Revisiting physics of quantum bits

Advancements in the technology of quantum bits invoke more precise calculations for decoherence and dissipative effects. In this thesis, the physics of truncated two level systems is revisited and it is shown that in some systems, such as in triple-junction superconducting flux qubit, environmental noises that are usually recognized to only have dissipative effects and the trivial resultant dephasing, can also cause pure dephasing. Furthermore, it is demonstrated that, in the current domain of interest for quantum computation purposes, the effective Hamiltonian of single molecule magnets in interaction with a spin bath differs from the commonly quoted result in the literature. It is also shown that the topological effects in such problems are as small as transitional effects to higher excited states beyond the two-level picture. Finally, a simulation of the quantum noise by the classical noise and a simulation of spin bath by oscillator bath for a quantum bit as the principal system are presented