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Optics Express

Experimental demonstration of the time-dependent source side-channel and countermeasures in polarization-based BB84 QKD systems

Wanli Zhang, Cong Jiang, Fei Zhou, Yang Liu, Fang Li, Xiang‐Bin Wang

Published Sep 23, 2024• 4 citations
Publisher

Abstract

High-speed is one development tendency of the practical and commercial quantum key distribution (QKD) systems, and the gain-switched semiconductor laser (GSSL) and Sagnac interferometer have been popular components for high-speed commercial polarization-based BB84 QKD systems thanks to their stability, compactness and low cost. However, due to the finite extinction ratio (ER) of the GSSL, the time-dependent source side-channel, which is theoretically presented in [Phys. Rev. A106(6): 062618 (2022)10.1103/PhysRevA.106.062618], also exists in these QKD systems. In this article, we experimentally investigate this side-channel in the polarization-based BB84 QKD system. The results show an obvious correlation in the output polarization states between the weak leakage light and its adjacent pulses, which will cause information to leak. More importantly, we have proposed several countermeasures, retested the side channel, and developed a mathematical framework to quantitatively assess the impact of the time-dependent side channel and these countermeasures on the secure key rate. Finally, we offer a trade-off analysis that considers defense effectiveness, pulse impact, and cost for various countermeasures in QKD. This provides a practical recommendation for improving the security of QKD systems.

DOI
10.1364/oe.528171

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High-speed is one development tendency of the practical and commercial quantum key distribution (QKD) systems, and the gain-switched semiconductor laser (GSSL) and Sagnac interferometer have been popular components for high-speed commercial polarization-based BB84 QKD systems thanks to their stability, compactness and low cost. However, due to the finite extinction ratio (ER) of the GSSL, the time-dependent source side-channel, which is theoretically presented in [Phys. Rev. A106(6): 062618 (2022)10.1103/PhysRevA.106.062618], also exists in these QKD systems. In this article, we experimentally investigate this side-channel in the polarization-based BB84 QKD system. The results show an obvious correlation in the output polarization states between the weak leakage light and its adjacent pulses, which will cause information to leak. More importantly, we have proposed several countermeasures, retested the side channel, and developed a mathematical framework to quantitatively assess the impact of the time-dependent side channel and these countermeasures on the secure key rate. Finally, we offer a trade-off analysis that considers defense effectiveness, pulse impact, and cost for various countermeasures in QKD. This provides a practical recommendation for improving the security of QKD systems.
While measurement-device-independent (MDI) quantum key distribution (QKD) allows two trusted parties to establish a shared secret key from a distance without needing to trust a central detection node, their quantum sources must be well characterized, with side channels at the source posing the greatest loophole to the protocol's security. In this paper, we identify a time-dependent side channel in a common polarization-based QKD source that employs a Faraday mirror for phase stabilization. We apply a recently developed numerical proof technique [Phys. Rev. A 99, 062332 (2019)] to quantify the sensitivity of the secret key rate to the quantum optical model for the side channel, and to develop strategies to mitigate the information leakage. In particular, we find that the MDI three-state and BB84 protocols, while yielding the same key rate under ideal conditions, have diverging results in the presence of a side channel, with BB84 proving more advantageous. While we consider only a representative case example, we expect the strategies developed and key rate analysis method to be broadly applicable to other leaky sources.
The importance of secure and high-rate information transmission cannot be understated in today's society. Unfortunately, a security crisis is impending due to the development of quantum computing technologies which can break current encryption schemes. A promising solution is quantum key distribution (QKD), which offers information-theoretic security. However, practical implementations of QKD do not conform to the idealized security assumptions that are made in QKD security proofs and therefore form security loopholes. The discovery of the measurement-device-independent QKD (MDI QKD) protocol presented a rapid leap in QKD research, as it effectively closes all side channels at the detection end of QKD setups, while maintaining experimental feasibility. Naturally, a major remaining obstacle in QKD research is the vulnerability of practical sources. As the clock rate of QKD systems advances into the GHz range, specific imperfections at the source arise. The goal of this PhD thesis is to conduct characterizations of source imperfections (especially those arising from GHz modulation), quantify their impact on security, and find methods to mitigate them. Specifically, this thesis presents the following studies. A) Identification and analysis of a novel side channel in sources using a Faraday mirror for temperature-stabilized polarization modulation, leading to time-dependent passive leakage of quantum states. We demonstrate that precise side-channel characterization (rather than the pessimistic approximations commonly used in the literature) can improve the estimated secret key rate by multiple orders of magnitude. B) Identification and analysis of a new class of source side channels (hidden multidimensional modulation side channels), using high-speed time-varying encoding as an illustrative case example (which we characterize using experimental and simulation-based methods). We demonstrate that hidden degrees of freedom, such as time, may be coupled to the encoding process, enabling unambiguous state discrimination attacks. In our illustrative case example, this causes the maximum secure transmission distance to be reduced by over 50%. C) Experimental and simulation-based characterization of phase correlations at GHz repetition rates when using electro-optic phase modulation for phase/polarization encoding. Our findings indicate that pattern-dependent phase deviations are on the order of 0.01pi. D) Design and experimental demonstration of a path-selection modulation source -- a source using no active phase modulation components (to mitigate the side channels arising from such components). Phase randomization is achieved through gain-switching and polarization encoding is achieved by randomly selecting between multiple paths, each path corresponding to one of the desired encoded states. Overall, by identifying and characterizing multiple source side channels, this thesis underscores the importance of avoiding unwarranted assumptions regarding the security of QKD sources. Hidden imperfections can introduce significant security vulnerabilities, and these must be continually identified and analyzed. Furthermore, this thesis highlights the value of advancing key rate analysis techniques that can accommodate a broad range of implementation flaws while producing tight security bounds, thereby enabling precise characterization of the information leakage arising from such imperfections. Finally, this thesis emphasizes that active modulators, in general, constitute a major source of potential side channels, highlighting the importance of exploring modulator-free sources. The path-selection modulation scheme presented here provides a practically feasible example of such a source.
Constructing high-rate quantum key distribution (QKD) systems is an effective way to increase secure key rates. However, in such systems, modulation strengthens the correlation between signaling and non-signaling states, allowing an eavesdropper to extract information from the latter. While frequency-domain side-channel leakage from light sources has been recognized, leakage arising from modulation in high-rate QKD systems has not yet been reported. In this paper, we first constructed BB84 phase and polarization coding units and observed frequency-domain variations in increasing rates of lithium niobate (LN) modulation. Using an information-theoretic model, we evaluated the correlation between frequency and coded bits, finding side-channel mutual information of 2.44 × 10 −2 and 8.81 × 10 −3 bits/pulse at 594 MHz and 668 MHz for phase and polarization units, respectively. Information leakage increased with higher modulation rate and voltage, and potential defense schemes are briefly discussed.
Side channel effects such as temporal disparity and intensity fluctuation of photon pulses caused by random bit generation with multiple laser diodes in high-speed polarization-based BB84 quantum key distribution (QKD) systems can be eliminated by increasing DC bias current condition. However, background photons caused by the spontaneous emission process under high DC bias current degrade the performance of the QKD systems. In this study, we investigated, for the first time, the effects of spontaneously emitted photons on the system performance in a high-speed QKD system at a clock rate of 400 MHz. Also, we further show improvements of system performance without side channel effects by utilizing temporal filtering technique with real-time FPGA signal processing.
Most polarization-based BB84 quantum key distribution (QKD) systems utilize multiple lasers to generate one of four polarization quantum states randomly. However, random bit generation with multiple lasers can potentially open critical side channels that significantly endangers the security of QKD systems. In this paper, we show unnoticed side channels of temporal disparity and intensity fluctuation, which possibly exist in the operation of multiple semiconductor laser diodes. Experimental results show that the side channels can enormously degrade security performance of QKD systems. An important system issue for the improvement of quantum bit error rate (QBER) related with laser driving condition is further addressed with experimental results.