Theoretical And Experimental Research Of Underwater Quantum Optical Communication Based On The Polarization Encoding

During recent years, quantum information, which is based on the principles ofquantum mechanics, has been drawing much attention. As an important branch ofquantum information, quantum key distribution overcomes the security risk oftraditional cryptography, and it has been proved to be absolutely safe. Besides,underwater wireless communication has been developing rapidly in recent years, forwhich underwater quantum key distribution can provide absolute security. The studyof underwater communication is of importance both for the development ofcommunication and quantum technology. As a result, it is necessary to study theeffects of ocean channel on photons carrying the key information. Besides, for theexperiment system, as the optical devices are not perfect, they will have effects onthe polarization states of photons and then bring errors. To ensure the security ofquantum communication, the study of the effects of optical devices and waterchannel on the polarization states of photons is an important part of this thesis.Based on the above reasons, we have finished two researches as follows:1)Research on the transmission of polarized photons in seawater.2) Study of theeffects of optical devices and water on polarization states of photons.For the first research, we carry out the simulation of the transmission ofpolarized photons in seawater by Monte Carlo method, which is combined with theoptical property of sea channel and quantum mechanics. Firstly, we set up a logicalocean channel modeling, and then simulate the scattering process of polarizedphotons in ocean water. We get the data of the number of photons, Stokes vectorsand the coordinates of photons etc. with computer simulation. Then we study theattenuation of photons when propagating underwater, the depolarization ofscattered photons and the quantum bit error rate for different weather condition byanalyzing the data. Our results show that it is possible to accomplish underwaterquantum key distribution in the clearest ocean, for which the distance is with a level of100meters.For the experiment part, we set up the quantum key distribution experimentsystem, and study the depolarization property of the optical devices and waterchannel for this experiment system. We design reasonable experimental program tostudy the effect of Polarization-independent beam splitter on transmitted light andreflected light separately, and study the change of polarization states of polarizedphotons propagating in water channel. We get the number of photons with differentpolarization states through single photon detector, and obtained the effects ofPolarization-independent beam splitter and water channel on the polarization statesof photons separately.