| Free-space quantum communications have a variety of advantages, such as thelight weight and small size of transmitting terminal, high security, high-capacityinformation transmission and low power consumption. However, as the informationtransmission channel is turbulent atmosphere, communications quality is inevitablyinfluenced. Therefore, the impact must be analyzed and compensated.Now, space quantum communications researches mainly focus on inter-groundexperiments and feasibility analysis of quantum key distribution between low orbitsatellite and ground station. The complexity of the atmosphere and changing of theposition and orientation of the satellite make the polarization change continually andtherefore make it difficult to capture a single photon. So after the emitting of a singlephoton, how to guarantee the arrival of the photon is a key problem for quantumcommunication.To solve issues mentioned above, this paper first utilizes the extended HuygensFresnel principle and Rytov perturbation approximation to calculate axial singlephoton acquisition probability density of the fundamental mode Gaussian beams,TEM01mode Hermite-Gaussian, and TEM01Laguerre Gaussian under the influenceof isotropic, homogeneous atmospheric turbulence. Influence of turbulence intensity,transmitting terminal pore size and light wavelength are analyzed and mathematicalexpressions of axial photon acquisition probability distribution of Gaussian beam arederived. Then, based on the extended Huygens-Fresnel principle, assuming modifiedvon Karman spectrum of turbulent density in space and von Karman spectrum intime spread, average Gaussian beam intensity are analyzed and long term beamspread is studied. Single photon acquisition probability density of differenttransmission distance for uplink and downlink is given. To solve the polarizationchanging issue, the paper establishes the corresponding control system to compensatepolarization deviation. Compensation experiments are performed. |
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