Pietro Centorrino

Free space quantum key distribution (QKD) has now achieved a groundbreaking advancement in secure communication, enabling long-distance private key exchange and ensuring unbreakable encryption. However, complete compatibility between fiber and free-space infrastructures remains a challenge for a fully integrated QKD system. Indeed, free space and fiber-based QKD commonly utilize different wavelengths and qubit encoding schemes that optimize photon transmission in their respective channels. Free-space QKD state generators usually employ visible light due to their lower beam divergence compared to longer wavelengths and polarization encoding for their resilience against turbulence. In contrast, fiber-based QKD primarily utilizes the C-band, which exhibits the lowest losses in silica fibers, and employs time-bin encoding to mitigate the effects of polarization instability in optical fibers. In our field trial, we demonstrate the viability of performinging QKD from a remote sender (Alice) to a fiber-based receiver (Bob) using the same signal without any wavelength or encoding conversion. We employ a time-bin encoded QKD protocol operating in the C-band through horizontally turbulent free-space channels and a pre-existing dark fiber infrastructure. We tested the setup over 50 m and 500 m free space long links, reaching an average secure key rate of 793 kbps and 40 kbps during several hours of measurement. The results put a step forward the interoperability between free-space and fiber-based infrastructures, opening new possibilities for connecting terminal users with satellites in hybrid systems.