare them to a region achievable by a conjectured joint detection strategy. In these latter cases, we determine achievable rate regions if the receivers employ a recently discovered min-entropy quantum simultaneous decoder.