Free Space Continuous-Variable Quantum Key Distribution Under Actual Meteorological Conditions

The rapid development of information technology and network system puts forward higher requirements for information network security.Quantum key distribution(QKD)based on quantum mechanics can guarantee the security of information unconditionally in theory.According to different preparation methods of states,QKD is divided into discrete-variable quantum key distribution(DVQKD)and continuous-variable quantum key distribution(CVQKD).CVQKD has attracted great attention due to its characteristics of simple preparation of quantum states,high detection efficiency and strong compatibility.In order to realize the globalization of quantum communication,free space CVQKD has been prevailing.It solves the problem that the optical fiber channel is limited by the geographical environment.However,the performance of CVQKD protocol in free space will be affected by rain,fog,atmospheric turbulence and other actual factors.Therefore,it is of great practical significance to study the effects of the actual meteorological conditions on the CVQKD.This thesis focuses on the research and analysis of CVQKD in free space under the actual meteorological conditions with temperature,high concentration of sea salt,rainy and foggy weather,and composite channel.The specific work is as follows:1.Research on the effect of temperature on the atmospheric channel CVQKD.In free space atmospheric channel,turbulent motion always exists,but due to the irregularity of turbulent motion,it is difficult to obtain the effects of turbulence on the protocol directly.As a very crucial parameter to describe the turbulent motion,temperature can describe the effects of turbulent motion on the atmospheric channel CVQKD more vividly.Therefore,in this thesis,the effects of temperature on the atmospheric channel CVQKD are studied.By analyzing the effects of temperature on transmittance and interruption probability,the transmittance and interruption probability formulas are given,and the covariance matrix is constructed.In addition,according to the characteristics of actual temperature changes,the influence of diurnal and seasonal temperature variation on the performance of the protocol is studied.The numerical simulation results demonstrate that,without considering the fluctuation of other factors,the temperature has a positive correlation with the performance of the protocol.The higher the temperature is,the greater the transmittance and secret key rate are.This research provides a basis for evaluating the CVQKD protocol in free space atmospheric channel.2.Research on the effect of sea salt on the atmospheric channel CVQKD.The difficulty of free space CVQKD analysis lies in the construction of model for the channel analysis based on various complex parameters.In this thesis,the CVQKD model of shore-to-reef atmospheric channel is established according to the characteristics of sea salt in high concentration in actual meteorological conditions.The extinction effect of sea salt particles on the laser beam is analyzed in detail,and the formula of transmittance under the model is given,then the formula of secret key rate under the condition of sea salt is given,and the effect of sea salt on the secret key rate is numerically analyzed.The results show that there is a negative correlation between the radius size of sea salt particles and the performance of the protocol.The increase of radius of sea salt particles can remarkably enhance the negative effect on the protocol.In addition,with the extension of transmission distance,the secret key rate tends to decrease.This work provides a theoretical basis for the deployment of sea-related atmospheric CVQKD and ideas for the research of other impacts brought by the oceanic atmospheric channel.3.Research on the effect of rainy and foggy weather on the atmospheric channel CVQKD.Variations in weather conditions play an important role in determining the environmental conditions of free space protocols.Then the performance of the protocol is determined by the merits and demerits of the communication environment.Therefore,the free space atmospheric channel CVQKD is studied based on the rainy and foggy weather.This research aims to move closer to the actual battlefield environment.Combined with the flexible deployment of the atmospheric channel continuous-variable measurement-device-independent quantum key distribution(CV-MDI-QKD)protocol,the free space CV-MDI-QKD model in rain and fog is established.Then the formula of secret key rate under the rainy and foggy conditions is obtained.Basing on this model,numerical simulation is carried out.Simulation results show that rainy and foggy weather conditions will seriously affect the performance of free space CVQKD protocol,that is,the size of raindrops and fog droplet has a negative effect on the protocol.The increasing of the size of raindrop and fog droplet can remarkably weaken the performance of protocol.This research provides a theoretical basis for the normal operation of free space CV-MDI-QKD protocol in rainy and foggy environment.4.Research on the performance of CVQKD in composite channel.Based on the demand of ocean strategy for China and the goal of establishing a global quantum information network,it is urgent to develop atmosphere-sea surface-underwater quantum communication.Therefore,the CVQKD channel model of atmosphere-sea-underwater is established in this thesis.The expression of the channel transmittance under the condition of the composite channel is given,and the transmission characteristics of the entanglement state in the composite channel are studied.Based on this model,the entanglement of photon pairs is simulated numerically.The simulation results show that the beam wandering,the angle of incidence of the beam and the concentration of chlorophyll in the sea all lead to the change of the entanglement.The increase of chlorophyll concentration in seawater has a relatively large effect on the entanglement,and the concentration of chlorophyll has a negative correlation with the entanglement.In addition,expanding the aperture of the receiving telescope will increase the entanglement under this condition.This research is of great significance to the transmission of quantum entanglement in free space,and provides a theoretical basis for the application of free space CVQKD and a strong support for the realization of globalization for quantum communication.