The Theoretical Study Of Quantum Secure Direct Communication

Quantum communication is an important subject of quantum information. It utilizes the basic laws of quantum physics, such as the uncertainty principle and the non-cloning theorem to provide a new way for secure transmission of information. Compared with conventional cryptosystem, it has two important characters, unconditional security and eavesdropping detection. Quantum communication could solve the computational complexities in classical cryptosystem.With the development of quantum communication, quantum secure direct communication (QSDC) has become a very important branch of quantum communication. During the process, the secret message could be transmitted directly between two communication parties in the form of different photon states. Different from Quantum Key Distribution (QKD), the key point of QSDC is transmitting important messages directly without first establishing random keys. So this communication system requires a higher security than QKD which could not implement a simple rejection.My work involves in designing new theoretical methods of quantum secure direct communication. We have proposed two feasible QSDC protocols and one de-coherence free subspace state generation protocol.Firstly, we propose two quantum secure direct communication methods with authentication. The authentication key expansion method is introduced to improve the life of the keys with security. In the first scheme, the third party, called Trent is introduced to authenticate the users that participate in the communication. He sends the polarized photons in blocks to authenticate communication parties Alice and Bob using the authentication keys. In the communication process, polarized single photons are used to serve as the carriers, which transmit the secret messages directly. The second QSDC process with authentication just between two parties is also discussed. Secondly, an improved quantum secure direct communication protocol is proposed in this paper. Blocks of entangled photon pairs are transmitted in two steps in which secret messages are transmitted directly. The single logical quantum bits and unitary operations under de-coherence free subspaces are presented and the generalized Bell states are constructed which are immune to the collective noise. We present a de-coherence free subspace state generation protocol using cross-Kerr nonlinearities and quantum logical gates. Also we discussed the success probabilities and fidelities of our proposed protocol.