Dynamics of Dissipative Two-Level Systems in the Stochastic Approximation

The dynamics of the spin-boson Hamiltonian is considered in the stochastic approximation. The Hamiltonian describes a two-level system coupled to an environment and is widely used in physics, chemistry and the theory of quantum measurement. We demonstrate that the method of the stochastic approximation which is a general method of consideration of dynamics of an arbitrary system interacting with environment is powerful enough to reproduce qualitatively striking results by Leggett at al. found earlier for this model. The result include an exact expression of the dynamics in terms of the spectral density and show an appearance of two most interesting regimes for the system, i.e. pure oscillating and pure damping ones. Correlators describing environment are also computed.