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arXiv:1908.06230 · Physical Review Applied

One-Time Shot-Noise Unit Calibration Method for Continuous-Variable Quantum Key Distribution

Yichen Zhang, Yundi Huang, Ziyang Chen, Zhengyu Li, Song Yu, Hong Mei Guo

Published Feb 21, 2020• 44 citations
arXivHTMLPDF

Abstract

The shot-noise unit in continuous-variable quantum key distribution plays an important and fundamental role in experimental implementation, as it is used as a normalization parameter that contributes to performing security analyses and distilling key information. However, the traditional calibration procedure and detector model cannot cover all system noise in practical applications, which results in loopholes and influences the practical security. What is more, the traditional procedure is also rather complicated and has difficulty in providing compatibility with automatic operating systems. In this paper, we propose a calibration model based on a proposed trusted detector model, which could naturally close the loopholes in practical applications. It can help to identify the shot-noise unit in only one step, which can not only effectively simplify the evaluation process but also reduce the statistical fluctuations, while two steps are needed in the traditional method. We prove its feasibility and derive a complete version of the corresponding entanglement-based model. A detailed security analysis against arbitrary collective attacks and numerous simulation results in both the asymptotic-limit regime and the finite-size regime are provided. A proof-of-principle experiment is implemented, and the results indicate that our one-time calibration model can be employed as a powerful alternative method for calibrating the shot-noise unit. Our method paves the way for the deployment of continuous-variable quantum key distribution with real-time calibration and automatic operation.

DOI
10.1103/physrevapplied.13.024058

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The shot-noise unit in continuous-variable quantum key distribution plays an important and fundamental role in experimental implementation, as it is used as a normalization parameter that contributes to performing security analyses and distilling key information. However, the traditional calibration procedure and detector model cannot cover all system noise in practical applications, which results in loopholes and influences the practical security. What is more, the traditional procedure is also rather complicated and has difficulty in providing compatibility with automatic operating systems. In this paper, we propose a calibration model based on a proposed trusted detector model, which could naturally close the loopholes in practical applications. It can help to identify the shot-noise unit in only one step, which can not only effectively simplify the evaluation process but also reduce the statistical fluctuations, while two steps are needed in the traditional method. We prove its feasibility and derive a complete version of the corresponding entanglement-based model. A detailed security analysis against arbitrary collective attacks and numerous simulation results in both the asymptotic-limit regime and the finite-size regime are provided. A proof-of-principle experiment is implemented, and the results indicate that our one-time calibration model can be employed as a powerful alternative method for calibrating the shot-noise unit. Our method paves the way for the deployment of continuous-variable quantum key distribution with real-time calibration and automatic operation.
Imperfections in practical detectors, including limited detection efficiency, and inherent electronic noise, can seriously decrease the transmission distance of continuous-variable measurement-device-independent quantum key distribution systems. Owing to the difficulties inherent in realizing a high-efficiency fiber homodyne detector, challenges still exist in continuous-variable measurement-device-independent quantum key distribution system implementation. We offer an alternative approach in an attempt to solve these difficulties and improve the potential for system implementation. Here, a novel practical detector modeling method is utilized, which is combined with a one-time shot-noise-unit calibration method for the purpose of system realization. The new modeling method benefits greatly from taking advantage of one-time shot-noise-unit calibration methods, such as measuring electronic noise and shot noise directly to a novel shot-noise unit, so as to eliminate the statistical fluctuations found in previous methods; this makes the implementation of such systems simpler, and the calibration progress more accurate. We provide a simulation of the secret key rate versus distance with different parameters. In addition, the minimal detection efficiency required at each distance, as well as the contrast between the two methods, are also shown, so as to provide a reference in terms of system realization.
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 used as a constant during key distribution. Because the measurement result of quadrature is normalized with the calibration SNU but scaled with practical SNU, which could open loopholes for the eavesdropper to intercept the secret key. In this paper, we report a quantum hacking method to control the practical SNU by using the limited compensation rate for polarization drift. Since the polarization of local oscillator pulses is partially measured, the attack is implemented by manipulating the polarization of the local oscillator pulses without measurement when the system is running. The simulation and experiment results indicate that the practical SNU can be manipulated by the eavesdropper. By making the difference between the calibration and the practical SNU, the excess noise estimated by Alice and Bob could always be lower than the practice which is introduced by the eavesdropper and the distributed keys are not secure.
We propose a rigorous calibration method for homodyne detection efficiency, which combines all the factors that affect detection efficiency to calibrate together through the actual homodyne detection. With this method, the transmittance converted from electronic noise in the one-time calibration method of the shot noise can be attributed to the detection inefficiency. Thus, a trusted detection noise-free model for continuous-variable quantum key distribution (CV-QKD) can be established, which simplifies the calibration of shot noise while having the same performance as the trusted detection noise model. We demonstrate this calibration method with a balanced detector based on a transimpedance amplifier. Experimental results show that detection efficiency will be overestimated if the integration factor of the detector is overlooked. The overestimation of the detection efficiency leads to an underestimation of modulation variance and excess noise when the modulation variance is monitored by the balanced detector, which opens security loopholes. Our method may prove a necessary method in the calibration of detection efficiency for CV-QKD.
In the safety assessment of the actual CVQKD (continuous-variable quantum key distribution) system,the preparation measurement model is generally equivalent to the entanglement-based model,whose major drawback is that the shot noise variance is treated as a constant.As the attacks on the LO (local oscillator) from the Eve,the shot noise variance will change with LO.And in the process of safety analysis based on the shot noise variance calibration technology,there are loopholes in which the shot noise variance for calculating secret key rate is obtained by the linear relationship between the shot noise variance and the LO before distributing the quantum key.However,the shot noise variance is not accurate nor real-time.In the security analysis of system,all the noise parameters of the system are normalized to the shot noise variance.The Eve can reduce the shot noise variance by controlling the strength of LO,thus actual excess noise of system will increase.But legal communicating parties are still normalized based on previous larger shot noise variance,so that the excess noise of system is substantially underestimated.As a consequence,the Eve can obtain secret key information without attracting the attention of legal communicating parties by adopting some attacks, such as intercept-resend attack.Thus it is an essential factor for ensuring the system security to evaluate real-time shot noise variance accurately.In order to effectively resist the above mentioned attacks on the LO from the Eve,a scheme of CVQKD system based on real-time shot noise variance monitoring is presented to improve the security of CVQKD system.The shot noise variance calibration technology is adopted in this system.By adding the real-time shot noise variance monitoring modules to the primary CVQKD system,the real-time shot noise variance is assessed by the linear relationship between the shot noise variance and the LO.In the hardware system,independent clocks are adopted. Sampling in peak algorithm is applied to software system,and this effectively solves the problem that CVQKD system with LO clock source is at risk of shot noise variance calibration attack.The scheme prevents the hazards that the Eve changes previously calibrated linear relationship by regulating the pulse delay of the LO,and thus judges whether the system is safe through calculating the accurate and real-time secret key rate.The system can analyze the real-time security of quantum key distribution and display safety status of system.The experimental results show that this system can defend effectively the LO attacks from the Eve and improve the security performance of the CVQKD system.
Digital signal processing technology has paved the way for the realization of high-speed continuous-variable quantum key distribution systems. However, existing security proofs are limited to static digital signal processing algorithms, while practical systems rely on dynamic multiple-input multiple-output algorithms to compensate for time-varying channel impairments. Our analysis reveals that the conventional dynamic algorithm, due to its non-unitary nature, systematically underestimates the excess noise, which in turn leads to security issues and the generation of insecure keys. To close this gap, we propose a secure algorithm model, mapping the dynamic algorithm to an equivalent physical optical model whose security can be rigorously assessed. Simulations illustrate the algorithm's non-unitary property and provide a quantitative analysis of the excess noise underestimation caused by the conventional algorithm. We further experimentally validate the necessity of the proposed modeling for dynamic digital signal processing, achieving a secret key rate of 14.4 Mbps based on estimated excess noise of 0.07 shot noise unit; whereas the conventional algorithm would have dangerously overestimated the key rate to 28.2 Mbps with noise of 0.008 shot noise unit. This work provides the essential security framework for dynamic digital signal processing, overcoming a critical impediment for the development of high-performance continuous-variable quantum key distribution systems.