Electrons on superfluid helium: towards single electron control

Abstract

Electrons floating on the surface of superfluid helium have been suggested as promising mobile spin qubits. While no experimental measurements have been made, theoretical calculations have led to the conclusion that these spins will have long spin decoherence and relaxation times. In this thesis we study how well electrons can be transported on the surface of helium using two types of devices consisting of micron-sized helium-filled channels.

The first type of device is fabricated using the standard silicon CMOS processing, which has underlying gates along the channels for clocking electrons. The electrons are photoemitted above channels, which are filled with superfluid helium by capillary action. Electron packets in the parallel channels can be efficiently transported over a billion pixels without detectable errors using the gates connected as a 3-phase charge-coupled device (CCD). To reliably obtain a single electron per channel the electrons are clocked into a turnstile region, where the channel narrows and the gates in the region allow us to repeatedly split these electron packets. The results show a plateau in the electron signal as a function of the applied gate voltages, indicating quantization of the number of electrons per channel.

The second type of device moves electrons between channels and a thin film of helium by creating a smooth transition between the two regions. Bringing electrons onto a film above a metallic layer will expedite thermalization, while also allowing for electrical measurements of electron densities when they are above the channels. The transport measurements suggest that the electrons can be transported from one channel, across a helium-coated metal layer, to the neighboring channel.

The extreme efficiency in clocking electrons over pixels and isolating them proves the concept of scalable mobile qubit systems. The ability to move the electrons on and off the thin film region can be used for electron thermalization, as well as a basis for combining transport experiments and future experiments requiring more precise gate control such as controlling electrons over quantum dots.