Sebastiano Cocchi

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.

Ensuring information privacy in modern communication systems has become increasingly critical. Quantum key distribution (QKD), leveraging the principles of quantum mechanics, provides information-theoretically secure key sharing and has matured into the most advanced quantum communication application. Despite successful demonstrations and emerging commercial deployments, the widespread adoption of QKD is hindered by the high cost of building dedicated quantum networks. A promising and cost-effective alternative is the integration of QKD into classical fiber-optic infrastructure, particularly using standard single-mode fibers. However, this approach is limited by noise and nonlinear effects such as spontaneous Raman scattering. Recent advancements in space-division multiplexing (SDM) have led to the development of uncoupled-core multi-core fibers (MCFs), which offer spatial separation between quantum and classical signals, mitigating interference. While previous QKD-MCF coexistence studies have been restricted to lab environments and non-standard large-diameter fibers, we demonstrate, for the first time, the coexistence of QKD and classical communication channels, in a realistic field-deployed scenario. One of the cores was dedicated to QKD and the other cores to classical transmission. The system was tested with 110-Tb/s traffic over 25.2 km of field-deployed MCF with a 125-µm cladding. Our results mark a significant step forward in integrating QKD with classical communication based on uncoupled-core MCF technology.