Piezoacoustics for flying electron qubits on helium.

Piezoelectric surface acoustic waves (SAWs) are powerful for investigating and controlling elementary and collective excitations in condensed matter. In semiconductors SAWs have been used to reveal the underlying spatial and temporal structure of quantum phases of electronic matter, produce quantized charge pumping, transfer quantum information, and coherently control single electron states, culminating in the possibility of SAW-driven electronic flying qubits. However important challenges remain in protecting the coherence of propagating electrons from uncontrolled background nuclear spins, chemical dopants and spin-orbit crystal fields. In contrast, electrons trapped on the surface of superfluid helium form a ultra-clean low-dimensional electron system devoid of the imperfections limiting the coherence in semiconductors. However, SAWs have not, to-date, been employed to manipulate electrons in this system or to explore the development of flying-qubits. Additionally, electrons on helium form strongly-interacting Coulomb liquid and solid states having collective dynamics that can couple to SAWs. Here we report on the first coupling of electrons on helium to an evanescent piezoelectric SAW. The electrons surf with the piezoelectric wave and we demonstrate high-frequency charge pumping in this system for the first time. With this piezoacoustic method we can precisely transport as little as ~0.01% of the total electron density, opening the door to quantized charge pumping, the possibility of electrical metrology, and ultimately single electron state transfer with electrons on helium. We also show that SAWs are a route to directly investigating the high-frequency dynamical response, and relaxational processes, of collective excitations of the electronic liquid and solid phases of electrons on helium.