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Security of advanced quantum key distribution protocols in realistic conditions

Currás Lorenzo, José Guillermo

Published May 1, 2021• 0 citations
OpenAlex

Abstract

Quantum key distribution (QKD) allows two users to generate a random secret key, which they can use to securely exchange a message. Unlike many other cryptographic schemes, QKD offers information-theoretical security based on the laws of physics. In recent years, major theoretical and experimental advancements have been made. Among these are two novel protocols, memory-assisted (MA) QKD and twin-field (TF) QKD, which can both improve the secret-key rate scaling with channel length, potentially allowing QKD to be performed at longer distances. The main motivation of this thesis is to incorporate more realistic assumptions into the security proofs and performance analyses of these new protocols. \n \n \nOne common assumption made in QKD security proofs is that the protocol is run for an infinitely long time, which allows the users to obtain a perfect statistical characterisation of the quantum channel. In this thesis, we drop this assumption for a TF-QKD variant that is well suited for experimental implementation, proving its security in the finite-key regime. We also analyse the finite-key performance of MA-QKD, concluding that it is particularly resistant to its statistical fluctuation effects. Moreover, we develop an alternative finite-key security analysis approach based on random sampling theory, and apply it to the loss-tolerant protocol, which can ensure security in the presence of flawed sources. Compared to previous finite-key security proofs of the protocol, our analysis offers better performance. \n \n \nAnother common assumption is that the users can emit laser pulses with a continuous random phase. In practice, this is difficult to achieve, and the phase is often randomised discretely. In this thesis, we prove the security of a TF-QKD variant that relies on discrete phase randomisation, and show that, using certain post-selection techniques, it can provide higher secret-key rates than an equivalent continuously-randomised protocol.

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Quantum key distribution (QKD) allows two users to generate a random secret key, which they can use to securely exchange a message. Unlike many other cryptographic schemes, QKD offers information-theoretical security based on the laws of physics. In recent years, major theoretical and experimental advancements have been made. Among these are two novel protocols, memory-assisted (MA) QKD and twin-field (TF) QKD, which can both improve the secret-key rate scaling with channel length, potentially allowing QKD to be performed at longer distances. The main motivation of this thesis is to incorporate more realistic assumptions into the security proofs and performance analyses of these new protocols. \n \n \nOne common assumption made in QKD security proofs is that the protocol is run for an infinitely long time, which allows the users to obtain a perfect statistical characterisation of the quantum channel. In this thesis, we drop this assumption for a TF-QKD variant that is well suited for experimental implementation, proving its security in the finite-key regime. We also analyse the finite-key performance of MA-QKD, concluding that it is particularly resistant to its statistical fluctuation effects. Moreover, we develop an alternative finite-key security analysis approach based on random sampling theory, and apply it to the loss-tolerant protocol, which can ensure security in the presence of flawed sources. Compared to previous finite-key security proofs of the protocol, our analysis offers better performance. \n \n \nAnother common assumption is that the users can emit laser pulses with a continuous random phase. In practice, this is difficult to achieve, and the phase is often randomised discretely. In this thesis, we prove the security of a TF-QKD variant that relies on discrete phase randomisation, and show that, using certain post-selection techniques, it can provide higher secret-key rates than an equivalent continuously-randomised protocol.
Twin-field (TF) quantum key distribution (QKD) can overcome fundamental secret-key-rate bounds on point-to-point QKD links, allowing us to reach longer distances than ever before. Since its introduction, several TFQKD variants have been proposed, and some of them have already been implemented experimentally. Most of them assume that the users can emit weak coherent pulses with a continuous random phase. In practice, this assumption is often not satisfied, which could open up security loopholes in their implementations. To close this loophole, we propose and prove the security of a TFQKD variant that relies exclusively on discrete phase randomization. Remarkably, our results show that it can also provide higher secret-key rates than an equivalent continuous-phase-randomized protocol.
Twin-field (TF) quantum key distribution (QKD) can overcome fundamental secret-key-rate bounds on point-to-point QKD links, allowing us to reach longer distances than ever before. Since its introduction, several TFQKD variants have been proposed, and some of them have already been implemented experimentally. Most of them assume that the users can emit weak coherent pulses with a continuous random phase. In practice, this assumption is often not satisfied, which could open up security loopholes in their implementations. To close this loophole, we propose and prove the security of a TFQKD variant that relies exclusively on discrete phase randomization. Remarkably, our results show that it can also provide higher secret-key rates than an equivalent continuous-phase-randomized protocol.
Long-distance quantum key distribution (QKD) has long time seriously relied on trusted relay or quantum repeater, which either has security threat or is far from practical implementation. Recently, a solution called twin-field (TF) QKD and its variants have been proposed to overcome this challenge. However, most security proofs are complicated, a majority of which could only ensure security against collective attacks. Until now, the full and simple security proof can only be provided with asymptotic resource assumption. Here, we provide a composable finite-key analysis for coherent-state-based TF-QKD with rigorous security proof against general attacks. Furthermore, we develop the optimal statistical fluctuation analysis method to significantly improve secret key rate in high-loss regime. The results show that coherent-state-based TF-QKD is practical and feasible, with the potential to apply over nearly one thousand kilometers.
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.
Quantum key distribution (QKD) allows two remote parties to share information-theoretic secret keys. Many QKD protocols assume the phase of encoding state can be continuous randomized from 0 to 2π, which, however, may be questionable in the experiment. This is particularly the case in the recently proposed twin-field (TF) QKD, which has received a lot of attention since it can increase the key rate significantly and even beat some theoretical rate-loss limits. As an intuitive solution, one may introduce discrete-phase randomization instead of continuous randomization. However, a security proof for a QKD protocol with discrete-phase randomization in the finite-key region is still missing. Here, we develop a technique based on conjugate measurement and quantum state distinguishment to analyze the security in this case. Our results show that TF-QKD with a reasonable number of discrete random phases, e.g., 8 phases from {0,π/4,π/2,…,7π/4}, can achieve satisfactory performance. On the other hand, we find the finite-size effects become more notable than before, which implies that more pulses should be emit in this case. More importantly, as a the first proof for TF-QKD with discrete-phase randomization in the finite-key region, our method is also applicable in other QKD protocols.
Quantum Key Distribution (QKD) offers information-theoretic security grounded in the laws of quantum mechanics rather than computational assumptions. While its theoretical foundations are robust, practical implementations necessarily deviate from the idealized models assumed in security analyses, opening the door to side-channel attacks and implementation loopholes. In particular, most research to date has focused on mitigating vulnerabilities in the measurement devices, leading to the development of Measurement-Device-Independent QKD (MDI-QKD) and Twin-Field QKD (TF-QKD), which effectively close all receiver-side loopholes. By contrast, the security of the source has remained less explored, despite being critical for the overall integrity of QKD systems. The goal of this thesis is to develop security proofs that explicitly account for realistic imperfections in QKD sources. Special emphasis is placed on three main aspects: correlations between emitted pulses that violate independence assumptions, imperfect phase randomization arising from practical modulation schemes, and leakage of internal settings through passive or active side channels. By formulating rigorous methods to analyze these imperfections, the work aims to provide a framework that enables practitioners to quantify and mitigate implementation risks systematically. Additionally, this thesis seeks to clarify under which conditions the commonly used decoy-state method remains secure, and to what extent source assumptions can be relaxed without compromising performance. This thesis aims to advance the understanding of how real-world deviations impact QKD security and to contribute to the development of protocols and devices. Ultimately, this work is motivated by the ambition to help bring QKD from a theoretically promising idea to a mature technology deployable with confidence in practical scenarios.