Lucas A. Zischler

We analyse the secret-key rate (SKR) of continuous-variable quantum key distribution (CV-QKD) when the quantum channel shares a fibre with classical DWDM traffic. Using a physical model of an 88-channel, 50 GHz-spaced C-band link that captures four-wave mixing, spontaneous Raman scattering, Rayleigh backscatter, and out-of-band effects with full frequency dependence, we compute the asymptotic SKR under Gaussian collective attacks across guardband size, quantum-channel position, classical launch power, link distance, and propagation direction. We report three results. First, placing the quantum channel at the band edge with a 150 GHz guardband yields a 108% SKR gain at 3.4% classical capacity loss, half the cost of symmetric centre-band protection. Second, counter-propagation without any guardband produces the same SKR as guardbanded co-propagation at the same operating point. Third, a power threshold near -2 dBm/ch separates two operating regimes: below threshold the link is Raman-limited and the SKR is essentially insensitive to the guardband, while above threshold four-wave mixing dominates and the SKR depends sharply on guardband size and band-edge proximity. These rules apply across the operating regimes typical of installed DWDM metropolitan fibre.

Ensuring information privacy in modern communication systems has become increasingly critical. Quantum key distribution (QKD), leveraging the principles of quantum mechanics, provides information-theoretically secure key sharing and has matured into the most advanced quantum communication application. Despite successful demonstrations and emerging commercial deployments, the widespread adoption of QKD is hindered by the high cost of building dedicated quantum networks. A promising and cost-effective alternative is the integration of QKD into classical fiber-optic infrastructure, particularly using standard single-mode fibers. However, this approach is limited by noise and nonlinear effects such as spontaneous Raman scattering. Recent advancements in space-division multiplexing (SDM) have led to the development of uncoupled-core multi-core fibers (MCFs), which offer spatial separation between quantum and classical signals, mitigating interference. While previous QKD-MCF coexistence studies have been restricted to lab environments and non-standard large-diameter fibers, we demonstrate, for the first time, the coexistence of QKD and classical communication channels, in a realistic field-deployed scenario. One of the cores was dedicated to QKD and the other cores to classical transmission. The system was tested with 110-Tb/s traffic over 25.2 km of field-deployed MCF with a 125-µm cladding. Our results mark a significant step forward in integrating QKD with classical communication based on uncoupled-core MCF technology.