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Frontiers of Physics

Twin-field quantum key distribution with coherent single-photon state

Sheng-Tao Lyu, Rong-Zheng Liu, Hua‐Lei Yin

Published Jan 1, 2026• 1 citation
Publisher

Abstract

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.

DOI
10.15302/frontphys.2026.083204

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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.
Photon-number superposition states are a valuable resource for quantum information processing and can be deterministically generated using advanced quantum-dot (QD) platforms. Here, we propose a twin-field quantum key distribution (TF-QKD) protocol that employs photon-number superposition states of the form 1− t |0⟩+ e i ϕ t |1⟩. Numerical simulations show that the protocol outperforms existing laser-based TF-QKD schemes in secret-key rate and transmission distance, surpasses the repeaterless bound, enabling secure communication beyond 210 km under experimentally achievable conditions. A key advantage is its direct compatibility with existing QD technology. As integrated, solid-state, on-demand single-photon sources, QDs inherently provide high stability, scalability, and photon indistinguishability, making them well suited for deployment in large-scale quantum networks. Overall, our work demonstrates a practical pathway toward high-performance long-distance TF-QKD and highlights the broader potential of solid-state non-classical light sources for quantum networking architectures.
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.
Twin-field quantum key distribution (TFQKD), using single-photon-type interference, offers a way to exceed the rate-distance limit without quantum repeaters. However, it still suffers from photon losses and dark counts, which impose an ultimate limit on its transmission distance. In this Letter, we propose a scheme to implement TFQKD with an entangled coherent state source in the middle to increase its range, as well as comparing its performance under coherent attacks with that of TFQKD variants. Simulations show that our protocol has a theoretical distance advantage of 400 km. Moreover, the scheme has great robustness against the misalignment error and finite-size effects. Our work is a promising step toward long-distance secure communication and is greatly compatible with future global quantum networks.
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 quantum key distribution (TF-QKD) can overcome the fundamental rate-loss limit in the point-to-point QKD without a quantum repeater and achieve high key rates over long distances. However, TF-QKD protocol and its variants commonly use weak coherent pulses (WCPs) as light sources. The vacuum part of WCP accounts for a large proportion, resulting in the lower photon detection rate of the communication receiver, which leads to a shorter transmission distance of the secret key. In this paper, we propose an active-decoy TF-QKD protocol with heralded single-photon source (HSPS), which can achieve higher key rate and longer transmission distance than the active-decoy TF-QKD protocol with WCPs. Also, in order to resist the side channel attacks caused by actively modulating intensity of the pulses, we propose a passive-decoy TF-QKD protocol with HSPS to enhance the security, which can perform almost as good as the active-decoy TF-QKD protocol with WCPs.
Large-scale quantum communication networks are still a huge challenge due to the rate-distance limit of quantum key distribution (QKD). Recently, twin-field (TF) QKD has been proposed to overcome this limit. Here, we prove that coherent-state-based TF-QKD is a time-reversed entanglement protocol, where the entanglement generation is realized with entanglement swapping operation via an entangled coherent state measurement. We propose a coherent-state-based TF-QKD with optimal secret key rate under symmetric and asymmetric channels by using coherent state and cat state coding. Furthermore, we show that our protocol can be converted to all recent coherent-state-based TF-QKD protocols by using our security proof. By using the entanglement purification with two-way classical communication, we improve the transmission distance of all coherent-state-based TF-QKD protocols.