Barcelona test shows 18.9km quantum key exchange over a city

Researchers at ICFO-Institut de Ciències Fotòniques in Barcelona have successfully demonstrated quantum key distribution over a free-space link spanning 18.9 kilometers within a densely populated urban area. The system, operating at 785 nm, achieved this feat without relying on adaptive optics at the receiver, simplifying the technology for potential wider use. This demonstration establishes the feasibility of inter-city free-space QKD using a comparatively simple architecture, and the team estimates an average finite-size secret key rate exceeding 11 kbit/s. BB84 QKD Achieves 18.9km Range Across Barcelona Population densities along the 18.9 kilometer free-space link ranged from approximately 1,500 to 13,000 inhabitants per square kilometer, demonstrating quantum key distribution in a realistically crowded environment. The experimental link connected Castelldefels and Barcelona, operating entirely within the optical-emission restriction zone of Barcelona-El Prat Airport, a constraint addressed by the system’s aviation-compliant optical architecture. 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. “These capabilities are not always simultaneously available in most reported field implementations,” the researchers note in their report. Collserola Tower, a major telecommunications hub in Barcelona, hosted the receiver node, providing direct fiber connectivity to critical infrastructure. This integration illustrates a pathway for building hybrid fiber-free-space quantum networks using trusted nodes. The free-space infrastructure itself was based on a commercial research platform developed by Quantum Technology Laboratories GmbH (QTLabs), adapted to meet aviation-safety constraints and operate within the 785-810 nm wavelength range. All authors contributed to the installation, preliminary testing and calibration of the deployed platform, ensuring a comprehensive evaluation of the system’s performance in a real-world setting.

The team reports that data underlying the results are available upon reasonable request. Decoy-State Protocol Enables Urban QKD Transmission The Barcelona demonstration employed a decoy-state BB84 protocol, enabling secure key exchange over an 18.9 kilometer free-space link within a metropolitan area. This hardware implementation allows for flexible adjustment of these probabilities, optimizing performance under varying conditions. The measured channel parameters enabled estimation of the secret key rate (SKR) using a standard decoy-state BB84 protocol with two decoy levels. Polarization and intensity selection probabilities mirrored those used for stability characterization. While a dedicated long-term statistical characterization of the channel remains a future research direction, initial measurements indicate a stable and reliable quantum link. Aviation-Compliant Optics for Free-Space Deployment This design prioritizes deployment flexibility, a key consideration for extending quantum communication beyond fixed installations. The architecture allows the quantum payload to connect to the free-space optical terminal without imposing strict hardware or operational demands, simplifying integration processes. This adaptability helps build hybrid quantum networks that combine the strengths of both fiber and free-space links. Beyond its modularity, the demonstrated system employed an aviation-compliant optical architecture, a feature designed to broaden potential deployment scenarios. The use of room-temperature single-photon detectors further simplifies the technology, potentially reducing costs and easing the burden on system cooling requirements. 19 dB Attenuation and 1.31% QBER Performance The system achieved 19 dB attenuation and 1.31% quantum bit error rate (QBER), figures indicating a robust link for quantum key distribution (QKD) in a complex urban environment. These measurements, taken over an 18.9 km free-space link, demonstrate performance consistency despite the challenges of atmospheric turbulence and signal loss inherent in city-wide deployments. The system’s ability to maintain a stable QBER at this attenuation level helps establish secure communication channels without relying on traditional fiber optic infrastructure. Detection gains for signal, decoy, and vacuum states were also evaluated, providing a comprehensive assessment of the system’s performance under varying conditions. This granular level of analysis is essential for optimizing key generation rates and ensuring long-term security. Measurements taken over a three-month period showed that the system consistently operated within a QBER range of 1% to 3% with attenuation fluctuating between 16 and 22 dB. This reproducibility, observed across multiple campaigns and varying atmospheric conditions, suggests the architecture is resilient to real-world environmental factors. Further long-term studies will focus on attenuation statistics over extended periods, the correlation between residual pointing errors and QBER, and continuous, unattended operation, including daytime performance. GPS Synchronization Supports Optical Infrastructure The demonstrated free-space quantum key distribution (QKD) system in Barcelona relied on precise timing maintained by an independent, GPS-disciplined synchronization architecture, a feature that eases integration with existing optical networks. This architecture allows the QKD transmitter to interface smoothly with versatile free-space optical terminals, moving beyond isolated quantum systems toward hybrid infrastructures. Unlike some field implementations, the system’s design did not require modification of terminal hardware when the quantum payload was connected, streamlining deployment. A 40-minute field test characterized by an average channel attenuation of approximately 19 dB and a mean QBER of 1.31% was performed. Room-Temperature Detectors Simplify QKD System The Barcelona demonstration used room-temperature silicon single-photon detectors, a departure from many field implementations relying on cryogenic or wavelength-conversion systems. This choice notably reduces system complexity and eases deployment, especially in locations with strict optical emission regulations, by removing the need for bulky cooling apparatus or auxiliary wavelength conversion stages. This combination allows for a comparatively streamlined architecture suitable for wider adoption beyond controlled laboratory settings. A key element of the successful 18.9 kilometer link was a two-stage deployment strategy designed to minimize risk and ensure a stable optical path. Initially, telescopes, mounts, and LED-based pointing systems were installed and the classical light channel characterized, confirming acceptable link attenuation before adding the more sensitive quantum components. 11 kbit/s Secret Key Rate Demonstrated The system’s ability to maintain secure communication at this speed over 18.9 km represents a step toward practical, long-distance quantum networks without reliance on traditional fiber optic cables. Each block was completed once the target of 2 million sifted detections was reached, with an average acquisition time of 116.4 seconds per block. “The average acquisition time per block was 116.4~s, yielding an average finite-size secure key rate of 11.17~kbit s–1,” the researchers report. This fluctuating rate, while averaging above 11 kbit/s, highlights the dynamic nature of free-space quantum communication and the need for robust error correction and key distillation techniques. Free-Space QKD Complements Fiber Networks This approach moves beyond reliance on dedicated point-to-point links, which can be costly to deploy, particularly where existing infrastructure is limited, and vulnerable to physical disruption. The system’s optical configuration incorporates a collimation stage adjacent to a tracking telescope, facilitating co-alignment of quantum and tracking channels, and a low-insertion-loss optical multiplexer combining beacon signals with the quantum signal itself. This configuration allows for a practical free-space QKD receiver based on high-efficiency, room-temperature silicon single-photon detectors, a key element in reducing system complexity and cost. “The FS infrastructure is based on a commercial research platform for FS quantum communication,” the team notes, highlighting the potential for scalability and wider adoption of this technology. The successful demonstration of this inter-city link establishes a foundation for hybrid fiber-free-space quantum networks, offering increased resilience and flexibility in securing communications. Source: https://arxiv.org/html/2611.16774v1 More like thisQuantum CryptographyNtt Research Builds Virtual Quantum Computer from Untrusted ServersQuantum CryptographySealsq highlights post-quantum security at BMO conferenceQuantum Computing Business NewsLumen invests in quantum networking research at Ohio StateQuantum NetworkingQunnect launches US open-access quantum network in New MexicoStay currentSee today’s quantum computing news on Quantum Zeitgeist for the latest breakthroughs in qubits, hardware, algorithms, and industry deals. Tags: