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}).
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}).
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.
Quantum key distribution (QKD) aims to generate secure private keys shared by two remote parties. With its security being protected by principles of quantum mechanics, some technology challenges remain towards practical application of QKD. The major one is the distance limit, which is caused by the fact that a quantum signal cannot be amplified while the channel loss is exponential with the distance for photon transmission in optical fiber. Here using the 3-intensity sending-or-not-sending protocol with the actively-odd-parity-pairing method, we demonstrate a fiber-based twin-field QKD over 1002 km. In our experiment, we developed a dual-band phase estimation and ultra-low noise superconducting nanowire single-photon detectors to suppress the system noise to around 0.02 Hz. The secure key rate is 9.53×10^{-12} per pulse through 1002 km fiber in the asymptotic regime, and 8.75×10^{-12} per pulse at 952 km considering the finite size effect. Our work constitutes a critical step towards the future large-scale quantum network.
Twin-field quantum key distribution can overcome the secret key capacity of repeaterless quantum key distribution via single-photon interference.However, to compensate for the channel fluctuations and lock the laser fluctuations, the techniques of phase tracking and phase locking are indispensable in experiment, which drastically increase experimental complexity and hinder free-space realization.We herein present an asynchronous measurement-device-independent quantum key distribution protocol that can surpass the secret key capacity even without phase tracking and phase locking.Leveraging the concept of time multiplexing, asynchronous two-photon Bell-state measurement is realized by postmatching two interference detection events.For a 1 GHz system, the new protocol reaches a transmission distance of 450 km without phase tracking.After further removing phase locking, our protocol is still capable of breaking the capacity at 270 km.Intriguingly, when using the same experimental techniques, our protocol has a higher key rate than the phase-matching-type twin-field protocol.In the presence of imperfect intensity modulation, it also has a significant advantage in terms of the transmission distance over the sending-or-not-sendingtype twin-field protocol.With high key rates and accessible technology, our work provides a promising candidate for practical scalable quantum communication networks.
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.
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.
Based on the novel idea of twin-field quantum key distribution [TF-QKD; Lucamarini et al., Nature (London) 557, 400 (2018)], we present a protocol named the ``sending or not sending TF-QKD'' protocol, which can tolerate large misalignment error. A revolutionary theoretical breakthrough in quantum communication, TF-QKD changes the channel-loss dependence of the key rate from linear to square root of channel transmittance. However, it demands the challenging technology of long-distance single-photon interference, and also, as stated in the original paper, the security proof was not finalized there due to the possible effects of the later announced phase information. Here we show by a concrete eavesdropping scheme that the later phase announcement does have important effects and the traditional formulas of the decoy-state method do not apply to the original protocol. We then present our ``sending or not sending'' protocol. Our protocol does not take postselection for the bits in Z-basis (signal pulses), and hence the traditional decoy-state method directly applies and automatically resolves the issue of security proof. Most importantly, our protocol presents a negligibly small error rate in Z-basis because it does not request any single-photon interference in this basis. Thus our protocol greatly improves the tolerable threshold of misalignment error in single-photon interference from the original a few percent to more than 45. As shown numerically, our protocol exceeds a secure distance of 700, 600, 500, or 300 km even though the single-photon interference misalignment error rate is as large as 15, or 45.