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Belief propagation with quantum messages for quantum-enhanced classical communications

Publication ,  Journal Article
Rengaswamy, N; Seshadreesan, KP; Guha, S; Pfister, HD
Published in: npj Quantum Information
December 1, 2021

For space-based laser communications, when the mean photon number per received optical pulse is much smaller than one, there is a large gap between communications capacity achievable with a receiver that performs individual pulse-by-pulse detection, and the quantum-optimal “joint-detection receiver” that acts collectively on long codeword-blocks of modulated pulses; an effect often termed “superadditive capacity”. In this paper, we consider the simplest scenario where a large superadditive capacity is known: a pure-loss channel with a coherent-state binary phase-shift keyed (BPSK) modulation. The two BPSK states can be mapped conceptually to two non-orthogonal states of a qubit, described by an inner product that is a function of the mean photon number per pulse. Using this map, we derive an explicit construction of the quantum circuit of a joint-detection receiver based on a recent idea of “belief-propagation with quantum messages” (BPQM). We quantify its performance improvement over the Dolinar receiver that performs optimal pulse-by-pulse detection, which represents the best “classical” approach. We analyze the scheme rigorously and show that it achieves the quantum limit of minimum average error probability in discriminating 8 (BPSK) codewords of a length-5 binary linear code with a tree factor graph. Our result suggests that a BPQM receiver might attain the Holevo capacity of this BPSK-modulated pure-loss channel. Moreover, our receiver circuit provides an alternative proposal for a quantum supremacy experiment, targeted at a specific application that can potentially be implemented on a small, special-purpose, photonic quantum computer capable of performing cat-basis universal qubit logic.

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Published In

npj Quantum Information

DOI

EISSN

2056-6387

Publication Date

December 1, 2021

Volume

7

Issue

1

Related Subject Headings

  • 5108 Quantum physics
  • 4902 Mathematical physics
  • 4613 Theory of computation
 

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Rengaswamy, N., Seshadreesan, K. P., Guha, S., & Pfister, H. D. (2021). Belief propagation with quantum messages for quantum-enhanced classical communications. Npj Quantum Information, 7(1). https://doi.org/10.1038/s41534-021-00422-1
Rengaswamy, N., K. P. Seshadreesan, S. Guha, and H. D. Pfister. “Belief propagation with quantum messages for quantum-enhanced classical communications.” Npj Quantum Information 7, no. 1 (December 1, 2021). https://doi.org/10.1038/s41534-021-00422-1.
Rengaswamy N, Seshadreesan KP, Guha S, Pfister HD. Belief propagation with quantum messages for quantum-enhanced classical communications. npj Quantum Information. 2021 Dec 1;7(1).
Rengaswamy, N., et al. “Belief propagation with quantum messages for quantum-enhanced classical communications.” Npj Quantum Information, vol. 7, no. 1, Dec. 2021. Scopus, doi:10.1038/s41534-021-00422-1.
Rengaswamy N, Seshadreesan KP, Guha S, Pfister HD. Belief propagation with quantum messages for quantum-enhanced classical communications. npj Quantum Information. 2021 Dec 1;7(1).

Published In

npj Quantum Information

DOI

EISSN

2056-6387

Publication Date

December 1, 2021

Volume

7

Issue

1

Related Subject Headings

  • 5108 Quantum physics
  • 4902 Mathematical physics
  • 4613 Theory of computation