Investigation Of Mixed States In Quantum Communications

Quantum information is a new-born subject, which has been developed rapidly in recent years. It consists of quantum computation and quantum communication. Its main object is to realize quantum information processing and quantum simulation via quantum system in a controlled way. However, decoherence will be induced more or less from the interaction between the quantum systems and their environment. This will affect quantum information processing, and thus cause the loss of quantum information. Therefore, we need to investigate quantum communication via mixed states.The cavity QED system is considered to be one of the most promising systems to physically realize quantum information processing and quantum computer. Therefore, in this thesis, we mainly explore the quantum information processing based on mixed states, and obtain the following result:1. Controlled quantum teleportation through noisy GHZ channel.We propose two physical schemes for teleporting an unknown atomic state through noisy channel in cavity QED. The quantum channel is a noisy one----a mixed GHZ state, which is more realistic in quantum information processing. We solve analytically a master equation in the Lindblad form with(k2,z,k3,z, k4,z)-type of noise in cavity QED. The comparison between the two protocols is also discussed. In the controlled teleportation scheme, the sender can choose neither of the receivers to reconstruct the original information.2. Quantum entanglement swapping through mixed states.(1) Quantum entanglement swapping through mixed Bell-state.We propose an entanglement swapping scheme via mixed states. The mixed states are mixed Bell state, which is more practical than the pure ones as quantum channel in quantum information processing. We solve analytically a master equation in the Lindblad form with (L2,z,L3,z)-type of noise in cavity QED. The fidelity and successful probability of these protocols are also explicitly discussed.(2) Quantum entanglement swapping in a non-Markovian environment.In a classical non-Markovian noise, which is modeled as the so-called Ornstein-Uhlenbeck processes, the two original Bell states will evolve to X-type mixed states before entanglement swapping. We investigate the entanglement swapping scheme based on such mixed states and obtain the fidelity and concurrence of the states after the swapping.