Observation of the Rydberg Resonance in Surface Electrons on Superfluid Helium Confined in a 4-μm Deep Channel

We report the first observation of the microwave-induced intersubband (Rydberg) resonance of surface-bound electrons on superfluid helium confined in a single 20- μ m wide and 100- μ m long channel. The resonance signal from a few thousand surface electrons comprising a Wigner Solid (WS) is detected by observing WS melting due to the microwave absorption. The transition frequency of electrons, in the range of 0.4–0.5 THz determined by the microwave source used in our experiment, is tuned via the Stark shift of energy levels by a voltage applied to the channel electrode, and the resolution of the transition line requires the depth of the channel to be about 4 μ m. The observed large broadening of the resonance on the order of 10 GHz, which is due to the inhomogeneous distribution of the pressing electric field in the microchannel, is in reasonable agreement with our finite-element modeling calculations. This study of the confined electrons is motivated by their potential use for charge and spin qubits.