  We discuss the dephasing induced by the internal classical chaotic motion in
the absence of any external environment. To this end we consider a suitable
extension of fidelity for mixed states which is measurable in a Ramsey
interferometry experiment. We then relate the dephasing to the decay of this
quantity which, in the semiclassical limit, is expressed in terms of an
appropriate classical correlation function. Our results are derived
analytically for the example of a nonlinear driven oscillator and then
numerically confirmed for the kicked rotor model.
