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

We investigate microwave excitation of helium 28 ${}_{}{}^3{}_{}{}^{}$S Rydberg atoms and find that a second microwave field lifts the degeneracy of certain Floquet states describing the 28 ${}_{}{}^3{}_{}{}^{}$S He atom plus 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 noise. We present two-frequency Floquet maps showing the origin of the interferences.