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Optics Express

Performance evaluation of twin-field quantum key distribution using avalanche photodiode single-photon detectors

Zhi-Lin Xie, Lei Zhou, Yan-Lin Tang, Dongdong Li, Li Zhang, Xi-Hao Wang, Mulan Xu, Ming Liu, Lian-Jun Jiang, Kaiping Xue, Shi-Biao Tang

Published Jan 7, 2026• 3 citations
Publisher

Abstract

Twin-field quantum key distribution (TF-QKD) surpasses the repeaterless key-rate bound, offering great promise for long-distance quantum communication. Avalanche-photodiode (APD) single-photon detectors are widely used in practical QKD systems, due to their compactness, cost-effectiveness, and room-temperature operation. However, their non-ideal characteristics-including afterpulse probability, dead time, and dark counts-can severely degrade TF-QKD performance. In this work, we develop a comprehensive TF-QKD simulation model incorporating these APD imperfections, enabling quantitative analysis of their individual impacts. Our results reveal that afterpulse induced by high-intensity reference pulses is the dominant performance-limiting factor. Under identical APD conditions, we compare three representative TF-QKD protocols. The sending-or-not-sending (SNS) protocol demonstrates superior long-distance performance due to its higher signal photon number and enhanced noise tolerance. This study provides practical guidance for the design of compact, APD-based TF-QKD systems and highlights key strategies for performance optimization.

DOI
10.1364/oe.582280

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Twin-field quantum key distribution (TF-QKD) surpasses the repeaterless key-rate bound, offering great promise for long-distance quantum communication. Avalanche-photodiode (APD) single-photon detectors are widely used in practical QKD systems, due to their compactness, cost-effectiveness, and room-temperature operation. However, their non-ideal characteristics-including afterpulse probability, dead time, and dark counts-can severely degrade TF-QKD performance. In this work, we develop a comprehensive TF-QKD simulation model incorporating these APD imperfections, enabling quantitative analysis of their individual impacts. Our results reveal that afterpulse induced by high-intensity reference pulses is the dominant performance-limiting factor. Under identical APD conditions, we compare three representative TF-QKD protocols. The sending-or-not-sending (SNS) protocol demonstrates superior long-distance performance due to its higher signal photon number and enhanced noise tolerance. This study provides practical guidance for the design of compact, APD-based TF-QKD systems and highlights key strategies for performance optimization.
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.
Twin-field (TF) quantum key distribution (QKD) can overcome the fundamental rate-loss bound of repeaterless QKD, enabling secure communication over long distances. However, in practical implementations, its security relies on accurate intensity modulation, which may be compromised by source imperfections and light-injection attacks. In this work, we investigate the security of the NPP TF-QKD protocol in the presence of imperfect intensity modulation, focusing on Trojan-horse attacks and induced photorefractive attacks. By employing the reference technique, we establish a finite-key security proof that explicitly incorporates this source-side information leakage. Numerical simulations show that with realistic transmitter isolation, the NPP TF-QKD protocol can still surpass the rate-loss bound and achieve secret-key rates close to the ideal case. Our results further demonstrate strong robustness against modulation deviations induced by photorefractive effects. These findings confirm the practical security of TF-QKD under realistic source imperfections and provide quantitative guidance for the design of secure quantum communication systems.
Twin-field quantum key distribution (TF-QKD) protocol can drastically increase the secure transmission distance of quantum key distribution, changing the channel-loss dependence of key rate fromO(η){O} (\eta)O(η)toO(η){O} (\sqrt {\eta })O(η​). To date, many revolutionary protocols have been invented to increase the practical performances of TF-QKD. However, most of them simply assume using symmetric detectors and ideal beam-splitters, which is inconsistent with the actual experiment. Here, we take experimental imperfections of the detection part in TF-QKD into account, for example, the two detectors own different detection efficiencies, different dark count rates, and the reflectivity of the beam-splitter is unbalanced. We employ the sending-or-not-sending twin-field quantum key distribution (SNS TF-QKD) protocol as an example, constructing a more systematic model to analyze its practical performance. Therefore, our work provides an useful reference for the practical implementation of TF-QKD.
Twin-field quantum key distribution (TF-QKD) is recognized as a promising approach to surpass the repeaterless key rate bound, playing a pivotal role in long-distance quantum communication. However, conventional TF-QKD implementations rely on laser coherence, where coherent-state sources differ fundamentally from the non-classical light sources required for future quantum networks. Recent advances demonstrate that quantum dot (QD) sources can generate high-purity coherent single-photon states on demand, exhibiting both perfect antibunching and long coherence times akin to laser source. Here, we propose a six-state TF-QKD protocol utilizing coherent single-photon states, which can be directly achieved through current experimental techniques. Simulation results indicate that our protocol achieves significantly enhanced key rates and extended transmission distances. Moreover, a data size on the order of 108 is sufficient to surpass the repeaterless key rate bound, demonstrating particular robustness against finite-size effects. Our work paves the way for the direct implementation of long-distance quantum key distribution utilizing future quantum network infrastructure.
Quantum key distribution endows people with information-theoretical security in communications. Twin-field quantum key distribution (TF-QKD) has attracted considerable attention because of its outstanding key rates over long distances. Recently, several demonstrations of TF-QKD have been realized. Nevertheless, those experiments are implemented in the laboratory, and therefore a critical question remains about whether the TF-QKD is feasible in real-world circumstances. Here, by adopting the sending-or-not-sending twin-field QKD (SNS-TF-QKD) with the method of actively odd parity pairing (AOPP), we demonstrate a field-test QKD over 428 km of deployed commercial fiber and two users are physically separated by about 300 km in a straight line. To this end, we explicitly measure the relevant properties of the deployed fiber and develop a carefully designed system with high stability. The secure key rate we achieved breaks the absolute key rate limit of repeaterless QKD. The result provides a new distance record for the field test of both TF-QKD and all types of fiber-based QKD systems. Our work bridges the gap of QKD between laboratory demonstrations and practical applications and paves the way for an intercity QKD network with measurement-device-independent security.
Quantum Key Distribution (QKD) offers information-theoretic security, making it a cornerstone of future cryptographic infrastructures in the era of quantum computing. However, long-distance deployment of conventional QKD protocols is limited by channel losses, detector noise, and the fundamental repeaterless Pirandola-Laurenza-Ottaviani-Banchi (PLOB) bound. To address these challenges, this paper investigates the Twin-Field QKD (TF-QKD) protocol, which enables secure key exchange over long-haul optical fibers by exploiting single-photon interference at an untrusted intermediate node. An elaborate system model is provided with optical channel impairments, imperfections of detector and phase stabilization conditions. Finite-key analysis, error correction as well as privacy amplification are placed and made together in order to increase robustness of protocol against practical attacks. The simulation data show that TFQKD can reach secure key rates of over 500 km and is out of reach of decoy-state QKD but nearly close to fundamental limits. These results demonstrate that TF-QKD has the potential to practicalize quantumsecure communication at large scale and in a costeffective way, which can constitute a sub-unit of the future quantum internet.