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. 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.