Simulation Analysis Of The Transmission Performance Of Continuous-Variable Quantum Key Distribution Free Space Channel

One of the important research directions of quantum key distribution(QKD)in the future is to establish satellite-to-ground communication and global quantum network.Free space,as a channel environment with great potential,has undoubtedly become the most suitable choice for researchers to expand the application scenarios of QKD system.The research and analysis on the performance of continuous-variable quantum key distribution(CV-QKD)in free space can provide theoretical help for its subsequent experimental progress.At the same time,in practical experiments,it can also provide theoretical reference for reducing the impact of atmospheric channels on QKD in free space and improving the system performance.When conducting CV-QKD in the atmospheric channel,there are mainly two coding methods,one is to modulate the phase of the beam,the other is to modulate the polarization.However,in previous polarization coding experiments,due to the limitation of free space polarization modulator,the stokes vector corresponding to the beam polarization states can only be taken in a limited range in the Poincare sphere space,which will restrict the modulation range of the signal states and affect the performance to a certain extent.Secondly,at present,the theoretical research on the performance of polarization coding in free space channel is incomplete,and there is a lack of comparative analysis of the protocol performance of polarization coding and phase coding.Meanwhile,the sudden extreme weather can cause extremely bad influence on the communication performance of free space CV-QKD.Therefore,the theoretical simulation analysis under extreme weather is helpful to provide a reference for improving the communication performance in practical experiments.To solve the above problems,this thesis has simulated and analyzed the performance of CV-QKD in the environment of atmospheric channel.The specific work is as follows:1.This thesis proposes a new polarization coding scheme based on the PolaRITETM Ⅱ polarization controller of General Photonics Co,which can make the stokes vector corresponding to the polarization states of the beam obtain any value in the Poincare sphere space.The modulation scheme proposed in this thesis expands the modulation range of the signal states and improves the performance to a certain degree.In the polarization coding scheme,Alice modulates the stokes component by changing the delay phase of different wave plates in the modulator,while Bob can directly obtain the value of the corresponding stokes component by measuring the light intensity.Based on the Miller matrix corresponding to the optical components,a detection structure which can directly measure the stokes component of the target is obtained through theoretical analysis.At the same time,in order to make the modulation data meet a certain probability density distribution,the probability density distribution required by the delay phase of different wave plates in the modulator is also studied theoretically.2.This thesis combines the ellipse model,the beam extinction model and the interruption probability model to simulate and analyze the freespace channel,and compares the performance of two protocols based on different modulation methods in free space from different aspects.Besides,the thesis also considers the influence of sudden extreme weather such as foggy weather on the performance of CV-QKD in the atmospheric channel.In particular,the appropriate fog channel attenuation model is selected in the analysis of different empirical models.Combined with the theoretical simulation of fog channel attenuation model,the influence of fog with different degrees of visibility on continuous variable quantum key distribution is also studied.Meanwhile,the effect of different wavelengths of laser on the performance of the protocol is studied when the visibility is low.In all kinds of extreme weather,the attenuation in foggy weather is the most serious.Therefore,the research on the transmission performance of CV-QKD in foggy weather is of great significance,which will help to provide theoretical guidance for the actual experiments to ensure the communication performance and communication stability in foggy weather.