Dissipative quantum systems modeled by a two-level-reservoir coupling

The coupling between a quantum dynamical system and a two-level-system reservoir is analyzed within the framework of the Feynman-Vernon theory. We stress the differences between this reservoir and the well-known bath of oscillators and show that, in order to obtain the Langevin equation for the system of interest in the high-temperature regime, we have to choose a spectral distribution function J({ω}) that is finite for {ω}=0.