Francesco Picciariello

Intermodal quantum key distribution enables the integration of fiber networks and free-space channels, essential components for developing a global quantum network. We conducted a field trial of an intermodal quantum key distribution system, featuring two polarization-based transmitters and a single receiver. In this trial, the active channel was alternately switched between a 620-meter free-space link and a 17-kilometer deployed fiber in the metropolitan area of Padova. The free-space channel's performance was assessed in relation to atmospheric turbulence strength. The field trial, conducted over several hours in daylight, demonstrated the intermodal functionality between fiber and free-space channels. Our switching system offers a cost-effective solution for a trusted quantum key distribution network, minimizing the number of necessary devices across different network topologies.

We describe a QKD field trial running on urban fibers deployed in Padua, Italy. This is the first validation outside of the laboratory environment of a new low-error and calibration-free polarization encoder, called iPOGNAC, which we also present here. Our system is resource- and cost-effective, and can be installed quickly in an existing fiber network.