Stable polarization entanglement based quantum key distribution over a deployed metropolitan fiber

We demonstrate a quantum key distribution (QKD) implementation over deployed dark telecom fibers with polarization-entangled photons generated at the O-band. One of the photons in the pairs is propagated through 10 km of deployed fiber, while the others are detected locally. Polarization drifts experienced by the photons propagating through the fibers are compensated with liquid crystal variable retarders. This ensures continuous and stable QKD operation with an average quantum bit error rate of 6.4% and a final key rate of 109 bits/s.