  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 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.
