Kadir Gumus

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.