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 ω is less than the Kepler frequency 1/n3. However, when ω>1/n3, i.e., at high scaled frequency Ω=ωn3>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≤Ω≤5 . Our measurements demonstrate the quantum suppression of classical diffusive ionization and show the ionization field to be n independent in this regime