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Field demonstration of phase-time quantum key distribution over an urban atmospheric link using a multimode delay interferometer
V V Tretiakov (Lomonosov Moscow State University, Moscow State University), Veronika M. Vakhrusheva (Lomonosov Moscow State University, Moscow State University), A V Borisova (Lomonosov Moscow State University, Moscow State University), Andrey N. Klimov (Lomonosov Moscow State University, Moscow State University), Kirill A. Balygin (Lomonosov Moscow State University, Moscow State University), I V Gritsenko, S. P. Kulik (Lomonosov Moscow State University, Moscow State University)
Résumé : Abstract We report the implementation and field testing of an atmospheric quantum key distribution (QKD) system obtained by adapting the commercially available fiber-based ViPNet-QTS platform. The phase-time state-preparation module, detector electronics, and post-processing procedures were retained, whereas the fiber channel was replaced by external optical devices forming a 400 m urban free-space link. The 1550 nm quantum-channel signal was coupled into a multimode fiber (MMF) at the receiver and analyzed by a free-space multimode delay interferometer, allowing the quantum-cryptographic equipment to remain inside trusted zones. Synchronization and bidirectional pointing were implemented using spectrally separated 850 and 780 nm auxiliary channels. We characterized the channel loss, background-count probability, atmospheric turbulence, and performance of the fast and low-bandwidth tracking loops. Although feedback based on fast steering mirrors strongly reduced the fluctuations of the tracking-beam position at the CMOS sensor, it increased the mean power coupled into the MMF by only 4.07% under the tested conditions. Long-term alignment was instead maintained by low-bandwidth feedback to the motorized mounts, demonstrating that continuous high-bandwidth fine tracking was not required for the investigated 400 m link. During an autonomous four-day QKD experiment, the system operated fully unattended, with no manual intervention in alignment, system control, or post-processing. Secret keys were generated over most of the measurement interval under changing day–night and weather conditions. Temporary interruptions in key generation coincided with strong daylight background, whereas the measured turbulence variations had no similarly direct effect on system operation. These results demonstrate a practical approach to extending existing fiber-based QKD platforms to atmospheric links using an MMF-coupled receiver and a comparatively simple tracking architecture.