量子密钥分发中偏振补偿算法

Polarization compensation algorithm for quantum key distribution

在以单模光纤作为量子信道,并采用光子偏振编码方式的量子密钥分发过程中,光纤的双折射效应会导致光子在光纤中传播时其偏振态发生随机变化,使安全密钥的最终成码率大幅度降低.利用两个四分之一波片和一个半波片的组合作为校正器,可以实现对任意偏振态的校正补偿.建立了一种以该类偏振校正器为执行机构的基于随机并行梯度下降控制算法的实时偏振补偿仿真控制模型,讨论了算法的随机扰动幅度、增益系数与收敛速度的关系,分析了算法对于偏振的校准能力.通过实验对算法的性能进行了验证.实验结果表明,经过一定次数的迭代后可将系统的偏振消光比校正到一个比较理想的状态.

In the process of the quantum key distribution based on the polarization coding of photons,the polarization state will be affected strongly and randomly by the birefringence effect of the single-mode fibers,which will lead to decrease the final secure key rate. Fortunately,this undesirable effect of single-mode fibers can be corrected by the combination of two quarter-wave plates and a half-wave plate.Utilizing the combination of three wave plates as the actuator,we build a simulation model of real-time polarization compensation based on stochastic parallel gradient descent algorithm. We study the relationship among the amplitude of the random disturbance,the gain factor and the convergent rate. Furthermore,we implement an experiment to demonstrate the algorithm we proposed. The results of the experiment showthat the polarization extinction ratio of the simulation system can be corrected well after a certain number of iterations.