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arXiv:2112.11165 · Physical Review A

Scalable high-rate twin-field quantum key distribution networks without constraint of probability and intensity

Yuan-Mei Xie, Chen-Xun Weng, Yu-Shuo Lu, Yao Fu, Yang Wang, Hua‐Lei Yin, Zeng‐Bing Chen

Published Apr 7, 2023• 23 citations
arXivHTMLPDF

Abstract

Implementation of a twin-field quantum key distribution network faces limitations, including the low tolerance of interference errors for phase-matching-type protocols and the strict constraint regarding intensity and probability for sending-or-not-sending-type protocols. Here we propose a two-photon twin-field quantum key distribution protocol and achieve twin-field-type two-photon interference through postmatching phase-correlated single-photon interference events. We exploit the noninterference mode as the code mode to highly tolerate interference errors and the two-photon interference naturally removes the intensity and probability constraint. Therefore, our protocol can transcend the above-mentioned limitations while breaking the secret key capacity of repeaterless quantum key distribution. Simulations show that for a four-user network, under which each node with fixed system parameters can dynamically switch different attenuation links, the key rates of our protocol for all six links can either exceed or approach the secret key capacity. However, the key rates of all links are lower than the key capacity when using phase-matching-type protocols. Additionally, four of the links could not extract the key when using sending-or-not-sending-type protocols. We anticipate that our protocol can facilitate the development of practical and efficient quantum networks.

DOI
10.1103/physreva.107.042603

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Implementation of a twin-field quantum key distribution network faces limitations, including the low tolerance of interference errors for phase-matching-type protocols and the strict constraint regarding intensity and probability for sending-or-not-sending-type protocols. Here we propose a two-photon twin-field quantum key distribution protocol and achieve twin-field-type two-photon interference through postmatching phase-correlated single-photon interference events. We exploit the noninterference mode as the code mode to highly tolerate interference errors and the two-photon interference naturally removes the intensity and probability constraint. Therefore, our protocol can transcend the above-mentioned limitations while breaking the secret key capacity of repeaterless quantum key distribution. Simulations show that for a four-user network, under which each node with fixed system parameters can dynamically switch different attenuation links, the key rates of our protocol for all six links can either exceed or approach the secret key capacity. However, the key rates of all links are lower than the key capacity when using phase-matching-type protocols. Additionally, four of the links could not extract the key when using sending-or-not-sending-type protocols. We anticipate that our protocol can facilitate the development of practical and efficient quantum networks.
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.
The twin-field quantum key distribution (TF-QKD) protocol has led to extensive investigation for its capacity of overcoming the rate-distance limit. One variant for TF-QKD named sending-or-not-sending (SNS) TF-QKD has also been proposed. The SNS-TF-QKD encodes the classical keys 0 and 1 in sending or not sending the phase-randomized coherent states, which helps it to tolerate large misalignment errors due to optical phase fluctuations. Nevertheless, the original SNS-TF-QKD scheme utilizes only single-photon interference in generating secret keys, which seems quite different from other TF-QKD protocols. In this paper, we prove that multiphoton interference in the SNS TF-QKD protocol can also contribute to key generation. Accordingly, an improved SNS-TF-QKD scheme with higher key rate is proposed, where events of not only single-photon but also two-photon emissions are considered as effective events.
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}).
Quantum key distribution (QKD) is a key application in quantum communication, enabling secure key exchange between parties using quantum states. Twin-field (TF) QKD offers a promising solution that surpasses the repeaterless limits, and its measurement-device-independent nature makes it suitable for star-type network architectures. In this work, we propose a scalable TF-QKD network with adaptable architecture, where users prepare quantum signals and send them to network nodes. These nodes use an optical switch to route the signals to multi-user measurement units, enabling secure key distribution among arbitrary users and adapting to complex connection demands of the network. A proof-of-principle demonstration with three users successfully achieved secure key sharing over simulated link losses of up to 303030 dB, with an average rate of 19.5719.5719.57 bit/s. Additionally, simulations show that the proposed architecture can achieve a total secure key rate of 4.84×1044.84 \times 10^{4}4.84×104 bit/s at 100100100 km in a symmetric 323232-user network. This approach represents a significant advancement in the topology of untrusted-node QKD networks and holds promise for practical, large-scale applications in secure communication.
Abstract Quantum key distribution (QKD) over a point-to-point link enables us to benefit from a genuine quantum effect even with conventional optics tools such as lasers and photon detectors, but its capacity is limited to a linear scaling of the repeaterless bound. Recently, twin-field (TF) QKD was conjectured to beat the limit by using an untrusted central station conducting a single-photon interference detection. So far, the effort to prove the conjecture was confined to the infinite key limit which neglected the time and cost for monitoring an adversary’s act. Here we propose a variant of TF-type QKD protocol equipped with a simple methodology of monitoring to reduce its cost and provide an information-theoretic security proof applicable to finite communication time. We simulate the key rate to show that the protocol beats the linear bound in a reasonable running time of sending 10 12 pulses, which positively solves the conjecture.
Abstract Twin-field (TF) quantum key distribution (QKD) represents a novel QKD approach whose principal merit is to beat the point-to-point private capacity of a lossy quantum channel, thanks to performing single-photon interference in an untrusted node. Indeed, recent security proofs of various TF-QKD type protocols have confirmed that the secret key rate of these schemes scales essentially as the square root of the transmittance of the channel. Here, we focus on the TF-QKD protocol introduced by Curty et al, whose secret key rate is nearly an order of magnitude higher than previous solutions. Its security relies on the estimation of the detection probabilities associated to various photon-number states through the decoy-state method. We derive analytical bounds on these quantities assuming that each party uses either two, three or four decoy intensity settings, and we investigate the protocol’s performance in this scenario. Our simulations show that two decoy intensity settings are enough to beat the point-to-point private capacity of the channel, and that the use of four decoys is already basically optimal, in the sense that it almost reproduces the ideal scenario of infinite decoys. We also observe that the protocol seems to be quite robust against intensity fluctuations of the optical pulses prepared by the parties.