  In recent years, notable progress has been made in the study of hole qubits
in planar germanium, and circuit quantum electrodynamics (circuit QED) has
emerged as a promising approach for achieving long-range coupling and scaling
up of qubits. Here, we demonstrate the coupling between holes in a planar
germanium double quantum dot (DQD) and photons in a microwave cavity.
Specifically, a real-time calibrated virtual gate method is developed to
characterize this hybrid system, which in turn allows us to determine the
typical parameters sequentially through single-parameter fitting instead of
conventional multi-parameter fitting with additional uncertainty, and gives the
hole-photon coupling rate of $g_0/2\pi$ = 21.7 MHz. This work is a step toward
further research on hole-photon interactions and long-range qubit coupling in
planar germanium. The experimental method developed in this work contributes to
the more accurate and efficient characterization of hybrid cavity-QED systems.
