Inter-Campus Converged Fiber/Free-Space C-Band QKD Link During Daylight

Abstract

We demonstrate a practical inter-campus Quantum Key Distribution (QKD) implementation utilizing C-band time-bin/phase encoding over a converged 750 m urban Free-Space Optical (FSO) and fiber link. To address the challenges of real-world deployment, the system was operated during daylight hours and evaluated in a coexistence scenario with an intense C-band classical laser signal. High spectral isolation was achieved through a multi-stage passive filtering architecture, enabling stable operation. We report a secure key rate (SKR) of 2.74 kbps in the absence of classical traffic and 0.377 kbps during quantum-classical coexistence, with Quantum Bit Error Rate (QBER) values maintained below 3%. The experimental results align closely with decoy-state BB84 theoretical models, validating the feasibility of integrating time-bin/phase QKD systems into converged fiber/free-space, multi-domain metropolitan networks.