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Journal of Lightwave Technology

Patterning-Effect Calibration Algorithm for Secure Decoy-State Quantum Key Distribution

Xiang Kang, Feng-Yu Lu, Shuang Wang, Jia-Lin Chen, Ze-Hao Wang, Zhen−Qiang Yin, De‐Yong He, Wei Chen, Guan‐Jie Fan‐Yuan, Guang‐Can Guo, Zheng‐Fu Han

Published Oct 3, 2022• 17 citations
Publisher

Abstract

The decoy-state method for the quantum key distribution (QKD) is a great countermeasure against the fatal photon number splitting attack. At present, the decoy-state method has been widely employed in practical QKD systems. However, the patterning effect, namely the correlated intensity fluctuations in practical QKD systems, violates the important assumption of the decoy-state method and seriously threatens the protocol security. The previous works investigated the patterning effect between the adjacent pulses and proposed the countermeasures from the degree of post selection and modulators. In this work, we investigate the patterning effect with larger correlation range and present an algorithm to calibrate the driving voltages to mitigate the patterning effect. According to our methods, we experimentally observe the higher-order patterning effect and successfully mitigate it by our calibration algorithm. The experiment results indicate that our methods efficiently improve the protocol security and are expected to be widely employed in practical decoy-state QKD systems.

DOI
10.1109/jlt.2022.3211442

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The decoy-state method for the quantum key distribution (QKD) is a great countermeasure against the fatal photon number splitting attack. At present, the decoy-state method has been widely employed in practical QKD systems. However, the patterning effect, namely the correlated intensity fluctuations in practical QKD systems, violates the important assumption of the decoy-state method and seriously threatens the protocol security. The previous works investigated the patterning effect between the adjacent pulses and proposed the countermeasures from the degree of post selection and modulators. In this work, we investigate the patterning effect with larger correlation range and present an algorithm to calibrate the driving voltages to mitigate the patterning effect. According to our methods, we experimentally observe the higher-order patterning effect and successfully mitigate it by our calibration algorithm. The experiment results indicate that our methods efficiently improve the protocol security and are expected to be widely employed in practical decoy-state QKD systems.
Quantum key distribution (QKD) is a technology that allows two users to exchange cryptographic keys securely. The decoy state technique enhances the technology, ensuring keys can be shared at high bit rates over long distances with information theoretic security. However, imperfections in the implementation, known as side-channels, threaten the perfect security of practical QKD protocols. Intensity modulators are required for high-rate decoy-state QKD systems, although these are unstable and can display a side channel where the intensity of a pulse is dependent on the previous pulse. Here we demonstrate the superior practicality of a tunable extinction ratio Sagnac-based intensity modulator (IM) for practical QKD systems. The ability to select low extinction ratios, alongside the immunity of Sagnac interferometers to DC drifts, ensures that random decoy state QKD patterns can be faithfully reproduced with the patterning effects mitigated. The inherent stability of Sagnac interferometers also ensures that the modulator output does not wander over time.
Decoy-state method has greatly filled in loopholes from multi-photon pulses in the field of quantum key distribution (QKD) and it has been employed in almost all practical QKD systems in recent years. However, security and performance of the practical decoy-state QKDs are still limited by inaccurate modulations such as random intensity fluctuation and correlated intensity fluctuation. In this work, an intensity tomography method is proposed as a countermeasure against these fluctuations. It has an ability to tightly bound the unknown density matrices of photon-number state and accurately calibrate the fluctuations by measuring the decoy states with a local single-photon detector. We verified the tomography method by experiments and simulations. The results indicated that our method can efficiently mitigate the flaws from these fluctuations to improve the protocol performance and practical security. Noting that these fluctuations are inevitable in practical QKDs, our method would pave the avenue for QKD's practical applications.
Intensity correlations between neighbouring pulses open a prevalent yet often overlooked security loophole in decoy-state quantum key distribution (QKD). As a solution, we present and experimentally demonstrate an intensity-correlation-tolerant QKD protocol that mitigates the negative effect that this phenomenon has on the secret key rate according to existing security analyses. Compared to previous approaches, our method significantly enhances the robustness against correlations, notably improving both the maximum transmission distances and the achievable secret key rates across different scenarios. By relaxing constraints on correlation parameters, our protocol enables practical devices to counter intensity correlations. We experimentally demonstrate this first practical solution that directly overcomes this security vulnerability, establish the feasibility and efficacy of our proposal, taking a major step towards loophole-free and high-performance QKD.
Abstract Quantum cryptography or, more precisely, quantum key distribution (QKD), is one of the advanced areas in the field of quantum technologies. The confidentiality of keys distributed with the use of QKD protocols is guaranteed by the fundamental laws of quantum mechanics. This paper is devoted to the decoy state method, a countermeasure against vulnerabilities caused by the use of coherent states of light for QKD protocols whose security is proved under the assumption of single-photon states. We give a formal security proof of the decoy state method against all possible attacks. We compare two widely known attacks on multiphoton pulses: photon-number splitting and beam splitting. Finally, we discuss the equivalence of polarization and phase coding.
One of the most prominent techniques to enhance the performance of practical quantum key distribution (QKD) systems with laser sources is the decoy-state method. Current decoy-state QKD setups operate at gigahertz repetition rates, a regime where memory effects in the modulators and electronics that control them create correlations between the intensities of the emitted pulses. This translates into information leakage about the selected intensities, which cripples a crucial premise of the decoy-state method, thus invalidating the use of standard security analyzes. To overcome this problem, a security proof that exploits the Cauchy-Schwarz constraint has been introduced recently. Its main drawback is, however, that the achievable key rate is significantly lower than that of the ideal scenario without intensity correlations. Here, we improve this security proof technique by combining it with a fine-grained decoy-state analysis, which can deliver a tight estimation of the relevant parameters that determine the secret key rate. This results in a notable performance enhancement, being now the attainable distance double that of previous analyzes for certain parameter regimes. Also, we show that when the probability density function of the intensity fluctuations, conditioned on the current and previous intensity choices, is known, our approach provides a key rate very similar to the ideal scenario, which highlights the importance of an accurate experimental characterization of the correlations.
The decoy-state protocol has been considered to be one of the most important methods to protect the security of quantum key distribution (QKD) with a weak coherent source. Here we test two experimental approaches to generating the decoy states with different intensities: modulation of the pump current of a semiconductor laser diode, and external modulation by an optical intensity modulator. The former approach shows a side channel in the time domain that allows an attacker to distinguish s signal state from a decoy state, breaking a basic assumption in the protocol. We model a photon-number-splitting attack based on our experimental data, and show that it compromises the system's security. Then, based on the work of Tamaki et al. [New J. Phys. 18, 065008 (2016)], we obtain two analytical formulas to estimate the yield and error rate of single-photon pulses when the signal and decoy states are distinguishable. The distinguishability reduces the secure key rate below that of a perfect decoy-state protocol. To mitigate this reduction, we propose to calibrate the transmittance of the receiver (Bob's) unit. We apply our method to three QKD systems and estimate their secure key rates.