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Optics Express

Forced carrier perturbation opens a loophole for chip-based continuous variable quantum key distribution system

Lang Li, Peng Huang, Tao Wang, Han-Wen Yin, Guihua Zeng

Published Aug 20, 2024• 3 citations
Publisher

Abstract

The integration of the continous-variable quantum key distribution(CVQKD) system is an important technical route with great potential value for constructing high-performance and low-cost CVQKD system. In all previous CVQKD studies, the quantum efficiency of the detector can be calibrated in advance and is considered to remain unchanged. But when the size of the system shrinks to the on-chip level, especially in the premise of non-uniform waveguide, heavy doping and other factors, effects such as free carrier absorption and scattering loss will become prominent, which will directly cause carriers undergo violent migration due to the tiny jitter of the local oscillator and further lead to dynamical variation of quantum efficiency of detection. In this paper, we propose a practical chip-based detector model, and numerous simulation results based on this model show that the practical system will face potential security threats due to the variable quantum efficiency. Moreover, two defense strategies are proposed to solve these practical security problems commonly exist in general chip-based CVQKD systems. This work breaks the inherent viewpoint that the quantum efficiency in the chip-based CVQKD system can still be calibrated in advance, and suggests a more rigorous consideration of practical security for development of chip-based CVQKD.

DOI
10.1364/oe.531996

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The integration of the continous-variable quantum key distribution(CVQKD) system is an important technical route with great potential value for constructing high-performance and low-cost CVQKD system. In all previous CVQKD studies, the quantum efficiency of the detector can be calibrated in advance and is considered to remain unchanged. But when the size of the system shrinks to the on-chip level, especially in the premise of non-uniform waveguide, heavy doping and other factors, effects such as free carrier absorption and scattering loss will become prominent, which will directly cause carriers undergo violent migration due to the tiny jitter of the local oscillator and further lead to dynamical variation of quantum efficiency of detection. In this paper, we propose a practical chip-based detector model, and numerous simulation results based on this model show that the practical system will face potential security threats due to the variable quantum efficiency. Moreover, two defense strategies are proposed to solve these practical security problems commonly exist in general chip-based CVQKD systems. This work breaks the inherent viewpoint that the quantum efficiency in the chip-based CVQKD system can still be calibrated in advance, and suggests a more rigorous consideration of practical security for development of chip-based CVQKD.
A chip-based continous-variable quantum-key-distribution (CVQKD) system with a high practical confidentiality performance is crucial for constructing quantum metropolitan communication networks, but imperfections in the chip-based modulation will threaten the practical security of the chip-based CVQKD system. In this paper, we combine the plasma dispersion effect of free carriers to model the carrier fluctuations and reveal the essential mechanism of carrier fluctuations' influence on the system. The simulations show that the chip-based CVQKD system may face potential loophole threats or its performance will dramatically decrease under different carrier fluctuations. Moreover, two preliminary defense strategies are proposed to completely solve the practical security problems commonly induced by modulators in general chip-based CVQKD systems. This work proposes a set of modeling and analysis methods for general chip-based CVQKD systems' modulators, which provides constructive methods to build the chip-based CVQKD system with more rigorous practical security.
In a practical continuous-variable quantum-key distribution (CVQKD), the fluctuations of the local oscillator (LO) not only make the normalization of Bob's measurement outcomes difficult, but also can change the signal-to-noise ratio (SNR) of an imperfect balanced homodyne detector (BHD), which may lead the security of a practical system of CVQKD to be compromised severely. In this paper, we propose that the LO intensity can be manipulated by the legitimate parties, i.e., being tuned and stabilized to a required constant value, to eliminate the impact of LO fluctuations and defeat Eve's potential attack on the LO. Moreover, we show that the secret key rate can be increased over a noisy channel, especially the channels of metropolitan QKD networks, by tuning the intensity of LO and thus the SNR of a practical BHD to an optimal value, and we find that, counterintuitively, the requirement on BHD (i.e., high detection efficiency and low electronic noise) can also be reduced in this case. To realize this manipulation, we give a schematic setup which, thus, can be used to enhance the security of a practical CVQKD system.
When developing a practical continuous-variable quantum key distribution (CVQKD), the detector is necessary at the receiver’s side. We investigate the practical security of the CVQKD system with an unbalanced heterodyne detector. The results show that unbalanced heterodyne detector introduces extra excess noise into the system and decreases the lower bound of the secret key rate without awareness of the legitimate communicators, which leaves loopholes for Eve to attack the system. In addition, we find that the secret key rate decreases more severely with the increase in the degree of imbalance and the excess noise induced by the imbalance is proportional to the intensity of the local oscillator (LO) under the same degree of imbalance. Finally, a countermeasure is proposed to resist these kinds of effects.
Continuous-variable quantum key distribution (CVQKD) has attracted extensive attention due to its compatibility and low costs. However, bandwidth mismatch exists to varying degrees between the transmitter and receiver. This may prevent frequency components carrying modulation information from being fully perceived by the legitimate party. In this paper, we identify a practical security loophole caused by bandwidth mismatch and propose a corresponding spectral attack scheme. Different from previous approaches that exploit security loopholes to conceal the excess noise introduced by intercept-resend attacks, this scheme can directly obtain raw-key information without introducing additional disturbances. A proof-of-principle attack on a CVQKD system with filtering operation is constructed to verify the feasibility. Experimental results indicate that Eve can obtain enough information to render the system insecure if this practical security loophole is ignored. Based on the identified security loophole, corresponding defense strategies are proposed. This work helps bridge the gap between theoretical models and practical implementations, providing a reference for defense design in practical quantum communication systems.
Integrated continuous-variable quantum key distribution (CV-QKD) systems based on photonic chips offer a promising avenue for establishing large-scale and low-cost global quantum networks. In such systems, the grating coupler (GC) serves as an interface for optical coupling, but its strongly wavelength-dependent efficiency creates a severe security loophole in practice. In this paper, we propose a wavelength-shifting attack targeting the wavelength-dependent GC in chip-based CV-QKD systems. By intercepting Alice’s quantum states and resending forged quantum states together with a local oscillator (LO), both at a shifted wavelength, this attack manipulates the receiver’s excess-noise estimation, effectively masking the excess noise introduced by Eve. Numerical simulations show that the attack can keep Bob’s estimated excess noise below the security threshold, leading Alice and Bob to infer a positive secret key rate while the actual secure key rate is zero. This research reveals how the integrated components can be exploited to mount side-channel attacks. To mitigate this vulnerability, we propose practical defenses, including on-chip wavelength monitoring and randomized shot-noise calibration. These findings provide important guidance for the secure development of next-generation integrated quantum communication systems.
In a practical continuous-variable quantum key distribution (CVQKD) system, the optical attenuator can adjust the Gaussian-modulated coherent states and the local oscillator signal to an optimal value for guaranteeing the security of the system and optimizing the performance of the system. However, the performance of the optical attenuator may deteriorate due to the intentional and unintentional damage of the device. In this paper, we investigate the practical security of a CVQKD system with reduced optical attenuation. We find that the secret key rate of the system may be overestimated based on the investigation of parameter estimation under the effects of reduced optical attenuation. This opens a security loophole for Eve to successfully perform an intercept-resend attack in a practical CVQKD system. To close this loophole, we add an optical fuse at Alice's output port and design a scheme to monitor the level of optical attenuation in real time, which can make the secret key rate of the system evaluated precisely. The analysis shows that these countermeasures can effectively resist this potential attack.