Time-bin QKD free space field-trial link in the third telecommunication window

We demonstrate the feasibility of time-bin encoding in the third telecommunication window within the frame of Quantum Key Distribution (QKD) protocol in Free Space Optical (FSO) horizontal links. Operating at a 0.6 GHz repetition rate, the QKD transmitter delivers time-bin qubits at 1558.98nm over a turbulent channel linking two nearby buildings in the city of Florence. To mitigate those effects, the receiver mounts a tip/tilt adaptive system which proves a better Free Space (FS) to fiber coupling ratio stability with respect to no active control. Furthermore, we show that the Photonic Integrated Circuit (PIC) unbalanced Mach-Zender Interferometer (uMZI) in the quantum detection scheme, combined with Superconducting Nano-Wire Single Photon Detectors (SNSPD), guarantees long stability and an high Secret Key Rate (SKR) in the order of the thousands of kilobits per second (kbps).