Microwave-driven He Rydberg atoms: Floquet-state degeneracy lifted by a second frequency, Stueckelberg oscillations, and their destruction by added noise

We investigate excitation of He 283S atoms and find that a 2nd microwave field lifts the degeneracy of certain Floquet states describing the [atom + single microwave field] system. As a result a microwave pulse produces two patterns of Stueckelberg oscillations. They depend sensitively on the frequencies and amplitudes of both microwave fields and are influenced differently by added by added noise. We present two-frequency (2-f) Floquet maps showing the origin of the intererferences. We are able theoretically to reproduce reasonably well our observed 2-f Stueckelberg oscillations for (stronger) 30 GHz and (much weaker) 36 GHz 2-field. The case of both fields strong is much more complicated. It still remains theoretically to explain why the noise influences the two patterns of Stueckelberg oscillations differently