QKD and frequency distribution cooperation: the Twin-field QKD case

The integration of Quantum Key Distribution (QKD) protocols in metropolitan networks, exploiting already deployed telecommunication fibers, requires particular attention due to the non-ideal environment in which the protocol operates. At present, Twin-field QKD is one of the most promising techniques to reach the extension of QKD transmission but its real-word exploitation requires that the communication channel is stable in terms of optical length and is free from background light that increase the errors in the transmitted keys. Adapting interferometry techniques derived from frequency metrology, we recently proposed a solution [1] for a tight channel length control, and demonstrated it on a 206 km metropolitan fiber with 65 dB loss. Here we analyze in details the effects of background photons and the solutions that we applied to reduce their contribution to a negligible level.