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Physica Scripta

Improved finite-key analysis for side-channel-secure quantum key distribution with de Finetti reduction

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

Published Nov 1, 2025• 1 citation
Publisher

Abstract

Abstract Quantum key distribution (QKD) enables the generation of secure keys between two distant users. However, since practical devices cannot operate with perfect accuracy, achieving unconditional security in real-world QKD implementations remains a challenge. Side-channel-secure (SCS) QKD gives a good solution to realize both high practical security and performance. It keeps the advantage of measurement-device-independent, and for the source part, it only requires to upper bound the vacuum projecting probability of the states. However, security proof for SCS QKD against general coherent attacks is challenging. The existing analysis based on postselection method gives a bad performance. To overcome this drawback, instead of directly calculating key rate with postselection method, we propose a method correlating the failure probabilities of phase-error estimation against collective and coherent attacks, enabling the usage of the independent and identically distributed assumption in parameter estimation against coherent attacks. Our simulation shows that we give a better performance than previous analysis.

DOI
10.1088/1402-4896/ae1fbb

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Abstract Quantum key distribution (QKD) enables the generation of secure keys between two distant users. However, since practical devices cannot operate with perfect accuracy, achieving unconditional security in real-world QKD implementations remains a challenge. Side-channel-secure (SCS) QKD gives a good solution to realize both high practical security and performance. It keeps the advantage of measurement-device-independent, and for the source part, it only requires to upper bound the vacuum projecting probability of the states. However, security proof for SCS QKD against general coherent attacks is challenging. The existing analysis based on postselection method gives a bad performance. To overcome this drawback, instead of directly calculating key rate with postselection method, we propose a method correlating the failure probabilities of phase-error estimation against collective and coherent attacks, enabling the usage of the independent and identically distributed assumption in parameter estimation against coherent attacks. Our simulation shows that we give a better performance than previous analysis.
Quantum key distribution (QKD) enables the generation of secure keys between two distant users. Security proof of QKD against general coherent attacks is challenging, while the one against collective attacks is much easier. As an effective and general solution, the postselection method tries to extend security analyses of collective attacks to be against coherent attacks. However, it gives a bad performance. To overcome this drawback, instead of directly calculating key rate by postselection method, we propose a method correlating the failure probabilities of phase error estimation against collective and coherent attacks, enabling the use of the independent and identically distributed assumption in parameter estimation against coherent attacks. Then the key rate can be obtained by uncertainty relation of entropy. Our method can be applied to various QKD protocols, providing better performance compared with the traditional postselection method. For instance, we give the finite-key analyses of the side-channel-secure (SCS) QKD and the no-phase-postselection (NPP) twin-field (TF) QKD to show their performance improvements with the proposed method.
We present a result of side-channel-secure (SCS) quantum key distribution (QKD) under fully realistic conditions. Our result is not only measurement-device independent but also effective with imperfect (and unstable) source devices including imperfect vacuum and imperfect coherent-state source. Applying the virtual mapping idea, we present a general security proof under whatever out-side-lab attack, including whatever side-channel coherent attack. As a byproduct, we also present an improved method for SCS protocols which can raise the key rate by 1-2 orders of magnitude. Using these results, we obtain a non-asymptotic key rate which is instantly useful with full realistic conditions.
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.
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.
By applying the framework of entropic uncertainty relation (EUR) and the Quantum Leftover Hash Lemma (QLHL), we introduce a security-proof method for variable-length side-channel-secure (SCS) quantum key distribution (QKD) against coherent attacks. This method reframes composable security as a statistical fluctuation problem of phase errors, enabling direct proofs against coherent attacks through observables and virtual observables. It yields tight key rates for the SCS protocol and reduces pulse requirements by over two orders of magnitude compared to prior works that employ the post-selection technique. {Utilizing the characteristic of the SCS protocol that it has no untagged-bit bit-flip errors, we prove that the secure key length for the SCS protocol can be determined after Cascade error correction, using its actual parity disclosures. We also explain how the same argument extends to protocols with compatible bit-error-pattern and phase-error measurements.} We further identify sufficient conditions under which the final key length may be determined after error correction in a broader class of QKD protocols. Under the framework of EUR and QLHL, we clarify the applicability of several commonly used concentration bounds to variable-length QKD and the appropriate manner of their implementation. This work enhances the practical value of the SCS protocol and clarifies the security justification of key-rate formulas used in practical variable-length QKD implementations.
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.