Curator's Take
AI Commentary
This article shows a major step toward real‑world deployment of free‑space quantum key distribution across cityscapes, achieving an 18.9 km link in dense Barcelona with modest hardware. By using room‑temperature detectors, no adaptive optics and a GPS‑disciplined synchronization scheme, the system delivers over 11 kbit/s of finite‑size secret key rate, demonstrating that high‑rate secure links are possible without exotic components and complementing recent fiber‑based network efforts. The work suggests metropolitan QKD could become a flexible add‑on to existing infrastructure, though its performance still depends on clear atmospheric conditions and line‑of‑sight geometry.
— Mark Eatherly
Summary
Terrestrial free-space quantum key distribution (QKD) provides a flexible complement to fiber-based networks, but its deployment over long horizontal paths in dense metropolitan environments remains challenging, as atmospheric turbulence, demanding pointing and tracking requirements, and operational constraints can substantially increase system complexity. We experimentally demonstrate the operation of a decoy-state BB84 system over an 18.9~km free-space link across a densely populated urban area in the metropolitan region of Barcelona using an aviation-compliant optical architecture. The system operates at 785~nm using room-temperature single-photon detectors and requires no adaptive optics at the receiver. The modular QKD transmitter, based on a deployment-oriented design, is interfaced with a versatile free-space optical terminal and an independent GPS-disciplined synchronization architecture, facilitating its integration into heterogeneous optical infrastructures. We present the transmitter performance and optical-link characteristics, together with a 40-minute field test characterized by an average channel attenuation of approximately 19~dB and a mean QBER of 1.31\%. From the measured channel parameters, we estimate an average finite-size secret key rate exceeding 11~kbit/s. These results demonstrate the feasibility of inter-city free-space QKD over dense urban areas using a comparatively simple architecture.