  We show that the quantum fidelity is accessible to cold atom experiments for
a large class of evolutions in periodical potentials, properly taking into
account the experimental initial conditions of the atomic ensemble. We prove
analytically that, at the fundamental quantum resonances of the atom-optics
kicked rotor, the fidelity saturates at a constant, time-independent value
after a small number of kicks. The latter saturation arises from the bulk of
the atomic ensemble, whilst for the resonantly accelerated atoms the fidelity
is predicted to decay slowly according to a power law.
