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

• Published in: Nature photonics Vol. 6; no. 6; pp. 374 - 379
• Main Authors: Chen, Jian, Habif, Jonathan L., Dutton, Zachary, Lazarus, Richard, Guha, Saikat
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.06.2012; Nature Publishing Group
• Subjects: 639/624/1075/187; 639/624/400/482; Applied and Technical Physics; Channels; Coherence; Demodulation; Noise levels; Nulling; Photons; Physics; Physics and Astronomy; Quantum Physics; Receivers
• ISSN: 1749-4885; 1749-4893
• DOI: 10.1038/nphoton.2012.113

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. Researchers experimentally demonstrate the first joint-detection receiver capable of performing a joint measurement over pulse-position-modulation codewords. This result — the largest improvement over the standard quantum limit reported to date — is accomplished by using a conditional nulling receiver, which uses optimized-amplitude coherent pulse nulling, single-photon detection and quantum feedforward.