Coupling an ensemble of electrons on superfluid helium to a superconducting circuit

TL;DR

This paper demonstrates a hybrid quantum system where many electrons on superfluid helium are coupled to a superconducting resonator, enabling sensitive detection and potential for scalable quantum computing.

Contribution

First experimental realization of a large ensemble of electrons on superfluid helium coupled to a superconducting circuit, showing strong collective coupling and agreement with models.

Findings

Large dispersive shifts observed, enabling sensitive measurements.

Ensemble coupling exceeds 1 MHz per electron, indicating potential for single-electron strong coupling.

Good agreement between experimental results and numerical models.

Abstract

The quantized lateral motional states and the spin states of electrons trapped on the surface of superfluid helium have been proposed as basic building blocks of a scalable quantum computer. Circuit quantum electrodynamics (cQED) allows strong dipole coupling between electrons and a high-Q superconducting microwave resonator, enabling such sensitive detection and manipulation of electron degrees of freedom. Here we present the first realization of a hybrid circuit in which a large number of electrons are trapped on the surface of superfluid helium inside a coplanar waveguide resonator. The high finesse of the resonator allows us to observe large dispersive shifts that are many times the linewidth and make fast and sensitive measurements on the collective vibrational modes of the electron ensemble, as well as the superfluid helium film underneath. Furthermore, a large ensemble coupling is observed in the dispersive regime during experiment, and it shows excellent agreement with our numeric model. The coupling strength of the ensemble to the cavity is found to be >1 MHz per electron, indicating the feasibility of achieving single electron strong coupling.