Capacity and SKR tradeoff in coexisting classical and CV-QKD metropolitan-reach optical links

We demonstrate power-regime-dependent guardband optimization for quantum-classical coexistence in metropolitan DWDM. Quantum channel at band-edge with 100-150 GHz guardbands achieves 108% SKR improvement at -1.5 dBm/ch, incurring 3.4% capacity loss versus 6.8% for band-center.

💡 Research Summary

This paper addresses the practical challenge of co‑existing continuous‑variable quantum key distribution (CV‑QKD) with classical dense‑wavelength‑division‑multiplexed (DWDM) traffic in metropolitan optical networks. While CV‑QKD is known to tolerate higher noise than discrete‑variable schemes, the strong classical channels inevitably generate nonlinear interference that degrades the quantum signal. The authors focus on two dominant nonlinear effects: spontaneous Raman scattering (SpRS), which scales linearly with launch power and spreads over a very wide spectral range, and four‑wave mixing (FWM), which scales cubically with power and is highly sensitive to the frequency separation between channels.

A comprehensive analytical model, originally presented in reference