Intermodal quantum key distribution field trial with active switching between fiber and free-space channels

Intermodal quantum key distribution enables the full interoperability of fiber networks and free-space channels, which are both necessary elements for the development of a global quantum network. We present a field trial of an intermodal quantum key distribution system in a simple 3-node heterogeneous quantum network - comprised of two polarization-based transmitters and a single receiver - in which the active channel is alternately switched between a free-space link of 620 m and a 17km-long deployed fiber in the metropolitan area of Padova. The performance of the free-space channel is evaluated against the atmospheric turbulence strength of the link. The field trial lasted for several hours in daylight conditions, attesting the interoperability between fiber and free-space channels, with a secret key rate of the order of kbps for both the channels. The QKD hardware and software require no different strategies to work over the two channels, even if the intrinsic characteristics of the links are clearly different. The switching system represents a cost-effective solution for a trusted quantum key distribution network, reducing the number of necessary devices in different network topologies.