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Quantum Computing, Communication, and Simulation

Twin-field QKD over a deployed optical network enabled by long-range phase locking and semiconductor detectors

Mirko Pittaluga, Yuen San Lo, Adam Brzosko, Robert I. Woodward, Davide Scalcon, Matthew S. Winnel, Thomas Roger, J. F. Dynes, Kim A. Owen, Sergio Juárez, Piotr Rydlichowski, Domenico Vicinanza, Guy F. Roberts, A. J. Shields

Published Mar 19, 2025• 0 citations
Publisher
DOI
10.1117/12.3043348

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Twin-field quantum key distribution (TF QKD) can overcome the fundamental rate loss limit of repeaterless quantum links, but its practical deployment has long been hindered by the requirement of global phase locking between two independent lasers. By revisiting the fundamental principles of optical interference, this work reveals that interference in TF QKD inherently relies only on the instantaneous phase alignment of two independent optical pulses at the moment they temporally overlap, rather than on continuous global phase synchronization. Guided by this insight, we propose and demonstrate a single-sideband-interference TF-QKD protocol that eliminates global phase locking. Each user employs an IQ modulator to generate a weak single sideband as the quantum signal, while the intrinsically phase-correlated optical carrier propagates as a real-time phase reference. Carrier interference at the receiver enables real-time phase extraction and feedback compensation for the sidebands. Unlike prior no phase locking approaches requiring second- or microsecond-level coherence, in principle, our scheme reduces this requirement to nanoseconds. We achieve 98% interference visibility over 100.8 km fibre and secure key rates surpassing the PLOB bound in the high-loss regime, providing a simpler route towards practical long-distance quantum communication networks.
Twin-field (TF) quantum key distribution (QKD) has been proved to overcome the linear rate-distance limit of repeaterless quantum communications and has the capability to generate high key rates over long distances compared with other QKD protocols. Photonic integration has proven a promising technique for the large-scale deployment of QKD. While it has been adopted to implement a range of linearly scaling QKD protocols, there is to date no demonstration of TF QKD using photonic integrated chips. One important challenge resides in establishing high coherence between remote laser chips. In this study, we exploit optical injection locking to implement phase dissemination to independent lasers integrated onto photonic circuits and further demonstrate complete phase control on the same chip using electro-optic phase modulators. We achieve high laser coherence between remote transmitter nodes, which lets us demonstrate a quadratic key-rate improvement of the secure key rate beyond 80 dB of channel attenuation. The experimental results are in excellent agreement with the theory in both finite size and asymptotic regimes, in which the fundamental rate-distance limit is overcome. Our work paves the way for the deployment of large-scale TF QKD networks enabled by compact, low-cost, and high-yield chip-based devices.
Twin-field quantum key distribution (TF-QKD) has emerged as a promising solution for practical quantum communication over long-haul fiber. However, previous demonstrations on TF-QKD require the phase locking technique to coherently control the twin light fields, inevitably complicating the system with extra fiber channels and peripheral hardware. Here, we propose and demonstrate an approach to recover the single-photon interference pattern and realize TF-QKD without phase locking. Our approach separates the communication time into reference frames and quantum frames, where the reference frames serve as a flexible scheme for establishing the global phase reference. To do so, we develop a tailored algorithm based on fast Fourier transform to efficiently reconcile the phase reference via data postprocessing. We demonstrate no-phase-locking TF-QKD from short to long distances over standard optical fibers. At 50-km standard fiber, we produce a high secret key rate (SKR) of 1.27 Mbit/s, while at 504-km standard fiber, we obtain the repeaterlike key rate scaling with a SKR of 34 times higher than the repeaterless secret key capacity. Our work provides a scalable and practical solution to TF-QKD, thus representing an important step towards its wide applications.