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.
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.
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.
Practical security of the continuous-variable quantum key distribution (CVQKD) system with finite sampling bandwidth of analog-to-digital converter (ADC) at the receiver's side is investigated. We find that the finite sampling bandwidth effects may decrease the lower bound of secret key rate without awareness of the legitimate communicators. This leaves security loopholes for Eve to attack the system. In addition, this effect may restrains the linear relationship of secret key bit rate with repetition rate of the system; subsequently, there is a saturation value for the secret key bit rate with the repetition rate. To resist such kind of effects, we propose a dual sampling detection approach in which two ADCs are employed so that the finite sampling bandwidth effects are removed.
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, 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.
How to guarantee the practical security of continuous-variable quantum key distribution (CVQKD) system has been an important issue in the quantum cryptography applications. In contrast to the previous practical security strategies, which focus on the intercept-resend attack or the Gaussian attack, we investigate the practical security strategy based on a general attack, i.e., an arbitrated individual attack or collective attack on the system by Eve in this paper. The low bound of intensity disturbance of the local oscillator signal for eavesdropper successfully concealing herself is obtained, considering all noises can be used by Eve in the practical environment. Furthermore, we obtain an optimal monitoring condition for the practical CVQKD system so that legitimate communicators can monitor the general attack in real-time. As examples, practical security of two special systems, i.e., the Gaussian modulated coherent state CVQKD system and the middle-based CVQKD system, are investigated under the intercept-resend attacks.
Recently, several practical attacks on continuous-variable quantum key distribution (CVQKD) were proposed based on faking the estimated value of channel excess noise to hide the intercept-and-resend eavesdropping strategy, including the local oscillator (LO) fluctuation, calibration, wavelength, and saturation attacks. However, the known countermeasures against all these practical attacks will inevitably increase the complexity of the implementation of CVQKD and affect its performance. We develop here an asynchronous countermeasure strategy without structural modifications of the conventional CVQKD scheme. In particular, two robust countermeasures are proposed by adding peak-valley seeking and Gaussian postselection steps in conventional data postprocessing procedure. The analysis shows that the peak-valley seeking method naturally make the schemes immune to all known types of calibration attacks even when Eve simultaneously performs wavelength or LO fluctuation attacks and exhibit simpler implementation and better performance than the known countermeasures. Meanwhile, since the Gaussian postselection is able to resist the saturation attacks, the proposed schemes are secure against all known types of practical attacks.