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Physical Review Applied

Independent-optical-frequency-comb-powered 546-km field test of twin-field quantum key distribution

Lai Zhou, Jinping Lin, Chengfang Ge, Yuanbin Fan, Zhiliang L. Yuan, Hao Dong, Yang Liu, Di Ma, Jiu-Peng Chen, Cong Jiang, Xiang‐Bin Wang, Lixing You, Qiang Zhang, Jian-Wei Pan

Published Dec 16, 2024• 15 citations
Publisher

Abstract

Twin-field quantum key distribution exploits single-photon interference to provide long-haul secure communication. However, real-time phase tracking between optical signals sent from opposite ends of the communication link is necessary to realize this scheme. Here the authors show that it is feasible to apply independent optical frequency combs in a quantum communication field trial, using a coherent dual-band stabilization technique. A practical finite-size-secure-key rate of 0.53 bit/s is recorded over a 546-km link connecting two major cities in China. This study represents a significant setup toward the integration of long-distance fiber links into quantum networks.

DOI
10.1103/physrevapplied.22.064057

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Twin-field quantum key distribution exploits single-photon interference to provide long-haul secure communication. However, real-time phase tracking between optical signals sent from opposite ends of the communication link is necessary to realize this scheme. Here the authors show that it is feasible to apply independent optical frequency combs in a quantum communication field trial, using a coherent dual-band stabilization technique. A practical finite-size-secure-key rate of 0.53 bit/s is recorded over a 546-km link connecting two major cities in China. This study represents a significant setup toward the integration of long-distance fiber links into quantum networks.
Owing to its repeater-like rate-loss scaling, twin-field quantum key distribution (TF-QKD) has repeatedly exhibited in laboratory its superiority for secure communication over record fiber lengths. Field trials pose a new set of challenges however, which must be addressed before the technology's roll-out into real-world. Here, we verify in field the viability of using independent optical frequency combs -- installed at sites separated by a straight-line distance of 300~km -- to achieve a versatile TF-QKD setup that has no need for optical frequency dissemination and thus enables an open and network-friendly fiber configuration. Over 546 and 603 km symmetric links, we record a finite-size secure key rate (SKR) of 0.53~bit/s and an asymptotic SKR of 0.12 bit/s, respectively. Of practical importance, the setup is demonstrated to support 44~km fiber asymmetry in the 452 km link. Our work marks an important step towards incorporation of long-haul fiber links into large quantum networks.
Twin-field (TF) quantum key distribution (QKD) has rapidly risen as the most viable solution to long-distance secure fibre communication thanks to its fundamentally repeater-like rate-loss scaling. However, its implementation complexity, if not successfully addressed, could impede or even prevent its advance into real-world. To satisfy its requirement for twin-field coherence, all present setups adopted essentially a gigantic, resource-inefficient interferometer structure that lacks scalability that mature QKD systems provide with simplex quantum links. Here we introduce a technique that can stabilise an open channel without using a closed interferometer and has general applicability to phase-sensitive quantum communications. Using locally generated frequency combs to establish mutual coherence, we develop a simple and versatile TF-QKD setup that does not need service fibre and can operate over links of 100 km asymmetry. We confirm the setup's repeater-like behaviour and obtain a finite-size rate of 0.32 bit/s at a distance of 615.6 km.
Quantum mechanics allows distribution of intrinsically secure encryption keys by optical means. Twin-field quantum key distribution is one of the most promising techniques for its implementation on long-distance fiber networks, but requires stabilizing the optical length of the communication channels between parties. In proof-of-principle experiments based on spooled fibers, this was achieved by interleaving the quantum communication with periodical stabilization frames. In this approach, longer duty cycles for the key streaming come at the cost of a looser control of channel length, and a successful key-transfer using this technique in real world remains a significant challenge. Using interferometry techniques derived from frequency metrology, we develop a solution for the simultaneous key streaming and channel length control, and demonstrate it on a 206 km field-deployed fiber with 65 dB loss. Our technique reduces the quantum-bit-error-rate contributed by channel length variations to <1%, representing an effective solution for real-world quantum communications.
Twin-field (TF) quantum key distribution (QKD) promises high key rates over long distances to beat the rate-distance limit. Here, applying the sending-or-not-sending TF QKD protocol, we experimentally demonstrate a secure key distribution that breaks the absolute key-rate limit of repeaterless QKD over a 509-km-long ultralow loss optical fiber. Two independent lasers are used as sources with remote-frequency-locking technique over the 500-km fiber distance. Practical optical fibers are used as the optical path with appropriate noise filtering; and finite-key effects are considered in the key-rate analysis. The secure key rate obtained at 509 km is more than seven times higher than the relative bound of repeaterless QKD for the same detection loss. The achieved secure key rate is also higher than that of a traditional QKD protocol running with a perfect repeaterless QKD device, even for an infinite number of sent pulses. Our result shows that the protocol and technologies applied in this experiment enable TF QKD to achieve a high secure key rate over a long distribution distance, and is therefore practically useful for field implementation of intercity QKD.
Abstract Twin-field quantum key distribution (TF-QKD) has attracted considerable attention in quantum communication for the extension of secure communication distance. In TF-QKD, precise photon phase control is required and has been realized using dual-band stabilization (DBS). In this study, we performed residual phase noise reduction in DBS using laser frequency stabilization. The achieved frequency stability of the light sources was better than 1 × 10 −12 when the averaging time is larger than 1 s, indicating that the laser frequency variation was less than 200 Hz at 1.5 μm. The integrated phase noise at 1 s was less than 200 mrad, and the corresponding quantum bit error rate at 1 s was less than 1% after applying laser frequency stabilization. This result demonstrates that the stringent phase stability requirements in TF-QKD can be achieved without relying on complex optical frequency combs, enabling a simpler and more cost-effective experimental setup.
Channel loss seems to be the most severe limitation on the practical application of long distance quantum key distribution. The idea of twin-field quantum key distribution can improve the key rate from the linear scale of channel loss in the traditional decoy-state method to the square root scale of the channel transmittance. However, the technical demands are rather tough because they require single photon level interference of two remote independent lasers. Here, we adopt the technology developed in the frequency and time transfer to lock two independent laser wavelengths and utilize additional phase reference light to estimate and compensate the fiber fluctuation. Further, with a single photon detector with a high detection rate, we demonstrate twin field quantum key distribution through the sending-or-not-sending protocol with a realistic phase drift over 300 km optical fiber spools. We calculate the secure key rates with the finite size effect. The secure key rate at 300 km (1.96×10^{-6}) is higher than that of the repeaterless secret key capacity (8.64×10^{-7}).