Strong coupling of a microwave photon to an electron on helium

Electrons bound to the surface of superfluid helium have been proposed for scalable charge and spin-based quantum computing. However single electron quantum measurement in this system has remained elusive. Here we use a hybrid circuit quantum electrodynamic (cQED) device that comprises a quantum dot and a high-impedance superconducting resonator to demonstrate, for the first time, strong coupling between the resonator microwave field and the motional quantum state of the electron. We find a coupling strength between the electron motion and a resonator photon of $g/2π=118$ MHz, exceeding both the electron motional state decoherence and the resonator loss. These experiments open new avenues for investigating light-matter interaction at the single electron level, and are a key step towards measurement and control of electrons on helium-based spin qubits.
Main manuscript: 7 pages, 4 figures Supplementary information: 19 pages, 16 figures, 3 tables