Quantum Information Process Based On Josephson Effect

Stimulated by the chance of developing quantum computing, quantum state engineering has become a field of general interest in physics. The basic unit of information is a two-state quantum system, called qubit, whose dynamics have to be controlled externally. For the purpose of quantum computing, a huge number of qubits have to be prepared, manipulated coherently, coupled and measured independently. In practice, these requirements are difficult to satisfy simultaneously and realistic proposals only fulfill a few of the demanded criteria. Microscopic systems like atoms or ions, for instance, have the advantage that they behave naturally quantum coherently. On the other hand, these systems may not be scalable.In this thesis, we concentrate on proposals based on superconducting nan-odevices. Being implemented in electronic circuits, these systems could be scaled up to a large number of qubits. At present many spectacular experiments including coherent operations on single and two coupled qubits have been performed in the Josephson junction systems. However, the project of superconducting quantum computing has to overcome some major obstacle: Since these systems are not naturally quantum coherent the coherence time is relatively short; In addition, the coupling between solid state devices are usually always-on, hence universal quantum gates can not be easily realized.In this work, we aim at attacking the decoherence and coupling problem in solid state quantum computing. Our results focus mainly on the following three issues:1.In many previous "always-on" quantum computing schemes the non-nearest-neighbor couplings are often omitted because of their weak strength compared with the nearest-neighbor coupling. we consider the problem of residual long-range interaction. We propose a QC scheme in which by using generalized "barrier spin" encoding method, the influence of the long-range coupling can be significantly suppressed. The universal quantum gates could be deterministic established. We also study the application of this scheme in Josephson junction system.2.We study a current-biased Josephson junction (CBJJ) as a tunable coupler for superconducting transmission line resonators (TLR). By modulating the bias current, the junction can be tuned in and out of resonance with the TLRs connected to it. Various inter-TLR quantum operations can be reliably implemented by controlling the mediating CBJJ. The main decoherence sources are analyzed in detail.3.The performance of Josephson charge qubits performances are strongly degraded by decoherence due to low frequency background noise, typically with a 1/f spectrum. We investigate the decoherence process of two Cooper pair boxes (CPB) coupled via a capacitor. Going beyond the common/uncorrelated noise models and the Bloch-Redfield formalism of previous works, we study the coupled system's quadratic dephasing on the condition of partially correlated noise sources. Based on reported experiments and generally accepted noise mechanism, we introduce a reasonable assumption for the noise correlation, with which the calculation of multiqubit decoherence can be simplified to a problem on the single qubit level. Our results demonstrate that the quadratic dephasing rates are not very sensitive to the spatial correlation of the noises. Furthermore, we discuss the feasibility and efficiency of dynamical decoupling in the coupled CPBs.