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.
@article{arxiv.2509.14506,
title = {Strong coupling of a microwave photon to an electron on helium},
author = {G. Koolstra and E. O. Glen and N. R. Beysengulov and H. Byeon and K. E. Castoria and M. Sammon and S. A. Lyon and D. G. Rees and J. Pollanen},
journal= {arXiv preprint arXiv:2509.14506},
year = {2025}
}