Giulia Guarda

We demonstrate an entanglement-based quantum key distribution (QKD) system employing frequency-bin encoding. The entangled state is generated using two independent high-finesse ring resonators fabricated on a silicon photonic chip. The system implements the BBM92 protocol with a passive basis selection scheme and enables simultaneous acquisition of sixteen projective measurements across two mutually unbiased bases. To counteract random phase fluctuations induced by thermal instabilities in the transmission fiber, we apply a real-time adaptive phase correction to the measurement basis. We achieve stable QKD over a 26 km fiber spool with a secure key rate exceeding 4.5 bit/s.

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.