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arXiv:2011.13092 · Advanced Quantum Technologies

Recent Advances on Quantum Key Distribution Overcoming the Linear Secret Key Capacity Bound

Yingqiu Mao, Pei Zeng, Teng‐Yun Chen

Published Nov 25, 2020• 17 citations
arXivHTMLPDF

Abstract

Abstract A crucial goal for quantum key distribution (QKD) is to transmit unconditionally secure keys over long distances. Previous studies show that the key rate of point‐to‐point QKD is limited by a secret key rate capacity bound, and higher key rates would require quantum repeaters. In 2018, the seminal twin‐field (TF) QKD protocol is proposed to provide a remarkable solution to overcoming the linear secret key capacity bound. This article presents an up‐to‐date survey on recent developments in this area, including the security proofs of phase‐matching QKD and other TF‐QKD type protocols, the theoretical examinations of these protocols under realistic conditions, and the recent experimental demonstrations.

DOI
10.1002/qute.202000084

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Abstract A crucial goal for quantum key distribution (QKD) is to transmit unconditionally secure keys over long distances. Previous studies show that the key rate of point‐to‐point QKD is limited by a secret key rate capacity bound, and higher key rates would require quantum repeaters. In 2018, the seminal twin‐field (TF) QKD protocol is proposed to provide a remarkable solution to overcoming the linear secret key capacity bound. This article presents an up‐to‐date survey on recent developments in this area, including the security proofs of phase‐matching QKD and other TF‐QKD type protocols, the theoretical examinations of these protocols under realistic conditions, and the recent experimental demonstrations.
Abstract Quantum key distribution (QKD) offers a reliable solution to communication problems that require long-term data security. For its widespread use, however, the rate and reach of QKD systems must be improved. Twin-field (TF) QKD is a step forward toward this direction, with early demonstrations suggesting it can beat the current rate-versus-distance records. A recently introduced variant of TF-QKD is particularly suited for experimental implementation, and has been shown to offer a higher key rate than other variants in the asymptotic regime, where users exchange an infinite number of signals. Here, we extend the security of this protocol to the finite-key regime, showing that it can overcome the fundamental bounds on point-to-point QKD with ~1010 transmitted signals. In many practical regimes of interest, our analysis offers higher key rates than those of alternative variants. Moreover, some of the techniques we develop are applicable to the finite-key analysis of other QKD protocols.
Abstract Twin-Field (TF) quantum key distribution (QKD) is a major candidate to be the new benchmark for far-distance QKD implementations, since its secret key rate can overcome the repeaterless bound by means of a simple interferometric measurement. Many variants of the original protocol have been recently proven to be secure. Here, we focus on the TF-QKD type protocol proposed by Curty et al (2019 NPJ Quantum Inf. 5 64), which can provide a high secret key rate and whose practical feasibility has been demonstrated in various recent experiments. The security of this protocol relies on the estimation of certain detection probabilities (yields) through the decoy-state technique. Analytical bounds on the relevant yields have been recently derived assuming that both parties use the same set of decoy intensities, thus providing sub-optimal key rates in asymmetric-loss scenarios. Here we derive new analytical bounds when the parties use either two, three or four independent decoy intensity settings each. With the new bounds we optimize the protocol’s performance in asymmetric-loss scenarios and show that the protocol is robust against uncorrelated intensity fluctuations affecting the parties’ lasers.
Quantum communications promise to provide information theoretic security in the exchange of information. However, unlike their classical counterpart, they utilise dim optical pulses whose amplification is prohibited. Consequently, their transmission rate and range is confined below a theoretical limit known as repeaterless secret key capacity. Overcoming this limit with today’s technology was believed to be impossible until the recent proposal of Twin-field (TF) quantum key distribution (QKD), a scheme that uses phase-coherent optical signals and an auxiliary measuring station to distribute quantum information. Here, TF-QKD and its main variations are initially explored and compared in simulations, to assess their performance in different attributes. Such schemes are also practically implemented for the first time in two experiments. The first is a proof-of-principle implementation over significant channel losses, in excess of 90 dB. In the second, the setup is developed further and the protocol is implemented over real fibre channels exceeding 600 km in length, representing the first fibre-based secure quantum communication beyond the barriers of 600 km and 100 dB. In both cases, in the high loss/distance regime, the resulting secure key rates exceed the repeaterless secret key capacity, a result never achieved before.
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.
With the help of quantum key distribution (QKD), two distant peers are able to share informationtheoretical secure key bits.Increasing the key rate is ultimately significant for the applications of QKD in the lossy channel.However, it has been proven that there is a fundamental rate-distance limit, called the linear bound, which restricts the performance of all existing repeaterless protocols and realizations.Surprisingly, a recently proposed protocol, called twin-field (TF) QKD, can beat the linear bound with no need for quantum repeaters.Here, we present one of the first implementations of the TF-QKD protocol and demonstrate its advantage of beating the linear bound at a channel distance of 300 km.In our experiment, a modified TF-QKD protocol that does not assume phase postselection is considered, and thus a higher key rate than the original one is expected.After controlling the phase evolution of the twin fields traveling through hundreds of kilometers of optical fibers, the implemented system achieves high-visibility singlephoton interference and allows stable and high-rate measurement-device-independent QKD.Our experimental demonstration and results confirm the feasibility of the TF-QKD protocol and its prominent superiority in long-distance key distribution services.