Fidelity for kicked atoms with gravity near a quantum resonance

Kicked atoms under a constant Stark or gravity field are investigated for experimental setups with cold and ultra cold atoms. The parametric stability of the quantum dynamics is studied using the fidelity. In the case of a quantum resonance, it is shown that the behavior of the fidelity depends on arithmetic properties of the gravity parameter. Close to a quantum resonance, the long time asymptotics of the fidelity is studied by means of a {\em pseudo-classical} approximation first introduced by Fishman {\em et al.} [J. Stat. Phys. {\bf 110}, 911 (2003)]. The long-time decay of fidelity arises from the tunneling out of pseudo-classical stable islands, and a simple ansatz is proposed which satisfactorily reproduces the main features observed in numerical simulations.