On quantum limit of optical communications: concatenated codes and joint-detection receivers

TL;DR

This paper explores the quantum limits of optical communication channels, demonstrating that joint-detection receivers and concatenated codes can surpass traditional symbol-by-symbol detection, achieving higher channel capacities.

Contribution

It introduces concrete code designs and receiver architectures that approach the quantum capacity limits for free-space optical channels.

Findings

Joint-detection receivers outperform conventional methods.

Superadditive capacity gains demonstrated with practical receiver designs.

Tradeoff analysis between photon efficiency and spectral efficiency.

Abstract

When classical information is sent over a channel with quantum-state modulation alphabet, such as the free-space optical (FSO) channel, attaining the ultimate (Holevo) limit to channel capacity requires the receiver to make joint measurements over long codeword blocks. In recent work, we showed a receiver for a pure-state channel that can attain the ultimate capacity by applying a single-shot optical (unitary) transformation on the received codeword state followed by simultaneous (but separable) projective measurements on the single-modulation-symbol state spaces. In this paper, we study the ultimate tradeoff between photon efficiency and spectral efficiency for the FSO channel. Based on our general results for the pure-state quantum channel, we show some of the first concrete examples of codes and laboratory-realizable joint-detection optical receivers that can achieve fundamentally higher (superadditive) channel capacity than receivers that physically detect each modulation symbol one at a time, as is done by all conventional (coherent or direct-detection) optical receivers.