Experimental verification of fidelity decay: From perturbative to Fermi   Golden Rule regime
  The scattering matrix was measured for a flat microwave cavity with classically chaotic dynamics. The system can be perturbed by small changes of the geometry. We define the "scattering fidelity" in terms of parametric correlation functions of scattering matrix elements. In chaotic systems and for weak coupling the scattering fidelity approaches the fidelity of the closed system. Without free parameters the experimental results agree with random matrix theory in a wide range of perturbation strengths, reaching from the perturbative to the Fermi golden rule regime. 
R. Schaefer, T. Gorin, T. H. Seligman, H.-J. Stoeckmann