Journal of Statistical Physics, Vol. 51, Nos. 1/2, 1988. p. 77-94.
We study the relaxation of a degenerate two-level system interacting with a heat bath, assuming a random-matrix model for the system bath interaction. For times larger than the duration of a collision and smaller than the Poincare recurrence time, the survival probability of still finding the system at time t in the same state in which it was prepared at t = 0 is exactly calculated.