Description and control of decoherence in quantum bit systems
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. \nAt first, we derive a microscopic, perturbation theoretical approach for Lindblad master equations of a spin-Boson model at low temperatures.\nA different sort of decoherence is investigate by means of the bistable fluctuator model.\nFor this particular but nevertheless for solid state qubits relevant noise source, we present a suitably designed dynamical decoupling method (so-called quantum bang-bang).\nThis 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.\nFurthermore, we investigate the behaviour of a two coupled spin system exposed to collective and localized bath.\nFor this dressed-spin system we receive by means of scaling-analysis in first order a quantum phase diagram. \nOn that we can identify the various quantum dynamical and entanglement phases.