Qiao, Wenling2013-01-252013-01-252013-01-252012http://hdl.handle.net/10012/7294This thesis studies a spin-star model and an open spin-star model. The spin-star model is exactly solvable, and the approximation methods can be applied and compared with the exact solution. As this model is shown to be non-Gaussian and non-Markovian, Born-Markovian approximation is not valid. The comparison of Nakajima-Zwanzig and time-convolutionless methods show that the performances of those two techniques depend on the specific property of the model. The open spin-star model is not exactly solvable and the analytical solution under Gaussian approximation is obtained.ennon-Markoviannon-GaussiandecoherenceMaster ThesisPhysics