We consider the security of practical continuous-variable quantum-key-distribution implementation with the local oscillator (LO) fluctuating in time, which opens a loophole for Eve to intercept the secret key. We show that Eve can simulate this fluctuation to hide her Gaussian collective attack by reducing the intensity of the LO. Numerical simulations demonstrate that if Bob does not monitor the LO intensity and does not scale his measurements with the instantaneous intensity values of LO, the secret key rate will be compromised severely.
We consider the security of practical continuous-variable quantum-key-distribution implementation with the local oscillator (LO) fluctuating in time, which opens a loophole for Eve to intercept the secret key. We show that Eve can simulate this fluctuation to hide her Gaussian collective attack by reducing the intensity of the LO. Numerical simulations demonstrate that if Bob does not monitor the LO intensity and does not scale his measurements with the instantaneous intensity values of LO, the secret key rate will be compromised severely.
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.
In practical continuous-variable quantum key distribution (CVQKD) systems, due to environmental disturbance or some intrinsic imperfections of devices, inevitably the local oscillator (LO) employed in a coherent detection always fluctuates arbitrarily over time, which compromises the security and performance of practical CVQKD systems. In this paper, we investigate the performance of practical CVQKD systems with LO fluctuating randomly. By revising the measurement result of balanced homodyne detection and embedding fluctuation parameters into security analysis, we find that in addition to the average LO intensity, the fluctuation variance also severely affects the secret key rate. No secret key can be obtained if fluctuation variance is relatively large. This indicates that in a practical CVQKD, LO intensity should be well monitored and stabilized. Our research can be directly applied to improve the robustness of a practical CVQKD system as well as be used to optimize CVQKD protocols.
In a practical continuous-variable quantum-key-distribution (CVQKD) system, a strong local oscillator (LO) signal in a quantum channel may be manipulated by eavesdropper Eve to steal information about secret keys through general attacks (e.g., entangling cloner attack or intercept-resend attack) without being detected, which is an effective quantum hacking strategy, i.e., LO attacks. To guarantee the practical security of the system, a general monitoring scheme has been previously demonstrated to resist this quantum hacking attack, where the channel transmittance was regarded as a fixed value related to transmission distance. However, practical communication environments are complex, which may result in the time-varying transmittance. This deviation may affect the effectiveness of this monitoring scheme. In this paper, we investigate the monitoring of CVQKD systems running in complex communication environments. We first model the LO attacks on practical CVQKD systems in complex environments, where the channel transmittance is assumed to obey a fixed distribution. Then, the low bound of intensity disturbance of the LO signal for Eve successfully concealing herself is obtained based on this model, where we consider all noise that can be used by Eve in complex communication environments. Simultaneously, we obtain an optimal monitoring condition to resist the LO attacks. Our numerical analysis confirms that the proposed monitoring scheme can effectively resist the LO attacks on practical CVQKD systems in complex environments. Subsequently, the feasibility of this way is again demonstrated by using a quantitative example. It is important that this monitoring scheme can also be extended to resist other quantum hacking attacks.
Abstract In the practical continuous-variable source-independent quantum random number generator, it is usually necessary to introduce a local oscillator to amplify the received quantum signal due to its weak intensity. However, the intensity fluctuation of the local oscillator itself will lead to a deviation in evaluating the randomness without monitoring the local oscillator in real time and calibrating each output measurement, which will inevitably open a loophole for the eavesdropper to intercept the information of generated random numbers. Here, we present an eavesdropping attack utilizing this loophole. The results of numerical simulation indicate that when the proposed eavesdropping attack is performed, the evaluated extractable randomness value will be compromised severely, which will impact the communication security under the practical conditions. Meanwhile, we analyze the effects of practical imperfections in the presence of local oscillator fluctuation. Taking this as a reference, we may consider how to perfect the experimental system in practice and how to deal with this kind of eavesdropping attack.
In practical continuous-variable quantum key distribution system using local local oscillator (LLO CV-QKD), the phase noise related to coherent detection and phase-reference pulse intensity can be locally calibrated at the receiver side. Therefore, it can be regarded as trusted noise in order to obtain a higher secret key rate and transmission distance. However, security loopholes will be opened for the eavesdropper to launch attacks without appropriate countermeasures. To this end, we propose two practical phase-reference pulse intensity attack strategies, where Eve can reduce the trusted phase noise by manipulating the intensity of the phase-reference pulse, thereby hiding her attack on the signal pulse. The performance of the LLO CV-QKD system under these attacks has been analyzed. We show that precisely monitoring the intensity of the phase-reference pulse in real time is an essential countermeasure to prevent the proposed attacks. Moreover, an intensity-monitoring scheme for the phase-reference pulse is proposed to strengthen the security of the practical LLO CV-QKD system and make the trusted phase noise model more robust.
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.