Pure dephasing due to bistable quantum impurities

We study the dynamics of a spin (qubit) coupled to a bistable quantum impurity interacting with a Gaussian bath modeled by a ohmic spin boson model. For white noise the complete dynamics in the four-dimensional Hilbert space is analyzed within a Lindblad formalism. For ohmic damping at finite temperatures we resort to a functional integral approach. We show how different dynamical regimes and crossover of the nonlinear spin boson system are probed by the qubit dynamics.