are obtained,
where 
,
and H0 and H1 are
matrices of rank N, taken from the Gaussian unitary ensemble
(GUE) or the Gaussian orthogonal ensemble (GOE), respectively. For
small perturbation strengths 
a Gaussian decay
of the fidelity amplitude is observed, whereas for stronger
perturbations a change to a single-exponential decay takes place.
Close to the Heisenberg time 
, however, a partial revival
of the fidelity is found. It can be interpreted in terms of an
spectral analogue of the Debye-Waller factor, describing in X-ray
spectroscopy the decrease of von Laue reflexes with increasing
temperature.
Key words: random matrix theory, fidelity
