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Gutmann, Henryk (2005): Description and control of decoherence in quantum bit systems. Dissertation, LMU München: Fakultät für Physik



The description and control of decoherence of quantum bit systems have become a field of increasing interest during the last decade. We discuss different techniques to estimate and model decoherence sources of solid state quantum bit realizations. At first, we derive a microscopic, perturbation theoretical approach for Lindblad master equations of a spin-Boson model at low temperatures. A different sort of decoherence is investigate by means of the bistable fluctuator model. For this particular but nevertheless for solid state qubits relevant noise source, we present a suitably designed dynamical decoupling method (so-called quantum bang-bang). This works as a high-pass filter, suppressing low frequency parts of the noise most effectively and thus being a promising method to compensate the ubiquituous 1/f noise. Furthermore, we investigate the behaviour of a two coupled spin system exposed to collective and localized bath. For this dressed-spin system we receive by means of scaling-analysis in first order a quantum phase diagram. On that we can identify the various quantum dynamical and entanglement phases.