Polarization Degradation In Free Space QKD And Improvement For System Design

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.