Continuous-variable (CV) measurement-device-independent (MDI) quantum key distribution (QKD) can remove the feasible side-channel attacks on detectors based on the accurate Bell-state measurement (BSM), where an optical amplitude modulator (AM) plays a crucial role in managing the intensity of the transmitted light pulse. However, the AM-involved practical security has remained elusive as the operating frequency of the AM usually determines the actual secret key rate of the CV-MDI-QKD system. We find that an imperfect pulse generated from the AM at high speed can lead to a challenge to the practical security as a minor intensity change of the light pulse can bring about a potential information leakage. Taking advantage of this flaw, we suggest an attack strategy targeting the embedded AM in CV-MDI-QKD without sending the local oscillator (LO). This attack can damage the AM and thus decrease the estimated secret key rate of the system even when the orthogonal local LO (LLO) scheme is carried out. To assess the practical security risk resulting from the leaked information from the AM, we conduct numerical simulations to demonstrate the influence of the AM on the CVMDI-QKD system.
Continuous-variable (CV) measurement-device-independent (MDI) quantum key distribution (QKD) can remove the feasible side-channel attacks on detectors based on the accurate Bell-state measurement (BSM), where an optical amplitude modulator (AM) plays a crucial role in managing the intensity of the transmitted light pulse. However, the AM-involved practical security has remained elusive as the operating frequency of the AM usually determines the actual secret key rate of the CV-MDI-QKD system. We find that an imperfect pulse generated from the AM at high speed can lead to a challenge to the practical security as a minor intensity change of the light pulse can bring about a potential information leakage. Taking advantage of this flaw, we suggest an attack strategy targeting the embedded AM in CV-MDI-QKD without sending the local oscillator (LO). This attack can damage the AM and thus decrease the estimated secret key rate of the system even when the orthogonal local LO (LLO) scheme is carried out. To assess the practical security risk resulting from the leaked information from the AM, we conduct numerical simulations to demonstrate the influence of the AM on the CVMDI-QKD system.
An amplitude modulator plays an essential role in the implementation of continuous-variable quantum key distribution (CVQKD), whereas it may bring about a potential security loophole in the practical system. The high-frequency modulation of the actual transmitter usually results in the high rate of the system. However, an imperfect amplitude modulator (AM) can give birth to a potential information leakage from the modulation of the transmitter. To reveal a potential security loophole from the high-frequency AM embedded in the transmitter, we demonstrate an influence on the practical security of the system in terms of the secret key rate and maximal transmission distance. The results indicate the risk of this security loophole in the imperfect AM-embedded transmitter. Fortunately, the legal participants can trace back the potential information leakage that has been produced from the imperfect transmitter at high frequencies, which can be used for defeating the leakage attack in CVQKD. We find the limitations of the imperfect AM-embedded transmitter of the high-frequency quantum system, and hence, we have to trade off the practical security and the modulation frequency of the AM-embedded transmitter while considering its implementation in a practical environment.
Continuous-variable measurement-device-independent quantum key distribution (CV-MDI QKD) can defend all detection-side attacks effectively. Therefore, the source side is the final battlefield for performing quantum hacking attacks. This paper investigates the practical security of a CV-MDI QKD system under a light-injection attack. Here, we first describe two different light-injection attacks, i.e., the induced-photorefractive attack and the strong-power injection attack. Then, we consider three attack cases where Eve only attacks one of the parties or both parties of the CV-MDI QKD system. Based on the analysis of the parameter estimation, we find that the legitimate communication parties will overestimate the secret key rate of the system under the effect of a light-injection attack. This opens a security loophole for Eve to successfully obtain secret key information in a practical CV-MDI QKD system. In particular, compared to the laser-damage attack, the above attacks use a lower power of injected light and have a more serious effect on the security of the system. To eliminate the above effects, we can enhance the practical security of the system by doping the lithium niobate material with various impurities or by using protective devices, such as optical isolators, circulators, optical power limiters, and narrow-band filters. Apart from these, we can also use an intensity monitor or a photodetector to detect the light-injection attack.
Continuous-variable measurement-device-independent quantum key distribution (CV-MDI-QKD) is immune to all detector side channel attacks, however its security requires trusted sources. We investigate the effects of source intensity errors, the most crucial source error, on the security of CV-MDI-QKD protocol with Gaussian-modulated coherent states. We establish a general source intensity errors model and derive the secret key rate based on different assumptions on the abilities of legal parties Alice and Bob. To avoid the adverse effects of the sources' intensity errors on CV-MDI-QKD, we present different data-processing schemes. We also assess the security of the protocol against collective Gaussian attacks in the finite-size scenario with composable security. Taking source intensity errors into consideration, we achieve improved security compared to previous demonstrations. Our results will provide a useful reference for practical implementations of CV-MDI-QKD.
Continuous-variable measurement-device-independent quantum key distribution (CV-MDI QKD) can fundamentally defend all attacks on the measurement devices. Consequently, higher requirements are put forward for the source of a CV-MDI QKD system. However, the imperfections of the actual source brings practical security risks to the CV-MDI QKD system. Therefore, the characteristics of the realistic source must be controlled in real time to guarantee the practical security of the CV-MDI QKD system. Here we propose a countermeasure for negative impact introduced by the actual source in the CV-MDI QKD system based on the one-time-calibration method, not only eliminating the loophole induced from the relative intensity noise (RIN), which is part of the source noise, but also modeling the source noise thus improves the performance. In particular, three cases in terms of whether the preparation noise of the practical sources are defined or not, where only one of the users or both of the two users operate monitoring on their respective source outputs, are investigated. The simulation results show that the estimated secret key rate without our proposed scheme is about 10.7 times higher than the realistic rate at 18 km transmission distance when the variance of RIN is only 0.4. What is worse, the difference becomes greater and greater with the increase of the variance of RIN. Thus, our proposed scheme makes sense in further completing the practical security of the CV-MDI QKD system. In other words, our work enables the CV-MDI QKD system not only to resist all attacks against detectors, but also to close the vulnerability caused by the actual source, thus making the scheme closer to practical security.
Measurement-device-independent quantum key distribution (MDI-QKD) can remove all side-channel attacks on detectors. In the context of the dramatic progress of discrete-variable MDI-QKD and twin-field QKD, owing to the critical challenge of continuous-variable (CV) Bell-state measurement (BSM) of two remote independent quantum states, experimental demonstration of CV-MDI-QKD over optical fiber has remained elusive. To solve this problem, a technology for CV-BSM of remote independent quantum states is developed that consists of optical phase locking, phase estimation, real-time phase feedback, and quadrature remapping in the present work. With this technology, CV-BSM is accurately implemented, and the first CV-MDI-QKD over optical fiber is demonstrated, to our knowledge. The achieved secret key rates are 0.43 (0.19) bits per pulse over a 5-km (10-km) optical fiber. Our work shows that it is feasible to build a CV-MDI-QKD system over optical fiber. Further, the results pave the way towards realization of a high secret key rate and low-cost metropolitan MDI-QKD network, and serve as a stepping stone to a CV quantum repeater.
Continuous-variable measurement-device-independent quantum key distribution (CV-MDI-QKD) allows remote parties to share information-theoretical secure keys while defending all the side-channel attacks on measurement devices. However, the secure transmission distance and the secret key rate are quite limited due to the high untrusted equivalent excess noise in the Gaussian modulation. More particularly, extremely high-efficiency homodyne detections are required for even non-zero secure transmission distances, which directly restrict its practical realization. Here, we propose a CV-MDI-QKD protocol by encoding the key information into matched discrete phases of two groups of coherent states, which decreases the required detection efficiency for ideally asymmetric cases, and makes it possible to practically achieve secure key distribution with current low-efficiency homodyne detections. Besides, a proof-of-principle experiment with a locally generated oscillator is implemented, which, for the first time, demonstrates the realizability of CV-MDI-QKD using all fiber-based devices. The discrete-modulated phase-matching method provides an alternative direction of an applicable quantum key distribution with practical security.