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.
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.
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.
Quantum key distribution (QKD) provides an information-theoretic secure method for communication. Measurement-device-independent QKD (MDI-QKD) demonstrates immunity to all detection-side vulnerabilities while offering exceptional security and practicality. Nevertheless, imperfections in source-side devices may still introduce security risks that necessitate precise characterization and evaluation in practical security analyses. In this study, we demonstrate that the inherent frequency-shifting properties of the Mach-Zehnder intensity modulator can induce distinguishable features between signal and decoy states in the frequency domain, fundamentally undermining the core premise of the decoy-state protocol. Experimental characterization reveals that such spectral distinguishability exhibits strong dependence on three key parameters: modulation voltage, pulse width, and modulation scheme. Leveraging this vulnerability, we develop a frequency side channel attack framework applicable to both MDI-QKD and its variant mode-pairing QKD (MP-QKD), evaluate its impact on protocol security, and ultimately devise corresponding countermeasures.
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.
Nowadays, quantum key distribution (QKD) has gradually moved from the laboratory to practical applications. However, the inherent imperfections of practical QKD devices would cause side channels. Among them, there is one critical side channel vulnerability that is often overlooked, i.e., information leakage of the electromagnetic field (LEMF). In this work, we carry out systematic investigations on the LEMF by utilizing a practical phase-coding BB84 QKD system as an example. We demonstrate that LEMF from the widely used modulators could be exploited by a malicious eavesdropper to obtain corresponding encoding information assisted with artificial intelligence algorithms, posing a serious threat for practical QKD systems. To solve this problem, we conduct security analysis on LEMF and build a mathematical model on the final secure key rate. Moreover, we give out corresponding countermeasures to reduce information leakage and thus enhance practical security of QKD systems. Therefore, our current work can provide important theoretical and experimental foundations for the future development of secure large-scale quantum communication networks.
In recent years, quantum communication has received extensive attention due to its high security of transmitted information. Quantum key distribution (QKD), an important branch of quantum information, is developing rapidly and has been gradually moving toward practicality and networking. The use of phase coding through fiber optic channels is the basis for the implementation of QKD systems. In QKD systems, electro-optical modulation techniques are mainly used to change the photon phase through phase modulators to realize the phase coding scheme. Among them, lithium niobate is a common material for making phase modulators in QKD systems. Lithium niobate (LN) crystals are an optical material with excellent acousto-optical and electro-optical properties. It has good physical and chemical stability, a wide optical low-loss window, a large electro-optical coefficient and an excellent second-order nonlinear effect. It has a wide range of applications in high-speed electro-optical tuning, holographic storage, nonlinear frequency conversion, etc. Thin-film lithium niobate (LNOI), as a new integrated optical material, can well combine the excellent electro-optical, acousto-optical and nonlinear properties of the material with a compact optical waveguide. It also has the advantages of a small waveguide cross-section size, high electric field density, strong nonlinear effect, low half-wave voltage length product, and small size. It has significant advantages in the integration of optoelectronic devices. In the phase-coded QKD system, the coding object of the information is the phase of the optical signal. The polarization state of the optical signal can have a serious impact on the system. The phase-encoded QKD system based on the Faraday-Michaelson interference loop is able to self-compensate for the polarization variations in the system to remove the relevant effects of polarization variations on the QKD system. The application of a phase modulator based on thin film lithium niobate preparation in quantum key distribution can effectively enhance the rate of the quantum key distribution system. However, there is still a need to study the transmission and modulation characteristics of LNOI waveguides on polarized optical signals. In this paper, we develop a phase modulator based on thin-film lithium niobate for high-speed QKD systems. Simulation and analysis of the polarization mode of the optical signal transmitted in the optical waveguide. Test and study the transmission loss and modulation efficiency difference of a thin-film lithium niobate optical waveguide for TE and TM polarization state optical signals. To build a test system for application to the measurement and modulation test of the polarization state of optical signals in a high-speed phase-encoded QKD system.
In polarization-encoding quantum key distribution (QKD) systems, achieving high-speed, robust, and low-error polarization modulation helps to increase the secure key rate (SKR). Moreover, the advanced QKD protocols place higher demands on the system’s photon preparation capability, which implies providing additional polarization states for practical implementation. Here, we redesign the Sagnac interferometer and utilize the non-reciprocity of the lithium niobate modulator to eliminate the reverse modulation that limits the modulation speed. The proposed scheme enables high-speed, robust, and low-error modulation with arbitrary polarization states. The average bit error rate (BER) for the 10 min measurement is 0.2332% with a system repetition frequency of 16.5 GHz.