We present measurements of the resonant microwave excitation of Rydberg energy levels for surface-state electrons on superfluid helium. The temperature-dependent contribution to the linewidth $\gamma (T)$ agrees with theoretical predictions and is very small below 700 mK, in the ripplon scattering regime. 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 $\Omega$ was determined as a function of microwave power. High values of the ratio $\Omega /\gamma$ confirm this system as an excellent candidate for creating qubits.

• Received 31 July 2002

DOI:https://doi.org/10.1103/PhysRevLett.89.245301