【作者】 吴建荣；

【导师】 朱冰；

【作者基本信息】 中国科学技术大学 ， 电磁场与微波技术， 2011， 博士

【摘要】 在这个信息呈爆炸式增长的年代,信息的安全问题日益突出,保密通信的需求日益增长。安全密钥的获取是保密通信的关键。量子密钥分配由于可以提供大量安全密钥的分发而受到人们的青睐。经过二十余年的发展,在量子密钥分配的两大关键指标,即实现密钥分配的距离和成钥速率两方面都有了巨大的进步。但是,从目前的空间量子密钥分配实验结果来看,当传输距离大于10公里时,误码率都在5%以上,最终的成钥率只有几百bit/s。由于空间量子密钥分配一般采用偏振态编码方式,如果信号光子的偏振态在产生或传输的过程中偏离了密钥分配协议所要求的理想偏振态,即偏振发生劣化,将导致密钥误码率的增加。因此,有效解决空间量子密钥分配中的偏振劣化问题,是降低系统误码率、提升密钥分配效率的关键之一。本论文围绕空间量子密钥分配中的偏振劣化,包括系统中光学器件的不理想所导致的信号偏振光子的偏振态的转换以及大气传输所带来的信号光子的退偏振效应,进行了以下几个方面的研究:(1)分析了空间量子密钥分配系统中偏振劣化的器件来源。指出光在光学器件的介质分界面处折、反射时的偏振相关特性是系统中偏振劣化的物理根源。用来分/合束的偏振无关分束器(NPBS)的偏振相移特性的误差是系统退偏振的重要来源。采用琼斯矩阵法就NPBS对系统误码率的影响进行了分析。提出对光路布置设计的改进方案,在这一方案中,只对NPBS的透射p光与s光的相移差有性能要求,可以降低NPBS的偏振相移误差对误码率的影响。并改进设计了满足系统设计要求的新型NPBS。分别选用一般NPBS和改进设计过的NPBS构建空间量子密钥分配系统,进行室外200米及1.3公里的空间量子密钥分配对比实验。实验结果与理论预期符合基本一致,改进NPBS的设计及接收端光学系统的光路布置使系统误码率降低了2.6%,证明了对系统设计方案改进的有效性。(2)提出了基于正交对称结构的空间量子密钥分配系统的改进设计方案。在这一方案中,发射端和接收端采用同一种NPBS,两者相互呈90°放置,使得经过第一个NPBS的p光相对于第二个NPBS是s光,而经过第一个NPBS的s光相对于第二个NPBS是p光,从而抵消NPBS对透射p光和s光的相移差。这使得无需对NPBS的透射p光与s光的相移差作性能要求,就可以降低NPBS所带来的偏振劣化对误码率的影响。采用琼斯矩阵方法对这一方案的有效性进行了理论验证,并给出实验系统的示意图。(3)采用蒙特卡罗方法详细分析了偏振光在云雾大气中的退偏振及传输损耗特性。分别针对不同浓度及不同平均粒子半径的云雾模型,在不同的空间量子密钥分配系统的接收机参数下,进行了数值模拟。得到了在不同的接收机参数下,偏振光经过云雾大气传输后的传输损耗及退偏振变化规律,为分析穿越云雾的空间量子密钥分配提供了理论基础。搭建了测量偏振光经过大气传输后的退偏振效应的测试装置,给出了在合肥地区测得的累积实验结果。(4)进行了云雾大气中空间量子密钥分配的误码率仿真计算,讨论了不同云雾大气下空间量子密钥分配的可行性。结果表明,在实际接收机系统参数下,多次散射引起的光子的退偏振对系统误码率的影响相对于探测器暗记数与背景光噪声之和对误码率的影响是可以忽略的。通过计算得出了穿越云雾的量子密钥分配的误码率与云雾浓度的关系,论证了可以进行有效空间量子密钥分配的云雾浓度上限和对应的气象视距。更多还原

【Abstract】 Nowadays, with the explosive growth of information, the information security problem becomes more and more prominent and the demand for secure communication is constantly increasing. The critical point of secure communication is the acquisition of the keys to encrypt the information. Quantum key distribution (QKD) has attracted great attention as it can offer great amount of secure key. After more than twenty years development, QKD has made great achievements in its two critical specifications of distance and the secure key generation rate.However, from the current experimental result of free space QKD, the error bit is always above 5% and the secure key generation rate is only several 100bps when the distance is greater than 10km. Because the free space QKD usually adopts polarization coding, the error bit rate for the quantum key will increase if the polarization state of the signal photon deviate from that required by the QKD protocol at the stage of the generation of the signal photon or at the stage of the transmission of the signal photon. So the key to reduce the error bit rate and increase the efficiency for the QKD is to solve the polarization degradation problem in free space QKD. This paper centers on the polarization degradation in free space QKD, including polarization state transfer for the signal polarized photon in the optical system due to imperfection for the optical devices and depolarization effect for the signal photon after transmission through the atmosphere. The main content of the paper is as follows:(1) The origin of the depolarization in the QKD system is analyzed. It indicates that the polarization dependent properties when the light reflects from or refracts through the optical device’s dielectric interface is the origin of the depolarization in the system. For the free space QKD system, the polarization dependent phase shift difference for the NPBS (nonpolarizing beamsplitter) is the main source of the depolarization in the system. The Jones matrix method is adopted to analyze the affect of NPBS on the system error bit rate. The improved design for the arrangement of the optical path is proposed in which the system’s error bit rate can be decreased by only declining the phase shift difference between the p light and the s light in the transmitting optical path for the NPBS. An commercially available NPBS and our improved NPBS is used respectively to setup a free space QKD system. Several outdoor 200m and 1.3km QKD experiment is conducted to prove the feasibility of the design for the NPBS and the QKD system. The result indicates that the system’s bit error rate is declined by 2.6% by improving the NPBS and the arrangement of the receiver’s optical path, thus proving the validity of the improved design for the system.(2) A second improved design scheme based on orthogonally symmetric structure for the free space QKD system is raised up. In this scheme, the transmitter and the receiver use the same NPBS, and the two NPBS rotates by 90°relative to each other. This cancels the phase shift difference between the p light and s light caused by using only one NPBS so that it can decrease the error bit rate by the depolarization in the system. The feasibility of the design scheme is improved by Jones matrix method and the illustrative experimental system is given out.(3) The Monte Carlo method is adopted to analyze the transmission loss and depolarization properties when the polarized light passes through the atmosphere. The simulation is conducted for the clouds and fogs with different drop concentration and different mean radius for different free space QKD receivers. The variation law of the transmission loss and depolarization is released and this makes basis for the analysis for the feasibility of QKD through cloud/fog. We set up the depolarization measurement system for polarized light transmitted through the atmosphereand released the depolarization measurement result in Hefei .(4) The feasibility for free space quantum key distribution through cloud/fog is analyzed. The result suggests that, for practical QKD receiver, the influence of depolarization from multiple scattering on the error bit rate of QKD is negligible compared with the influence by the detector’s dark count and the background light. By analyzing and calculating the error bit rate for the free space quantum key distribution system, the relationship between the error bit rate and the drop concentration of the cloud or fog for quantum key distribution through cloud or fog is released. It gives out the maximum drop concentration of the cloud or fog and the corresponding shortest meteorological range for effective free space quantum key distribution.更多还原