Microwave saturation of the Rydberg states of electrons on helium
  We present measurements of the resonant microwave excitation of the Rydberg energy levels of surface state electrons on superfluid helium. The temperature dependent linewidth agrees well with theoretical predictions and is very small below 300 mK. Absorption saturation and power broadening were observed as the fraction of electrons in the first excited state was increased to 0.49, close to the thermal excitation limit of 0.5. The Rabi frequency was determined as a function of microwave power. The high values of the ratio of the Rabi frequency to linewidth confirm this system as an excellent candidate for creating qubits. 
E. Collin, W. Bailey, P. Fozooni, P.G. Frayne, P. Glasson, K. Harrabi,   M.J. Lea and G. Papageorgiou