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Quantum Quadrature Amplitude Modulation with Photon-Added Gaussian States

Andrea Giani, Moe Z. Win, Andrea Conti

Published Dec 4, 2023• 2 citations
Publisher

Abstract

Quantum communication systems exchange information between a source and a destination by encoding such information into quantum states according to modulation techniques. The design of quantum modulation techniques, including quantum quadrature amplitude modulation (QAM), requires choosing and characterizing quantum states. This paper explores the use of non-Gaussian photon-added Gaussian states (PAGSs) for quantum QAM communications. First, we characterize pure PAGSs in the Fock space and derive a closed-form expression for their inner product. Then, we develop a quantum QAM technique employing PAGSs. In particular, we show how to construct quantum QAM constellations of PAGSs. Finally, we evaluate the performance of the developed quantum QAM technique with PAGSs, when a square root measurement (SRM) receiver is employed, and compare such performance with that of existing coherent states. Results show that PAGSs can provide a performance gain with respect to coherent states.

DOI
10.1109/globecom54140.2023.10437870

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Quantum communication systems exchange information between a source and a destination by encoding such information into quantum states according to modulation techniques. The design of quantum modulation techniques, including quantum quadrature amplitude modulation (QAM), requires choosing and characterizing quantum states. This paper explores the use of non-Gaussian photon-added Gaussian states (PAGSs) for quantum QAM communications. First, we characterize pure PAGSs in the Fock space and derive a closed-form expression for their inner product. Then, we develop a quantum QAM technique employing PAGSs. In particular, we show how to construct quantum QAM constellations of PAGSs. Finally, we evaluate the performance of the developed quantum QAM technique with PAGSs, when a square root measurement (SRM) receiver is employed, and compare such performance with that of existing coherent states. Results show that PAGSs can provide a performance gain with respect to coherent states.
The quadrature amplitude modulation (QAM) signal of coherent state of light is applied to the quantum stream cipher by Y-00 protocol. We first discuss on the performance of the square-root measurement (SRM) for the QAM signals in comparison with the optimum receiver. It is shown that the quantum stream cipher with the QAM signals is designed by using the SRM, taking account of the ciphertext-only attack and the known/chosen plain attack. Furthermore, the modification of the quantum stream cipher with the QAM signals is considered.
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Quantum sensing and communication (QSC) is pivotal for developing next-generation networks with unprecedented performance. Many implementations of existing QSC systems employ Gaussian states as they can be easily realized using current technologies. However, Gaussian states lack non-classical properties necessary to unleash the full potential of QSC. This motivates the use of non-Gaussian states, which have non-classical properties beneficial for QSC. This paper establishes a theoretical foundation for QSC employing photon-varied Gaussian states (PVGSs). The PVGSs are non-Gaussian states that can be generated from Gaussian states using current technologies. First, we derive a closed-form expression for the generalized bilinear generating function of ordinary Hermite polynomials and show how it can be used to describe PVGSs. Then, we characterize PVGSs by deriving their Fock representation and their inner product. We also determine equivalence conditions for Gaussian states obtained from arbitrary permutations of rotation, displacement, and squeezing operators. Finally, we explore the use of PVGSs for QSC in several case studies.
We present a quantum receiver for quadrature amplitude modulation (QAM) coherent-state discrimination with the homodyne-displacement hybrid structure. Our strategy is to carry out two successive measurements on parts of the quantum states. The homodyne result of the first measurement reveals partial information about the state and is forwarded to a displacement receiver, which finally identifies the input state by using feedback to adjust a reference field. Theoretical analysis and numerical simulation results show that for 16-QAM, the hybrid receiver could outperform the standard quantum limit using only on-off single-photon detectors with a reduced number of signal-state partitions, showing great potential toward implementing the receiver practically.
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