Strong coupling of a microwave photon to an electron on helium

Electrons confined above the surface of superfluid helium present a promising platform for scalable charge- and spin-based quantum computing, owing to their ultra-high mobility, compatibility with CMOS fabrication, and the long predicted coherence times of single spins in vacuum. Here, we demonstrate single-electron control and quantum measurement in a hybrid circuit quantum electrodynamic (cQED) device that integrates helium microchannels, an electron quantum dot, and a high-impedance superconducting resonator. This architecture enables strong coupling between the resonator's microwave field and the in-plane motional (charge) quantum states of a trapped electron, with a coupling strength of g/2π = 118 MHz, which exceeds both electron motional decoherence and resonator losses. Precise control of electron trapping and orbital frequency tuning shows excellent agreement with finite-element modeling, validating our device design and fabrication. These results mark a critical step toward coherent measurement and manipulation of electrons on helium.