Coexistence of classical and polarization-encoded QKD signals over deployed fiber

We report the first long-term field test of a polarization-encoding QKD system in C-band coexistence with classical signals over the same standard optical fiber. The system combines a self-stable polarization transmitter with a polarization control unit that stabilizes the link based on quantum metrics. This enables robust QKD operation over a metropolitan fiber link for several days, without requiring knowledge or control of the classical channels. The system can produce positive secret key rates of 422 kbps while coexisting with 0.7 mW classical power along a 50-km deployed link, achieving more than 98% uptime in the quantum channel.