  Electrons on helium form a unique two-dimensional electron system on the
interface of liquid helium and vacuum. On liquid helium, trapped electrons can
arrange into strongly correlated states known as Wigner molecules, which can be
used to study electron interactions in the absence of disorder, or as a highly
promising resource for quantum computation. Wigner molecules have orbital
frequencies in the microwave regime and can therefore be integrated with
circuit quantum electrodynamics (cQED), which studies light-matter interactions
using microwave photons. Here, we experimentally realize a cQED platform with
the orbital state of Wigner molecules on helium. We deterministically prepare
one to four-electron Wigner molecules on top of a microwave resonator, which
allows us to observe their unique spectra for the first time. Furthermore, we
find a single-electron-photon coupling strength of g/2$\pi$ = 4.8$\pm$0.3 MHz,
greatly exceeding the resonator linewidth $\kappa$/2$\pi$ = 0.5 MHz. These
results pave the way towards microwave studies of strongly correlated electron
states and coherent control of the orbital and spin state of Wigner molecules
on helium.}
