Barcelona test shows 18.9km quantum key exchange over a city

Urban Free‑Space QKD Across 18.9 km

A team from the Institut de Ciències Fotòniques (ICFO) in Barcelona has shown that quantum key distribution (QKD) can be performed over a nearly 19‑kilometre free‑space link inside a densely populated city. The experiment used a decoy‑state BB84 protocol at 785 nm, a modular transmitter, and room‑temperature silicon single‑photon detectors, avoiding the need for adaptive optics or cryogenic cooling.

Simple, Aviation‑Compliant Architecture

The transmitter was mounted on a commercial research platform from Quantum Technology Laboratories GmbH and adapted to meet the optical‑emission restrictions of Barcelona‑El Prat Airport. A GPS‑disciplined timing system kept the transmitter and receiver synchronized without requiring hardware changes to the free‑space terminal. The receiver, located on the Collserola Tower, was linked to existing fiber infrastructure, illustrating a hybrid approach that could combine fiber and free‑space links.

Performance in a Real‑World Urban Environment

• Attenuation: ~19 dB (range 16–22 dB across campaigns)
• Quantum Bit Error Rate (QBER): 1.31 % average (stable 1–3 % over three months)
• Secret Key Rate: >11 kbit/s finite‑size secure key, based on 2 million sifted detections per block and 116 s acquisition time.

These figures were obtained during a 40‑minute field test and reproduced over multiple measurement campaigns, indicating robustness against atmospheric turbulence and pointing errors.

Implications for Future Quantum Networks

The demonstration shows that a relatively simple, modular system can deliver secure keys over inter‑city distances without relying on extensive fiber deployment. By using room‑temperature detectors and an aviation‑safe optical design, the approach lowers cost and complexity, making it attractive for scaling to larger metropolitan or regional quantum networks. Ongoing work will explore continuous, unattended operation and daytime performance, further paving the way for practical quantum‑secure communications.

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The underlying pre‑print is available at the arXiv link below; data can be requested from the authors.