Quantum Decoherence In Quantum Repeaters And Circuit QED Systems

In quantum mechanics, quantum decoherence is the mechanism by which quantum systems interact with their environments to exhibit probabilistically additive behavior. Recently, quantum deoherence has received intense considerations in quantum information and quantum computation, where decoherence is regarded as a major obstacle and a bottleneck to experimental realizations of quantum computation and communication. In this thesis, we are concerned with decoherence in quantum repeaters based on CNOT gate and a circuit QED system consisting of a charge qubit and two superconducting transmission line resonators (TLRs).We study single-qubit and single-user quantum repeaters based on CNOT gates under decoherence using the Kraus-operator representations of decoherence. We investigate the influence of decoherence on the information-disturbance trade-off of quantum repeaters. It is found that decoherence may lead to the appearance of three subspaces, called as the normal subspace, the anomalous subspace, and the decoherence-free subspace (DFS), respectively. It is indicated that in the normal subspace decoherence decreases the transmission and estimation fidelities, in the anomalous subspace decoherence enhances these fidelities, and in the DFS these fidelities do not change. It is indicated that the quality factor can be efficiently controlled and manipulated by changing the initial state of the probe qubit. It is found that under certain conditions the quantum repeater can be optimal even in the presence of decoherence.We propos a scheme to realize decoherence-free quantum dynamics of the bipartite consisting of the charge qubit and one superconducting TLR by using another superconducting TLR as auxiliary subsystem. In this scheme one TLR and the charge qubit constitute the controlled target system while the other TLR is the auxiliary subsystem which acts as a tool to control the target system. The whole of them forms a triple-partite circuit-QED system. It is found that in the dispersive regime of the circuit QED system, decoherence-free quantum dynamics of the bipartite target system can be realized when the auxiliary TLR subsystem is initially prepared in proper number states. This implies that by controlling and manipulating the auxiliary subsystem, one can protect quantum system against decoherence. This provides fundamental insight into the control of decoherence in circuit QED systems.