Deployed quantum key distribution network: further, longer and more users

Abstract

Abstract Entanglement-based quantum links are intended to become the backbone of future quantum networks, enabling secure communication between distant cities. Implementing such networks requires addressing multiple practical challenges: time synchronization, interferometric system stabilization, dispersion, and loss management, as well as reliable automation. In this quest for practical QKD systems, we report the continuous exploitation of an operational link for more than 325 hours over 50 km between two remote locations, demonstrating the feasibility of long-duration key generation. We further extend secure key distribution up to a 100 km operational link connecting several campuses of the University Côte d'Azur to an optical ground station, a setup compatible with future quantum satellite connections. Finally, by employing a dense wavelength-division demultiplexing strategy to separate paired photons, we enable QKD across multiple standard channels, achieving secure key exchange via the BBM92 protocol and time-energy observables in a multiple-user scenario.