Secret Key Rate Limits in Coexisting Classical-Quantum Optical Links

TL;DR

This paper develops a model to evaluate interference in coexisting classical and quantum optical links, revealing optimal frequency placement for maximizing secret key rates in fiber-optic quantum communication systems.

Contribution

It introduces closed-form expressions for interference power evaluation and demonstrates improved secret key rates by strategic frequency allocation in classical-quantum coexistence.

Findings

Quantum channels perform better in the upper E-/lower S-band.

The model accurately captures interference evolution from physical phenomena.

Frequency allocation impacts secret key rates significantly.

Abstract

Classical-quantum coexistence enables cost-effective transmission of data and quantum signals over the same fiber-optic channel. Nevertheless, weak quantum-key distribution (QKD) signals are susceptible to non-linear interference generated from the classical traffic, primarily spontaneous Raman scattering (SpRS) and four-wave-mixing (FWM), as well as to unfiltered noise. In QKD protocols, increased channel loss and excess noise both reduce the secret key rates (SKRs), as illustrated in this work for the two-state BB84 and Gaussian-modulated coherent-states (GMCS) protocols. In this study, we derive closed-form expressions for evaluating the accumulated interference power from coexisting classical signals in a quantum frequency channel. Our model enables effective design of classical-quantum systems in single-mode fibers (SMFs), capturing the evolution of interference arising from the relevant physical phenomena. We utilize the model to examine frequency allocation in multiband transmission systems, demonstrating that, contrary to common practice of allocating QKD channels in the O-band, increased SKR is achieved by placing quantum channels in the upper E-/lower S-band across the relevant scenarios.