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

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. Here, we report the first experimental demonstration of a joint-detection receiver, demodulating quaternary pulse-position-modulation 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 pulse-position-modulation systems, such as in deep-space communication. We anticipate our experiment to motivate future work towards building Holevo-capacity-achieving joint-detection receivers.