Quantum Teleportation And Quantum Secure Direct Communication

Quantum information is a new crossed subject that is a combination of physics science and information science at the 80's in 20 centuries. Quantum information which is based on the basic theory of quantum mechanics mainly includes quantum communication and quantum computation, because of the subtle feature of quantum world, quantum information exhibits a number of advantages corresponding to its classical counterpart. Quantum communication is a very important branch of quantum information science. At present, it mainly contains quantum teleportation, quantum dense coding, quantum secret sharing,quantum key distribution and quantum secure direct communication. Quantum teleportation proposed by pioneer Bennett et al in 1993 is one of the most striking fields in quantum communication. Its reality will greatly promote course and speed of quantum communication. So, seeking more reasonable and more pefect quantum teleportation scheme will have a tremendous pushing role in the quantum information processing, quantum computer, quantum information control and quantum secure direct communication feild.This thesis mainly reseaches the theoretical scheme of deterministic and probabilistic quantum controlled teleportation and its application in quantum secure direct communications when multi-quantum channel and the general Bell measurement are utilized.Teleportation of multi-particle quantum states as an important basis of "one-to-many" and "many-to- many" communications in the network has been paid much attention. The decoherence may arise because of the interaction between quantum channel and environment. At that moment, quantum channel is always not a maximally entangled state. So, We proposed a "one-to-one" communication scheme that three-particle partly entangled unknown state is probabilisticly teleported under the third party control. In this scheme, a Non-maximally entangled five-particle state is utilized as the quantum channel, one of the particles in which is selected as the controlling particle. The sender Alice makes Bell states measurement once, Hadamard operation and {|0〉,|1〉} base measurements two times, and the controller Charlie performs a Hadamard and {|0〉,|1〉} base measurement, so the receiver can get the quantum state which Alice wants to send to her by performing appropriate unitary transformations and some essential projection measurements determined by the sender and controller's measurement results transmitted via classical channel. Then we also generalize into N-particle state's quantum teleportation. In the scheme, a Non-maximally entangled (N+2)-particle state is utilized as the quantum channel, one of the particles in which is selected as the controlling particle. The sender Alice makes Bell states measurement once and Hadamard operation and {|0〉,|1〉}base measurements N-1 times, and the controller Charlie performes a Hadamard operation and {|0〉,|1〉} base measurement, so the receiver can get the quantum state which Alice wants to send by performing appropriate unitary transformations determined by the sender and controller's measurement results transmitted via classical channel. The scheme is a controlled quantum teleportation based on Non-maximally entangled state with certain probability and successful teleportation probability is 2 |a|~2.Based on the principle of quantum secure direct communication, we propose the two schemes for controlled and secure direct communication by using GHZ state and W state respectively. The two schemes are all based on controlled quantum teleportation. After insuring the security of the quantum channel, the sender Alice encodes the secret messages directly on a sequence of particle states and transmits them to Bob supervised by Charlie using controlled quantum teleportation. Bob can read out the encoded messages directly by the measurement on his qubits. In these schemes, the controlled quantum teleportation transmits Alice's messages without revealing any information to a potential eavesdropper. Because there is not a transmission of the qubit carrying the secret message between Alice and Bob in the public channel, they are completely secure for controlled and direct secret communication if perfect quantum channel is used.