High Quality Two Layer LC Resonator at Liquid Helium Temperature

Lumped element resonators have recently been shown to have excellent sensitivity to nearby electron spins, achieving coupling rates in the MHz. However, these resonators probe the atomic structure of dopant atoms which are completely immobile in the semiconductor host. To trap mobile electrons, a second metal layer is required, which could possibly degrade the quality factor of the designed resonator structure.

Electrons on liquid Helium is a promising platform for quantum information processing. Vertically bound by its image charge and the Pauli exclusion principle, electrons form a 2D electron gas in vacuum above the smooth superfluid He. Analogous to a plunger gate in silicon qubits, we need a conductive layer underneath the inductor to trap electrons near it without damaging the quality factor. We explore such a structure using a two-layer lumped element LC resonator which has a measured quality factor of 1700 at 4.2K. The device consists of a superconducting Nb layer on top with a highly resistive NbSi layer underneath used for applying a DC bias to trap electrons on the surface. To maximize spin-resonator coupling, we minimize the resonator impedance and observe no change in quality factor when varying potentials between the top Nb layer and bottom NbSi layer.