Research On Transmission Of Cv-qkd In Actual Communication Environment

Quantum key distribution ensures that two distant legal parties can gen-erate secure keys over untrusted channels.Continuous variable quantum key distribution using Gaussian modulated coherent states can be realized based on commercial optical communication devices such as semiconductor lasers and photodiodes,which not only has outstanding advantages in reliability and cost,but also exhibits good performance with commercial optical communication networks.It also has become a research hotspot in the field of quantum infor-mation in the past decade.The continuous variable quantum key distribution protocol based on Gaussian modulation coherent state has not only been proved theoretically unconditionally safe,but the experimental system has also been extensively studied and matured.Since the protocol was proposed,several research groups have imple-mented continuous variable quantum key distribution in the laboratory.How-ever,in the laboratory environment,long-term stability,practical security,and noise introduced during transmission are generally not considered,so the lab-oratory system is difficult to apply directly in commercial fiber-optic commu-nication networks.When the system is working in the actual communication environment,the system not only needs to compensate the change factors such as polarization drift and phase noise in real time to maintain the long-term stability of the system performance,but also needs to consider the practical security introduced by existing devices and technology.Furthermore,in future large-scale applications,quantum signals will be transmitted along with classi-cal signals in optical communication networks,and thus the noise introduced by classical signals should be considered.Existing experimental systems still have practical security risks,and can not meet the requirements of secure key rate,transmission distance and stability in practical applications.It is diffi-cult to highlight the practical advantages of continuous variable quantum key distribution systems.This paper focuses on the problems encountered in the continuous variable quantum key distribution system transmission in the actual communication environment,and the following researchs are included.1.The dominant source of excess noise introduced by classical optical signals WDM networks continuous variable quantum key distribution systems is proved.The theoretical calculation and experiments compare the excess noise generated by the spontaneous Raman scattering and the four-wave mixing effect under different system and channel conditions.The results show that the four-wave mixing effect introduces a huge excess noise,resulting in a system secure key rate of 0.Thus,the four-wave mixing need to be avoided in the actual communication environment.Furthermore,the power fluctuation of local oscillator caused by the classical signal crosstalk is proposed.The range of local oscillator fluctuation is given by simulation and experiments,and the secure key rate when the local oscillator fluctuation exists is calculated.According to the research results,several methods for reducing crosstalk when accessing the classical optical communication network are proposed.2.We point out a security vulnerability in a continuous variable quantum key distribution system in an actual communication environment.In the actual communication environment,the polarization drift of the quantum state needs to be compensated at Bob’s side.In order to improve the efficiency of the system,polarization compensation only measures a limited number of pulses,which leads to an opportunity for the eavesdropper to control the practical shot noise in the system.Both theory and experiment have confirmed that the eavesdropper can control the practical shot noise by modulating orientation angle of the local oscillator.By applying the polarization attack,Eve can make Alice and Bob overestimate the secue key rate,so that the keys generated by Alice and Bob are no longer secure.According to the analysis of the polarizaiton attack,a scheme for ensuring security in the actual communication environment is designed.3.A reference signal preparation method is proposed to reduce the phase noise estimation error and improve the spectral efficiency of the system,thereby improving the secure key rate of the system.The stable variable quantum key distribution system needs to compensate the phase noise.The accuracy of the phase noise compensation is affected by the form,number,and signal-to-noise ratio of the reference signal.Increasing the number of reference signals can reduce residual phase noise in the system but reduce spectral efficiency,resulting in a lower final secure key rate.By apllying the method in the paper,the theoretical evaluation of the residual phase noise in the system can be realized,and the optimal reference signal can be designed according to the evaluation result.Experiment results prove that the system’s secure key rate can be improved by optimizing the reference signal.4.We design and build a continuous variable quantum key distribution system that can support the actual communication environment.By combining the above research results with the actual working environment,the system runs more stably and efficiently.Based on this system,the filed test in the actual communication environment was carried out.We test the perfor-mance and stability the system in different commercial fiber networks in Xi’an and Guangzhou.The high-speed key distribution of 7.43 kbps in the 50km(11dB)Fibre Channel is two orders of magnitude higher than the existing research results,and the system can support continuous sta-bilization of all-weather.This work further promotes applications of the continuous variable quantum key distribution in the actual communication environment.