Experimental study of quantum and classical limits in microwave ionization of rubidium Rydberg atoms

First experiments are described on the impact of additive noise on the ionization of Rb Rydberg atoms in microwave fields. Dynamical localization and its gradual destruction by noise are observed. The threshold and time dependence of the ionization process are investigated in the presence as well as in the absence of noise. Purely stochastic excitation induces a diffusionlike behavior that might be interpreted as restoration of the classical limit.