Realization of intermodal fiber/free-space quantum key distribution networks

Abstract

Intermodal quantum key distribution (IM-QKD) enables the integration of fiber networks and free-space connections, which can be ground-to-ground links or involve satellite nodes in orbit. IM-QKD permits to extend the reach of free-space links without trusting any additional node, but this requires to efficiently couple the free-space signal into a single-mode fiber (SMF). We present the implementation of different IM-QKD networks realized in Padova and Vienna, exploiting km-long deployed fibers and free-space channels up to 620 meters. We show that such an intermodal scheme is compatible with both in-house QKD systems and commercially available solutions exploiting polarization encoding at 1550 nm. Remarkably, we realized different QKD tests in daylight and also in rainy conditions.