  The transition rate for a two-state system interacting with a bosonic heat
bath, from the initial state `up' at time t=0 to `down' at time t>0, was
derived formally in the seminal paper [12] by Leggett, Chakravarty, Dorsey,
Fisher, Garg and Zwerger. The expression was obtained for small tunneling
parameter of the two-level system using time-dependent perturbation theory to
second order. It contains all orders in the parameter of coupling between the
system and the bath. We give a rigorous derivation of the transition rate,
using a quantum resonance approach.
