Quantum Suppression of Microwave Ionization of Rydberg Atoms at High Scaled Frequency

Microwave ionization of Rydberg atoms is well described as the onset of classical chaos when the microwave frequency omega is less than the Kepler frequency 1/n(3). However, when omega>1/n(3), i.e., at high scaled frequency Omega=omegan(3)>1, classical ionization is predicted to be suppressed by quantum interference, an analogue to Anderson localization in a solid. Using 17.55 GHz microwave fields we have observed the ionization of Sr Rydberg atoms in the regime 1</=Omega</=5 . Our measurements demonstrate the quantum suppression of classical diffusive ionization and show the ionization field to be n independent in this regime.