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Journal of Physics Conference Series

Passive Local-oscillator Monitoring Assisted Parameter Estimation for Continuous-variable Quantum Key Distribution

Yun Shao, Yan Pan, Heng Wang, Mingli Zhu, Yaodi Pi, Ting Ye, YaZhuo Jiang, Li Ma, Yang Li, Wei Huang, Bingjie Xu

Published Aug 1, 2026• 0 citations
Publisher

Abstract

Abstract Continuous-variable quantum key distribution (CV-QKD) relies on the local oscillator (LO) not only as the phase reference for coherent detection but also as the basis for shot-noise unit (SNU) calibration in parameter estimation. In practical implementations, random LO intensity fluctuations distort the SNU normalization and thus bias the estimation of channel transmittance and excess noise, leading to inaccurate secret-key-rate evaluation. In this paper, we propose a passive LO-monitoring assisted parameter-estimation method for Gaussian-modulated coherent-state (GMCS) CV-QKD protocol. In the proposed scheme, the SNU is calibrated once using the average LO intensity before key distribution, while the LO fluctuation variance is monitored in real time through a passive monitoring branch. By incorporating the monitored fluctuation information into the parameter-estimation model, the bias induced by LO intensity fluctuations can be compensated without resorting to active real-time SNU calibration. Numerical simulations show that the proposed method yields estimated secret key rates close to those obtained under ideal LO conditions and significantly outperforms the conventional non-real-time SNU calibration method when the LO fluctuations become pronounced. The method provides a simple and practical way to improve the robustness and reliability of GMCS CV-QKD systems.

DOI
10.1088/1742-6596/3293/1/012010

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Abstract Continuous-variable quantum key distribution (CV-QKD) relies on the local oscillator (LO) not only as the phase reference for coherent detection but also as the basis for shot-noise unit (SNU) calibration in parameter estimation. In practical implementations, random LO intensity fluctuations distort the SNU normalization and thus bias the estimation of channel transmittance and excess noise, leading to inaccurate secret-key-rate evaluation. In this paper, we propose a passive LO-monitoring assisted parameter-estimation method for Gaussian-modulated coherent-state (GMCS) CV-QKD protocol. In the proposed scheme, the SNU is calibrated once using the average LO intensity before key distribution, while the LO fluctuation variance is monitored in real time through a passive monitoring branch. By incorporating the monitored fluctuation information into the parameter-estimation model, the bias induced by LO intensity fluctuations can be compensated without resorting to active real-time SNU calibration. Numerical simulations show that the proposed method yields estimated secret key rates close to those obtained under ideal LO conditions and significantly outperforms the conventional non-real-time SNU calibration method when the LO fluctuations become pronounced. The method provides a simple and practical way to improve the robustness and reliability of GMCS CV-QKD systems.
Abstract Source monitoring is a matter of vital importance for continuous-variable quantum key distribution (QKD), since it is the first barrier to ensure the practical security. In a practical scenario, the difficulty arises with the existence of the laser intensity fluctuation, which not only contributes to the source noise but also causes the deviation of source monitoring result. We first illustrate the effect of the relative intensity noise (RIN) on the ultimate security by explicitly showing the parameter estimation procedure, and find that the secret key rate is overestimated in this situation. To defend the continuous-variable QKD system against potential attacks utilizing this impairment to conceal itself, a source monitoring scheme is devised by adopting the seminal one-time shot-noise unit calibration approach into the monitor module. It is shown that by using the practical source monitoring scheme the impact of the RIN can be modeled and monitored in real time to circumvent an overestimated secret key rate. This scheme also lowers the system complexity thus is suitable for widespread application.
Abstract The shot-noise unit (SNU) is a crucial factor for the practical security of a continuous-variable quantum key distribution system. In the most widely used experimental scheme, the SNU should be calibrated first and acts as a constant during the key distribution. However, the SNU of a practical system is dependent on the various parameters of the local oscillator (LO), which can be controlled by the eavesdropper in the open channel. In this paper, we report a quantum hacking method to control the practical SNU by using the limited compensation rate of the polarization compensation. Since the compensation is only based on of the polarization measurement results of part of LO pulses, the polarization of other unmeasured pulses may not be compensated correctly, which can be utilized by the eavesdropper to control the practical SNU. The simulation and experiment results indicate that the practical SNU can be controlled by the eavesdropper. Thus, the eavesdropper can use the fact that the practical SNU is no longer equals to the calibrated one to control the excess noise and final key rate.
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 that can be locally calibrated at the receiver side is considered to be trusted noise to improve the performance. However, if the intensity of the phase-reference pulse is not monitored precisely in real-time, a security loophole will be opened for the eavesdropper to launch attacks. 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 if the total excess noise remains unchanged. 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 continuous-variable quantum key distribution with a local local oscillator (LLO CV-QKD), the phase noise after phase compensation is mainly composed of three parts: the relative phase drift noise, the relative phase accumulated noise, and the phase-reference measurement noise. Here, we develop a practical phase noise model for LLO CV-QKD protocols, in which part of the phase-reference measurement noise that can be locally calibrated by Bob at the receiver side is considered to be trusted noise. It is reasonable to adopt this model because Bob”s apparatus is inaccessible to the eavesdropper. We show that the secret key rate and the transmission distance of the LLO CV-QKD system under the practical phase noise model is significantly improved in comparison to that under the conventional phase noise model. We show that the secret key rate is improved more than 65% and the transmission distance is increased more than 70% with some typical simulation parameters.
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 continuous-variable quantum key distribution system with a true local oscillator (LLO CV-QKD), part of the phase noise associated with the coherent detector and the phase-reference intensity can be considered as trusted because which can be locally calibrated at the receiver’s side. The trusted phase noise model can significantly improve the noise tolerance of the system since the phase noise is the major excess noise. However, the transmission of the phase-reference pulse through the insecure quantum channel in the LLO CV-QKD system may leave rooms for the eavesdropper to mount attacks. Here, we propose a practical and flexible phase-reference intensity attack scheme using a phaseinsensitive amplifier to amplify the intensity of the phase-reference pulse. In this case, the eavesdropper can compromise the security of the LLO CV-QKD system severely by lowering the trusted part of the phase noise to compensate her increased attack on the signal pulse while the total excess noise is unchanged. We simulate the secure key rate with respect to the transmission distance to show that precisely monitoring the instantaneous intensity of the phase-reference pulse in real time is of great importance to guarantee the security of the LLO CV-QKD system.