  We discuss the dephasing induced by the internal classical chaotic motion in
the absence of any external environment. To this end a new extension of
fidelity for mixed states is introduced, which we name {\it allegiance}. Such
quantity directly accounts for quantum interference and is measurable in a
Ramsey interferometry experiment. We show that in the semiclassical limit the
decay of the allegiance is exactly expressed, due to the dephasing, in terms of
an appropriate classical correlation function. Our results are derived
analytically for the case of a nonlinear driven oscillator and then numerically
confirmed for the kicked rotor model.
