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New Journal of Physics

Saturation attack of the microwave-propagated continuous variable quantum key distribution in open air

Wenqi Peng, Xuan Li, Yongxin Feng, Xuekai Chen, Ying Guo

Published Jul 1, 2026• 1 citation
Publisher

Abstract

Abstract Continuous variable quantum key distribution (CVQKD) can achieve the secure communication between two distant legitimate users. The microwave CVQKD exhibits stronger robustness than the optical CVQKD in transmitting quantum signals and accordingly has broad application prospects in quantum networks. Although the microwave CVQKD does not rely on the traditional optical fibers to transmit quantum signals, its practical security is still vulnerable to saturation attacks. In view of this security vulnerability, we generalize saturation attacks to microwave CVQKD and clarify the impact of saturation attacks on the parameter estimation of microwave systems. Two saturation attack schemes are further compared, one based on displacement control alone and the other on joint displacement and gain control. To mitigate the negative impacts caused by this attack, we propose a countermeasure based on the quantum k nearest neighbor algorithm, which can detect saturation attacks by measuring unknown quantum states. In addition, the influence of microwave antennas on the security of the communication system is assessed, confirming that antenna gain plays a key role in determining both the secret key rate and the maximum transmission distance.

DOI
10.1088/1367-2630/ae856d

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Abstract Continuous variable quantum key distribution (CVQKD) can achieve the secure communication between two distant legitimate users. The microwave CVQKD exhibits stronger robustness than the optical CVQKD in transmitting quantum signals and accordingly has broad application prospects in quantum networks. Although the microwave CVQKD does not rely on the traditional optical fibers to transmit quantum signals, its practical security is still vulnerable to saturation attacks. In view of this security vulnerability, we generalize saturation attacks to microwave CVQKD and clarify the impact of saturation attacks on the parameter estimation of microwave systems. Two saturation attack schemes are further compared, one based on displacement control alone and the other on joint displacement and gain control. To mitigate the negative impacts caused by this attack, we propose a countermeasure based on the quantum k nearest neighbor algorithm, which can detect saturation attacks by measuring unknown quantum states. In addition, the influence of microwave antennas on the security of the communication system is assessed, confirming that antenna gain plays a key role in determining both the secret key rate and the maximum transmission distance.
We identify and study a security loophole in continuous-variable quantum key distribution (CVQKD) implementations, related to the imperfect linearity of the homodyne detector. By exploiting this loophole, we propose an active side-channel attack on the Gaussian-modulated coherent-state CVQKD protocol combining an intercept-resend attack with an induced saturation of the homodyne detection on the receiver side (Bob). We show that an attacker can bias the excess noise estimation by displacing the quadratures of the coherent states received by Bob. We propose a saturation model that matches experimental measurements on the homodyne detection and use this model to study the impact of the saturation attack on parameter estimation in CVQKD. We demonstrate that this attack can bias the excess noise estimation beyond the null key threshold for any system parameter, thus leading to a full security break. If we consider an additional criterion imposing that the channel transmission estimation should not be affected by the attack, then the saturation attack can only be launched if the attenuation on the quantum channel is sufficient, corresponding to attenuations larger than approximately 6 dB. We moreover discuss the possible countermeasures against the saturation attack and propose a countermeasure based on Gaussian postselection that can be implemented by classical postprocessing and may allow one to distill the secret key when the raw measurement data are partly saturated.
Abstract Saturation attack of imperfect detectors compromises the practical security of continuous-variable quantum key distribution (CVQKD) in implementations. In this paper, we demonstrate the practical security of the CVQKD system with finite-size analysis when considering effects of saturation attack on imperfect detectors at the receiver. We suggest an approach to resisting this attack by embedding an adjustable optical filter in the detector, motivated by characteristics of saturation attack. Numerical simulations illustrate the practical security of the system with finite-size analysis in terms of the achieved secret key rate and maximal transmission distance. It indicates that this approach can be elegantly used for defeating the potential saturation attack in practical quantum networks.
Continuous-variable quantum key distribution (CV-QKD) enables the efficient distribution of unconditionally secure keys for both communicating parties. Despite significant advancements in optical-based CV-QKD, a persistent challenge is the limited resilience of optical quantum signals to atmospheric noise in free-space channels. This susceptibility leads to considerable degradation in the performance of optical CV-QKD systems, particularly under adverse climatic conditions. Meanwhile, recent advancements in superconducting quantum computing have driven the demand for quantum communication between spatially separated superconducting processors operating at microwave frequencies. To solve the problems, we propose a Gaussian-modulated coherent-state protocol operating at microwave frequencies, utilizing traveling-wave parametric amplifiers to generate entangled states. Our numerical analysis demonstrates that CV-QKD at microwave frequencies enables secure communication in a free-space channel at room temperature. Furthermore, we show that this protocol outperforms the THz frequency protocol over a distance of 20 m and exhibits stronger immunity to interference. Finally, we consider the impact of microwave antenna gain on the protocol's performance, highlighting the decisive role of antenna gain in determining the maximum transmission distance in practical application scenarios.
A saturation attack can be employed for compromising the practical security of continuous-variable quantum key distribution (CVQKD). In this paper, we suggest a countermeasure approach to resisting this attack by embedding an adjustable optical filter (AOF) in the CVQKD system. Numerical simulations illustrate the effects of the AOF-enabled countermeasure on the performance in terms of the secret key rate and transmission distance. The legal participants can trace back the information that has been eavesdropped by an attacker from the imperfect receiver, which indicates that this approach can be used for defeating a saturation attack in practical quantum communications.
Continuous-variable quantum key distribution (CVQKD) over an atmosphere channel provides the possibility of global coverage, which is threatened by the imperfection of actual devices. Exploiting imperfections of practical devices, an eavesdropper steals the information between legal participants by performing the potential saturation-induced attacks on the homodyne detector. In this paper, we demonstrate the feasibility of the potential saturation attack in discretely modulated (DM) CVQKD over atmospheric turbulence. Subsequently, a countermeasure is suggested by using an adjustable optical filter (AOF) to attenuate the estimated parameter with the measured value in the finite linearity domain, where the attenuated data can be compensated. Numerical simulations illustrate the effects of the saturation attack on the performance of the AOF-based CVQKD system. We find that legal participants evaluate the information that has been eavesdropped because the secret key rate is made negative.
Discretely modulated continuous-variable quantum key distribution (DM CV-QKD) protocols, while promising for extended transmission distances, face significant security threats in practice due to discrepancies between theoretical and actual system models. A critical vulnerability lies in the differential detector, which can be exploited by an eavesdropper to launch saturation attacks. In this paper, we conduct a comprehensive analysis of saturation attacks specifically targeting DM CV-QKD systems and propose a novel machine learning-based defense scheme utilizing the K-Nearest Neighbors (KNN) algorithm for attack detection. The simulation results confirmed that saturation attacks can undermine the actual security of the DM CVQKD system. The defense scheme proposed in this paper can accurately identify the probability of saturation attacks at 96.5%, and has been proven to be effective in defending against saturation attacks.