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

2012; Nature Portfolio; Volume: 6; Issue: 6 Linguagem: Inglês

10.1038/nphoton.2012.113

1749-4893

Jian Chen, Jonathan L. Habif, Zachary Dutton, Richard S. Lazarus, Saikat Guha,

Neural Networks and Reservoir Computing

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.