A simple system of polarization compensation

The polarization-encoded quantum key distribution system uses the polarization property of photons to realize secure key sharing. For fiber-based systems, however, the birefringence effect in single-mode fibers significantly affects the preservation of photon polarization states. Therefore, it is essential to carry out polarization compensation for polarization coding system. The current mainstream polarization compensation systems utilize Field Programmable Gate Array for data acquisition, calculation, and control. This method offers rapid and efficient performance but comes with high costs and complex designs. To address this problem, a polarization compensation system based on hardware equipment is proposed in this paper. The system not only reduces the cost of polarization compensation, but also is simple and easy to operate by using computer for data processing. To further demonstrate the feasibility of the system, a polarization compensation procedure based on a gradient algorithm is designed, with the error count rate of a single-photon detector serving as an evaluation parameter. In addition, this algorithm is applied to the aforementioned system for long-term and long-distance polarization monitoring. Over a period of approximately 24 hours, the average error count rate for a 20km transmission fiber is 0.59% and the average error rate for a 50km transmission fiber is 0.55%. These experimental results demonstrate that the system exhibits good polarization compensation performance and long running ability.