Design And Analysis Of Free Space Quantum Communication System With Different Wavelengths

Quantum communication is an important branch of quantum information science.Different from the classical communication system,quantum communication system has the advantages of high security and long transmission distance.According to different channels the quantum signals transmitted through,quantum communication system can be divided into optical fiber quantum communication system and free space quantum communication system.Limited by the loss of optical fiber,the signal in optical fiber quantum communication system can only be transmitted about hundreds of kilometers at one time,so it is necessary to use the quantum repeater to increase the transmission distance.In contrast,signals in free space quantum communication system can be transmitted thousands of kilometers at a time.Thus,a global quantum communication network can be established based on the free space quantum communication system and by using the satellite.In this paper,entangled photons are used as information transmission carriers,the appropriate wavelengths of entangled photons is studied and selected,and a free space quantum communication system which is suitable for space channel transmission is designed and analyzed.The main research contents of this paper are as follows:1.Firstly,according to the atmospheric transmission window and actual experimental devices and materials,several signal wavelengths suitable for the free space quantum communication system are selected within the wavelength range of 4000 nm.Then,the influences of background light noise,atmospheric absorption,scattering and turbulence,which are closely related to the signal wavelengths,on the entangled photons transmission through the space channel are analyzed.Finally,by analyzing and combining with the experimental feasibility,three signal wavelengths used in this paper are determined.2.According to the practical experimental equipment and materials,the generation system of entangled photons with different wavelengths is designed based on spontaneous parametric down-conversion process.Then,the generation process of entangled photons is analyzed by using the coupling wave equations.Finally,according to the power expression of entangled photons,the relationship between entangled photons’ power and pump power as well as the lengths of nonlinear crystal simulated.3.Using different nonlinear frequency conversion methods,the detection system for detecting entangled photons with different wavelengths is designed.The quantum properties of entangled photons in the process of frequency transformation are studied by using the quantum theory.The relationship between frequency conversion efficiency and pump power as well as the lengths of nonlinear crystal is simulated.Finally,based on the above research results,the design scheme of a complete free space quantum communication system with different wavelengths is proposed.