  Gate-tunable transmons (gatemons) employing semiconductor Josephson junctions
have recently emerged as building blocks for hybrid quantum circuits. In this
study, we present a gatemon fabricated in planar Germanium. We induce
superconductivity in a two-dimensional hole gas by evaporating aluminum atop a
thin spacer, which separates the superconductor from the Ge quantum well. The
Josephson junction is then integrated into an Xmon circuit and capacitively
coupled to a transmission line resonator. We showcase the qubit tunability in a
broad frequency range with resonator and two-tone spectroscopy. Time-domain
characterizations reveal energy relaxation and coherence times up to 75 ns. Our
results, combined with the recent advances in the spin qubit field, pave the
way towards novel hybrid and protected qubits in a group IV, CMOS-compatible
material.
