Study On The Theory Of Quantum Communication

Quantum communication provides a new way for transmitting message securely,and is one of the important parts of quantum information. Many researches on quantumcommunication have been done both in theory and experiment since the BB84 quantumkey distribution protocol in 1984. Many scientists are participating in the studies in this?eld. My study involves four areas in quantum communication, i.e., quantum key distri-bution (QKD), quantum secure direct communication (QSDC), quantum secret sharing(QSS) and quantum source coding. In the ?eld of quantum key distribution, we have developed three theoretical pro-tocols. First, we devised a novel technique, controlled-order-rearrangement-encryption(CORE) with which Bell states are used as quantum information carries. It has the ad-vantage of high intrinsic ef?ciency and saves much classical information. Secondly, weproposed a bi-directional quantum key distribution protocol with practical faint laserpulses. It is secure when each laser pulse contains no more than two photons. Thekey distribution task is completed in two transmissions. Bob ?rst sends the laser pulsesto Alice, and Alice encodes the key message through certain unitary operations andreturns the laser pulses to Bob. Security is achieved by placing eavesdropping checkprocedures in both transmissions. This protocol is secure and is closer to practicalcondition. In addition, it does not require the publication of measuring basis informa-tion. Thirdly, we designed a multi-user any-to-any QKD protocol on a passive opticalnetwork. Different from QKD whose objective is to establish a common random key, QSDCis to communicate the secret message directly without creating a key ?rst. Hence,QSDC requires more than QKD on the security. We established four criteria for secureQSDC. Furthermore, we have proposed three secure QSDC, i.e., the Two-Step QSDC,the Quantum-one-time- pad QSDC and Repeated-classical-one-time-pad QSDC proto-cols. In the Two-Step QSDC, an ordered set of EPR pairs is used as a data block forsending secret messages directly. It is divided into two particle sequences, a check-ing sequence and a message-coding sequence. After ascertaining the security of thetransmission of the checking sequence, Alice encodes the secret message directly on – II –Abstractthe message-coding sequence and sends them to Bob. The scheme is secure becausean eavesdropper cannot obtain both particle sequences simultaneously and read outthe messages Alice encodes. In the Quantum-one-time-pad QSDC, we exploited asequence of polarized single photons as the information carrier. Then the procedurefor preparing and measuring the quantum signal is very simple. Different from theQuantum-one-time-pad QSDC, the Repeated-classical-one-time-pad QSDC is a pro-tocol without storing the quantum states. Alice and Bob use a control key generatedwith BB84-QKD to make the secret message carried by quantum states be read outdirectly. Moreover, the quantum signal does not run the double distance between Aliceand Bob. The control key, mapped to a sequence of nonorthogonal states in quantumdirect communication can be used repeatedly after Alice and Bob ascertain that thequantum channel is secure. We have proposed three QSS protocols. One is a QSS protocol with EPR pairs,another is a QSS protocol based on polarized single photon and the third protocol is aCircle-QSS protocol. Moreover, an asymmetric and a symmetric coding for quantumsource in measuring-base-encrypted QKD are introduced. The capacity of this sourcecoding QKD is improved considerably.