Anomalous power law of quantum reversibility for classically regular dynamics

The Loschmidt Echo M(t) (defined as the squared overlap of wave packets evolving with two slightly different Hamiltonians) is a measure of quantum reversibility. We investigate its behavior for classically quasi-integrable systems. A dominant regime emerges where M(t) chi t(-alpha) with = alpha = 3d/2 depending solely on the dimension d of the system. This power law decay is faster than the result chi t(-d) for the decay of the overlap of classical phase space densities.