Energy-constrained two-way capacity bounds for noisy Gaussian channels

Abstract:A gap between achievable rates and converse bounds persists for energy-constrained bosonic communication in the presence of excess noise. We derive a unified adaptive weak-converse bound for thermal attenuation, noisy amplification, and additive Gaussian noise under an unconditional mean transmitted-photon-number constraint. The bound applies to arbitrary adaptive protocols with quantum memories and constrains the two-way quantum, entanglement-distribution, private, and secret-key capacities. It vanishes throughout the entanglement-breaking region and approaches the corresponding PLOB bound at infinite energy. At finite energy, it improves the evaluated Gaussian squashed-entanglement and PLOB bounds in relevant parameter regimes, while comparison with hashing rates quantifies the remaining gap to achievability. The result provides a common energy-dependent benchmark for noisy bosonic Gaussian channels and extends finite-energy converse methods beyond the quantum-limited setting.

Quantum Physics (quant-ph); Mathematical Physics (math-ph); Optics (physics.optics)

arXiv:2610.08786 [quant-ph]

(or arXiv:2610.08786v1 [quant-ph] for this version)

https://doi.org/10.48550/arXiv.2610.08786 Focus to learn more arXiv-issued DOI via DataCite (pending registration)

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