Research Of The Basis Detection And Compensation In Free-space Quantum Key Distribution

Based on Heisenberg uncertainty principle and quantum no-closing theorem,quantum key distribution (QKD) provides a theoretical absolute security channel forone-time pad keys. Based on satellite laser communication technology, free-spacequantum key distributions, especially star-to-ground QKDs, have reached greatsuccesses in recent decades, and become the best way to achieve global securecommunication network. Single-photon polarization QKDs such as QKDs based onBB84protocol is the easiest way of realizing free-space QKDs at present technology.According to BB84protocol, the sender of a QKD—Alice encodes one-time-padkeys by using the polarization state of the photons, then transmitted them throughquantum channel to the receiver—Bob. This procedure is the quantum keydistribution (QKD). After that Alice encrypts the information with the keys in"one-time pad" encryption way and generate cipher text. The cipher text is send toBob in classic channel, and the whole quantum communication completes. DuringQKD process the reference basses of Alice and Bob must keep in consistence toensure Bob detecting the photons correctly. However, in quantum communicationsystems including moving stations such as a low-earth-orbit satellite (LEOS), themotion of satellite will cause basis deviation between Alice and Bob, resulting inincreasing the quantum-bit error rate (QBER). QBER goes too big will make theQKD meaningless. Therefore, basis deviation needs to be compensated throughtechnical to ensure safe QKD.This article starts from the principles of quantum communication, analyzes theimportance of basis vectors used in photon-polarization detection, and deduces therelation-equation of basis deviation and QBER. The structural characteristics offree-space QKD system is also been researched to find out the relationship betweensatellite motions and basis deviation. Two basis compensation schemes are proposed.One is based on orbit testing data and the other one is based on orbit report data. Theformer scheme has been tested in zhoushan flight experiment. We also propose acompensation scheme based on BB84decode module, and analyzed all the factorsthat will decrease the precision of the module，including the infection of APDs andphoton counts. Furthermore, we testify this scheme’s accuracy in Zhoushan voyageexperiment, and get the compensation accuracy of2degrees. By these works we cansettle the basis-deviation problems caused by moving satellite in free-space QKD, and guarantee the communication going smoothly and reliably.Main tasks and innovations of this paper are:1) Deduce the fraction of QBER caused by basis deviation from the point ofview of polarimetry. Calculate the QBER as a function of basis deviation, and deucethe precision demanding of basis compensation.2) Analyze the impact of the optical stage and the satellite-gesture on basisdeviation. Deduce basis deviation formula on two-dimension stage in satelliteterminal. Propose basis compensation scheme base on satellite orbit testing. Test thisscheme in Zhoushan voyage experiment and the results show the scheme can satisfybasis compensation demanding.3) Propose orbit-data prediction compensation scheme. Applycoordination-exchanging scheme in using prediction data to calculating basisdeviation. This scheme tend to compensate the basis deviation without anyangle-testing and data-transmission, it can pre-calculate the basis-deviation andprovide an immediately compensation when the satellite arrived the predict position.4) Analyze the polarimetry of BB84decoding module. Using software to mendthe parameter of the BB84module and increase the polarimetry precise.Comprehensive analyze the factors affecting the accuracy of the detection module,and find out the restriction of the precision is APDs.5) Designed automatic basis compensation system based onsingle-photon-detection, and developed its software. Testified the scheme’s precisionand analyzed the precision of this system in different photon counts. This systemhave tested to owning2°compensation precision in Zhoushan voyage experiment.