Analysis Of Quantum Cryptography Based On Single Photon Source

As the prevalence of internet, digital signature, electronic commerce and digital cash, the security of information becomes more and more important. The most convenient way to protect information is cryptography. Classic cryptography which is widely used today can be eavesdropped without being discovered. Even the strongest protection strategy can't assure its security. Moreover it is threatened by the computational ability that is getting more and more powerful as time go on.QC (Quantum cryptography) uses single photon to carry information, and builds quantum keys between legal users. This process is called QKD (quantum key distribution). QC uses quantum keys generated by QKD to encrypt and decrypt classic information, which make it impossible to be decrypted and eavesdropped by any third party. The security of QKD is protected by the basic laws of quantum mechanics. QKD based on fiber channel is becoming mature while QKD based on free-space channel makes breakthrough everyday. And the feasibility of satellite-to-ground QKD has been proved both experimentally and theoretically. People are making effort to achieve practical QKD systems with longer transmission distance, higher key rate, and stronger security. The ultimate goal of QC is to achieve global quantum secure communication networks.Scatheless transmission of quantum states is significant to enlarge transmission distance of QKD systems, as well as achieving global quantum secure communication networks. Single photon sources and entangled photon sources are the main light sources of QKD systems. Single photon source is widely used because of its simplicity. WCP (weak coherent laser pulse) is used to substitute ideal single photon sources in experimental QKD systems. The channels that quanta(eg. photons) passing through are called quantum channels in QKD systems. Turbulent atmosphere(so called free space), fiber, and vacuum are the physical realizations of quantum channels. Energy and quality of WCP states are damaged when passing through random quantum channels which contain turbulent atmosphere, and it strongly limited transmission distance and application of free-space QKD systems. So it is important to analyse the transmission character of WCP states in random quantum channels. By now the study on quantum channels are focused mostly on relations between channel capacity and entanglement, or effect of interactions between light field and atom on fidelity of quantum information. And this type of analysis is for application of quantum computation(eg. quantum error coding) and quantum communication(eg. quantum teleportation). While the study of transmission character of signal states used by QKD photon source in random quantum channel is rare.Transmission process of quantum states in quantum channels are described by quantum operation. By quantum mechanics conceptions we deduce fidelity of quantum channels, which is turbulent atmosphere in free-space QKD systems, of WCP states. Performance of turbulent atmosphere as a quantum channel is analysed, and the relationship between random quantum channels of QKD and amplitude dampping channels is discussed. Quantum bit error rate of QKD systems takes consideration of turbulence is given as well as its upper bound in case of BB84 protocol and B92 protocol. Numerical calculation and analysis are done.Signals used by QKD systems are extremely weak and is easily flooded in noise which is dominant by background noise and dark counts of SPDs (single photon detectors). Effects of noise and improving methods are introduced. After pulsing effect of SPDs limits the maximal key generation rate of a practical QKD system. Way to improve after pulsing effect is given too. Moreover we put forward the challenge of spatial-light-to-multi-mode-fiber coupling when using commercial devices to built QKD systems. Requirement of coupling qualifications on tracking and pointting precision and beam divergency are given as well as effect of coupling efficiency on photon detection probability.The security of QKD systems is assured by principles of quantum mechanics, but the imperfectness of real devices makes it difficult to built a secure practical QKD system. The maximal secure transmission length and maximal secure transmission atmospheric attenuation of free-space QKD systems under PNS (photon number splitting) attacks are given in this paper. At the same time the maximal secure transmission zenith angle of a satellite-to-ground QKD system is given too. The content discussed and conclusions drawed in this paper can be a positive reference to design of practical QKD systems. Further more , we bring forward an multi-access satellite-to-ground QKD scheme, which can increase both the capacity and link available probability of QKD systems and can decrease power consumption, mass, and volume onboard at the same time. Similar studies haven't been reported yet.