C-Band Coexistence Demonstration of Polarization-Based QKD and Classical Signals across a 50-km Deployed Fiber Link

This study introduces an advanced discrete-variable (DV) polarization-state quantum key distribution (QKD) transmitter tailored for the BB84 protocol with decoy states at 1550 nm. Demonstrating an intrinsic quantum bit error rate (QBER) of 0.15 % at a 615 MHz repetition rate and 0.34 % across a 50-km standard single-mode fiber link, our implementation marks a significant stride towards practical quantum-secured communication. The system's reliance on commercial off-the-shelf (COTS) components and low-voltage operation, coupled with a self-stable design, ensures resilience against environmental perturbations. This research pioneers the evaluation of a polarization-state QKD transmitter's performance in scenarios that involve copropagation with high classical powers within the telecom C Band on shared optical links, observing the impact of spontaneous Raman scattering on the quantum states.