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The quantum states of two laser pulses---coherent states---are never mutuallyorthogonal, making perfect discrimination impossible. Even so, coherent statescan achieve the ultimate quantum limit for capacity of a classical channel, theHolevo capacity. Attaining this requires the receiver to make joint-detectionmeasurements on long codeword blocks, optical implementations of which remainunknown. We report the first experimental demonstration of a joint-detectionreceiver, demodulating quaternary pulse-position-modulation (PPM) codewords ata word error rate of up to 40% (2.2 dB) below that attained withdirect-detection, the largest error-rate improvement over the standard quantumlimit reported to date. This is accomplished with a conditional nullingreceiver, which uses optimized-amplitude coherent pulse nulling, single photondetection and quantum feedforward. We further show how this translates intocoding complexity improvements for practical PPM systems, such as in deep-spacecommunication. We anticipate our experiment to motivate future work towardsbuilding Holevo-capacity-achieving joint-detection receivers.

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