Research On Polarization Tracking Technique In Space Quantum Communication

Quantum communication is communication technique that uses the single-photon as information carrier to distribute the secret keys through the quantum channel and conventional channel, and its security is guaranteed by the fundamentals of the quantum mechanics. As a promising technology in free-space communication, quantum communication will play an important role in future space communication network.In space quantum communication system, when satellite and ground start communication, they must build polarization benchmark. Except for the influence of atmosphere and geomagnetic field, the instability of satellite in the system as well changes the polarization benchmark and generates polarization error. So, it is important to compensate polarization error, that is polarization tracking, which is also one of key techniques in free-space quantum communication. In this paper, we firstly talk about the free-space quantum communication experiments which have been finished, and then have an introduction to the satellite to ground quantum communication. The polarization tracking of satellite-to-ground is especially investigated. The main contents are listed below:Firstly, the theoretical model of polarization tracking is established based on electro-optic effect. The principle of polarization tracking based on the electro-optic effect is given and a polarization "zero" direction can be corrected by adjusting electric field intensity. The quantum key coding principle based on electro-optic effect is analyzed for the polarization tracking.This article has built the quantum communication links model, and analyzed the influence of transmission of the photon polarization affected by the geomagnetic field. We find that the plane of polarization can rotate in the case of geomagnetic field. The quantum of the rotation angle is 10-3rad by theoretical analysis and calculation. We has analyzed the flow chart of polarization tracking, and analyzed the source of polarization error. Then, we analyze the principle of polarization tracking system and how to compensate the polarization error.We have researched the theory of polarization compensation based on two-mirror model and given the conditions that polarization quantum bits need to meet. The principle of satellite-ground link polarization tracking based on two-mirror model is given, finally the implementation steps of polarization compensation based on half-wave plate are given in this paper.In this paper, the theoretical model of polarization tracking based on electro-optic effect and the theoretical analysis that the geomagnetic field influence polarization tracking the principle of polarization based on two-mirror model are the theoretical foundation for the polarization tracking technique in quantum communication.