We present measurements of the resonant microwave absorption between the Rydberg energy levels of surface-state electrons on the surface of superfluid liquid helium, in the frequency range 165-220 GHz. The resonant frequency was temperature dependent. The experiments are in agreement with recent theoretical calculations of the renormalization of the electron energy levels due to zero-point and thermal ripplons. The temperature-dependent contribution to the line width gamma(T) for excitation to the first excited state at 189.6 GHz is compared with other measurements and theoretical predictions.

Experiments are in progress to measure and control the quantum states of surface state electrons on liquid helium, both individually and in arrays, for potential use as qubits. This requires the fabrication of novel electronic devices using microstructured substrates, the excitation of Rydberg states using millimetric microwaves and the detection of individual electrons and their quantum states. This paper presents some preliminary experimental results.

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.

We present measurements of the conduction of nondegenerate free electrons along a low-dimensional channel at low temperatures, using surface-state electrons on liquid helium in novel microelectronic devices. Above 1 K, the electrons form an ideal classical Drude conductor. Below 1 K, Coulomb interactions produce electronic spatial order, leading to strong non-Ohmic effects and negative differential conductivity. Evidence is presented for self-organized current filaments in the channel, created by a nonequilibrium phase transition. Periodic conductance oscillations suggest an anisotropic spatial order with lines of electrons along the channel edges.

Microelectronic devices for surface state electrons on liquid helium are being developed using surface structures on quartz and GaAs substrates. Conduction occurs along suspended helium films, held by surface tension in channels, 1–2μm deep and 16–30μm wide. Low-frequency devices with source, drain and gate electrodes have been demonstrated. Preliminary measurements indicate that the electron mobility is similar to that on bulk helium.