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arXiv:2507.11243

Side-channel-secure quantum key distribution with correlated sources

Yang-Guang Shan, Jia-Xuan Li, Zhen−Qiang Yin, Shuang Wang, Wei Chen, De‐Yong He, Guang‐Can Guo, Zheng‐Fu Han

Published Jul 15, 2025• 0 citations
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Abstract

Quantum key distribution (QKD) offers theoretical security guarantees for sharing secure key, but its practical systems face challenges due to the imperfections of devices. Widespread quantum state preparation imperfections, such as correlations between multiple rounds, significantly undermine the real-world security of QKD. In this paper, we propose a protocol that is immune to almost all kinds of state-preparation imperfections over multiple correlated rounds arising from both encoding and unknown non-encoding dimensions. The protocol relies only on three assumptions: the imperfect encoding produces unknown product states rather than entangled ones, a lower bound on the vacuum components is known, and the correlation has a finite range. The proposed protocol is also measurement-device-independent, ensuring high security at both the source and measurement sides. We provide the finite-key security analysis against coherent attacks and conduct numerical simulations to see the performance. The results show that for small correlation ranges, the protocol achieves excellent performance with a maximal transmission loss exceeding 60 dB (>300 km in standard fiber). Even for extreme cases, where one encoding affects up to 500 neighboring rounds, the protocol can still generate secret keys over a 10 dB-loss channel.

DOI
10.48550/arxiv.2507.11243

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Quantum key distribution (QKD) offers theoretical security guarantees for sharing secure key, but its practical systems face challenges due to the imperfections of devices. Widespread quantum state preparation imperfections, such as correlations between multiple rounds, significantly undermine the real-world security of QKD. In this paper, we propose a protocol that is immune to almost all kinds of state-preparation imperfections over multiple correlated rounds arising from both encoding and unknown non-encoding dimensions. The protocol relies only on three assumptions: the imperfect encoding produces unknown product states rather than entangled ones, a lower bound on the vacuum components is known, and the correlation has a finite range. The proposed protocol is also measurement-device-independent, ensuring high security at both the source and measurement sides. We provide the finite-key security analysis against coherent attacks and conduct numerical simulations to see the performance. The results show that for small correlation ranges, the protocol achieves excellent performance with a maximal transmission loss exceeding 60 dB (>300 km in standard fiber). Even for extreme cases, where one encoding affects up to 500 neighboring rounds, the protocol can still generate secret keys over a 10 dB-loss channel.
Quantum key distribution (QKD) provides information-theoretic security based on quantum mechanics, yet its implementation is hindered by source imperfections. Among these, correlations between transmitted pulses present a critical but underexplored threat to QKD's theoretical security. We propose a general framework for QKD security under correlated sources, achieving the first finite-key analysis by extending and reorganizing QKD rounds using the generalized chain rule. Inspired by side-channel-secure QKD, we design a protocol secure against correlated leakage sources, requiring only the correlation range and lower bounds on the vacuum component of the prepared states. Our framework also applies to other QKD protocols, providing a general approach to address correlation-induced vulnerabilities. Simulations demonstrate the effectiveness of our protocol and its significantly superior tolerance to imperfections compared to existing protocols. This work provides a crucial step toward closing security loopholes in QKD, enhancing its practicality, and ensuring long-distance, high-performance secure communication under real-world constraints.
Based on the basic principles of quantum mechanics, quantum key distribution (QKD) provides unconditional security for long-distance communication. However, existing QKD with relevant source protocols have limited tolerance for source correlation, which greatly reduces the key generation rate and limits the secure transmission distance, thereby limiting their practical deployment. In this work, we propose an improved QKD with correlated source protocol to overcome these limitations by discarding the traditional loss-tolerant security frameworks. Our approach adopts the standard BB84 protocol for the security analysis, under the assumption that the source correlation has a bounded range and characterized inner product of the states. We theoretically analyze the performance of the improved protocol at different levels of source correlation and channel loss. Numerical simulations show that our protocol achieves a much higher secret key rate and longer transmission distance than traditional schemes. In the case of typical parameters and 0 dB loss, our protocol achieves about 1.5–3 times improvement in secret key rate. Additionally, the maximum tolerable loss is enhanced by about 2–6 dB. This highlights a promising direction for enhancing the robustness and practicality of QKD with correlated sources systems, paving the way for their deployment in real-world quantum communication networks.
The side-channel-secure (SCS) quantum key distribution (QKD), SCS QKD has attracted much attention due to its higher security compared to the measurement-device-independent protocols. It is not only measurement-device-independent secure, but also secure against source-state side channels provided that Eve has no access to Alice's or Bob's setups inside their labs. In the original protocol for SCS QKD, the side channels and the intensities of the coherent sources are allowed to be different in different time windows, but the vacuum sources of Alice and Bob have to be perfect, which is impossible in practice due to the finite extinction ratio of the intensity modulators. In this paper, we present a method that does not need the perfect vacuum source requested by the original protocol and assures the result as secure as the original protocol. With the upper bounds of the amplitudes of the nonvacuum part of the sources, the secure key rate of our SCS QKD protocol here with imperfect vacuum and unstable sources can be calculated. The numerical results show that, the secure distance of the SCS QKD protocol exceeds 150 km, provided that the intensity of the imperfect vacuum source is less than ${10}^{\ensuremath{-}8}$, which can be achieved in the experiment by a two-stage intensity modulator, where we also show that the active odd-parity pairing method and standard pairing method using two-way classical communication can be applied to the SCS QKD protocol to improve the key rate. Given the side-channel security based on the existing technological level, this work makes it possible to realize side-channel-secure QKD with real devices.
Based on the idea that there is no side channel in the vacuum state, the side-channel-free quantum key distribution (SCFQKD) protocol was proposed, which is immune to all attacks in the source side-channel space and all attacks in the detectors. In the original SCFQKD protocol, an important assumption is that Alice and Bob can produce the perfect vacuum pulses. But due to the finite extinction ratio of the intensity modulators, the perfect vacuum pulse is impossible in practice. In this paper, we solve this problem and make the quantum key distribution side-channel secure with real source device which does not emit perfect vacuum pulses. Our conclusion only depends on the upper bounds of the intensities of the sources. No other assumptions such as stable sources and stable side channels are needed. The numerical results show that, comparing with the results of SCFQKD protocol with perfect vacuum sources, the key rates and secure distance are only slightly decreased if the upper bound of the intensity of the imperfect vacuum source is less than 10−810^{-8}10−8 which can be achieved in experiment by two-stage intensity modulator. We also show that the two-way classical communication can be used to the data post-processing of SCFQKD protocol to improve the key rate. Specially, the active odd-parity pairing method can improve the key rates in all distances by about two times and the secure distance by about 40 km. Give that the side channel security based on imperfect vacuum, this work makes it possible to realize side channel secure QKD with real devices.
Quantum key distribution (QKD) enables two remote parties to share encryption keys with information-theoretic security guaranteed by physical laws. Side-channel-secure QKD (SCS-QKD) has attracted considerable attention because it simultaneously removes source and detector side-channel vulnerabilities. Although a recent experiment demonstrated SCS-QKD over 50 km, practical implementation remains challenging due to imperfect vacuum preparation and finite-key constraints under coherent attacks. Here, following the theoretical framework of Jiang et al. [Phys. Rev. Res. 6, 013266 (2024)], we experimentally implement a practical SCS-QKD protocol using an imperfect whole-space source and rigorous finite-key analysis. Benefiting from a stable GHz-level system operating at 1.25 GHz, we extend the transmission distance to 200 km and achieve high secure key rates of 18.31 kbps, 2.55 kbps, and 196.03 bps at 100 km, 150 km, and 200 km, respectively. Our results establish a new distance record for SCS-QKD and demonstrate the feasibility of high-speed, long-distance, and practically secure quantum key distribution.
Quantum key distribution (QKD) theoretically provides unconditional security between remote parties. However, guaranteeing practical security through device characterization alone is challenging in real-world implementations due to the multidimensional spaces in which the devices may be operated. The side channel secure (SCS) QKD protocol, which only requires bounding the upper limits of the intensities for the two states, theoretically provides a rigorous solution to the challenge and achieves measurement-device-independent security in detection and security for whatever multidimensional side channel attack in the source. Here, we demonstrate a practical implementation of SCS-QKD, achieving a secure key rate of 6.67 kbps through a 50.5 km fiber and a maximum distribution distance of 101.1 km while accounting for finite-size effects. Our experiment also represents an approximately 40 times improvement over the previous experiment.