Tianqi Dou

Quantum key distribution (QKD) ensures information-theoretic security and remains resilient against future quantum attacks. Hollow-core fiber (HCF), with its ultra-low loss, low nonlinearity, and low latency, offers a promising medium for the coexistence of QKD and classical communications. While prior studies have demonstrated such coexistence over short HCF links, challenges remain in scaling to longer distances due to noise and attenuation. In this study, we experimentally investigate the coexistence of commercial QKD and classical signals over a 20 km HCF for the first time. We evaluate noise photon levels and analyze the wavelength dependence of QKD performance under varying classical power conditions. Results show that spontaneous Raman scattering (SpRS) remains a significant source of noise, and increased classical power degrades the secure key rate. However, classical wavelength variations have minimal effect on QKD performance. These findings provide insights into the feasibility and limitations of long-distance QKD over HCF, guiding future integration with classical networks.