Optical codeword demodulation with error rates below standard quantum limit using a conditional nulling receiver

TL;DR

This paper demonstrates the first experimental joint-detection receiver for optical communication that surpasses the standard quantum limit, achieving lower error rates using a conditional nulling approach with practical implications for deep-space communication.

Contribution

The paper presents the first experimental implementation of a joint-detection receiver that surpasses the standard quantum limit using a conditional nulling method with optimized coherent pulse nulling and quantum feedforward.

Findings

Error rate 40% below standard quantum limit

Demonstration of a practical joint-detection receiver

Potential for improved deep-space communication systems

Abstract

The quantum states of two laser pulses---coherent states---are never mutually orthogonal, making perfect discrimination impossible. Even so, coherent states can achieve the ultimate quantum limit for capacity of a classical channel, the Holevo capacity. Attaining this requires the receiver to make joint-detection measurements on long codeword blocks, optical implementations of which remain unknown. We report the first experimental demonstration of a joint-detection receiver, demodulating quaternary pulse-position-modulation (PPM) codewords at a word error rate of up to 40% (2.2 dB) below that attained with direct-detection, the largest error-rate improvement over the standard quantum limit reported to date. This is accomplished with a conditional nulling receiver, which uses optimized-amplitude coherent pulse nulling, single photon detection and quantum feedforward. We further show how this translates into coding complexity improvements for practical PPM systems, such as in deep-space communication. We anticipate our experiment to motivate future work towards building Holevo-capacity-achieving joint-detection receivers.