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.
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This work was funded by the Defense Advanced Research Projects Agency (DARPA) Information in a Photon (InPho) programme (contract no. HR0011-10-C-0159). The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the DARPA or the US government. The authors thank M. Takeoka, National Institute of Information and Communications Technology, Japan, for useful discussions.
The authors declare no competing financial interests.
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Chen, J., Habif, J., Dutton, Z. et al. Optical codeword demodulation with error rates below the standard quantum limit using a conditional nulling receiver. Nature Photon 6, 374–379 (2012) doi:10.1038/nphoton.2012.113
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