Theoretical Study Of Quantum Communication

Since the 80's of the 20th century, quantum information theory has been thriving whichcombines quantum mechanics and information theory. The quantum information theory,mainly consisting of quantum communication and quantum computing, can accomplish sometasks by using quantum states of microcosmic particles as carriers of information and byexploiting some special properties of quantum mechanics, such as uncertainty, coherence,entanglement and so on, which the classical communication, computation and cryptographydo not possess. The quantum entanglement, the core of the quantum information theory, canbe used in almost all the quantum information processes. Therefore, the study of bothquantum entanglement and its applications in quantum information theory are necessary andvaluable not only to understand well the special properties of the quantum mechanics but alsoto develop and make use of the new information process methods.In quantum information theory, quantum communication can provide a new way fortransmitting message securely, and it is also the first one to put quantum information theoryinto practice. So three main aspects of quantum communication based on quantumentanglement are proposed: quantum key distribution, quantum secure direct communicationand quantum teleportation.Firstly, in the field of quantum key distribution, two theoretical protocols are developed.One quantum key distribution scheme uses Bell states as the quantum channel and canaccomplish quantum key distribution and quantum secure direct communicationsimultaneously. The legitimate communicators can secretly share four certain key bits andfour random key bits via three EPR pairs. The information transforming efficiency of thescheme is very high. In addition, the quantum key distribution scheme is bi-directional.Another quantum key distribution scheme can accomplish the secret share of a series of keybits between two legitimate communicators by operating joint Bell-state measurements andthe classical communication using the four-qubit W state as the quantum channel. The lastscheme expands the field of the useful entanglement sources.Secondly, three quantum secure direct communication schemes are constructed. The firstquantum secure direct communication scheme is included in the first quantum keydistribution scheme. The second one also uses the four-qubit W state as the quantum channel.If quantum channel is perfect, two legitimate communicators can directly transmit the secretmessages by operating joint Bell-state measurements and the classical communication. Even though the quantum channel is unsecured, it is still possible for the two communicators toperform their secure communication. The last one can let two legitimate communicatorstransmit probabilistically the secret information by using quantum teleportation and localmeasurements using the three-qubit W state as the quantum channel.Lastly, in the field of quantum teleportation, four theoretical schemes are proposed. Inthe first scheme, an arbitrary two-qubit quantum state can be probabilistically teleported fromthe sender to the receiver. One of 16 non-maximally entangled G states functions as thequantum channel. The probability of successful teleportation is determined by the smallestone among the coefficients' absolute values of the quantum channel. In the second one, toweaken the requirements for the quantum channel (the number of shared particle pairs),without loss of generality, two communicators only share a non-maximally entangledtwo-qubit quantum state to teleport probabilistically an unknown bipartite quantum state. Inthe third one, a unique three-qubit W state is employed as the quantum channel to teleport anarbitrary single qubit state probabilistically and a particular single qubit state with 100ï¼…probability. In the fourth one, the joint state of Bell states is used as the quantum channel toteleport a multipartite quantum state. In the last two teleportation schemes, QED cavitytechnology is adopted, which realizes the preparation of the entanglement sources (Bell states)and enhances the maneuverability of the two theoretical schemes by transforming the jointBell-state measurements, which is difficult to perform in practice, to single particlemeasurements.Some theoretical protocols of quantum communication are studied. These protocols maybe useful in practice. They are expected to be validated by experiments and to be perfected inthe future.