  Semiconductor quantum dots (QDs) in planar germanium (Ge) heterostructures
have emerged as frontrunners for future hole-based quantum processors. Notably,
the large spin-orbit interaction of holes offers rapid, coherent electrical
control of spin states, which can be further beneficial for interfacing hole
spins to microwave photons in superconducting circuits via coherent
charge-photon coupling. Here, we present strong coupling between a hole charge
qubit, defined in a double quantum dot (DQD) in a planar Ge, and microwave
photons in a high-impedance ($Z_\mathrm{r} = 1.3 ~ \mathrm{k}\Omega$)
superconducting quantum interference device (SQUID) array resonator. Our
investigation reveals vacuum-Rabi splittings with coupling strengths up to
$g_{0}/2\pi = 260 ~ \mathrm{MHz}$, and a cooperativity of $C \sim 100$,
dependent on DQD tuning, confirming the strong charge-photon coupling regime
within planar Ge. Furthermore, utilizing the frequency tunability of our
resonator, we explore the quenched energy splitting associated with
strongly-correlated Wigner molecule (WM) states that emerge in Ge QDs. The
observed enhanced coherence of the WM excited state signals the presence of
distinct symmetries within related spin functions, serving as a precursor to
the strong coupling between photons and spin-charge hybrid qubits in planar Ge.
This work paves the way towards coherent quantum connections between remote
hole qubits in planar Ge, required to scale up hole-based quantum processors.
