Coexistence of continuous-variable quantum key distribution and classical data over 120-km fiber

Abstract: Integrating quantum key distribution (QKD) with classical data transmission over the same fiber is crucial for scalable quantum-secured communication. However, noise from classical channels limits QKD distance. We demonstrate the longest-distance continuous-variable QKD (CV-QKD) over 120 km (20 dB loss) coexisting with a fully populated coarse wavelength-division multiplexing (CWDM) system. Natural mode filtering of the local oscillator and phase noise mitigation enabled this without additional filtering or wavelength reallocation. Benchmarking against a commercial discrete-variable QKD system and considering finite-size effects confirms the feasibility of CV-QKD as a plug-and-play solution for typical 80--100 km long-haul optical networks. Our results set a record distance for CV-QKD, showing its potential for cost-effective, large-scale deployment in existing network infrastructure.