Direct microwave spectroscopy of Andreev bound states in planar Ge Josephson junctions

We demonstrate microwave measurements of the Andreev bound state (ABS) spectrum in planar Josephson junctions (JJs) defined in Ge high-mobility two-dimensional hole gases contacted with superconducting platinum germanosilicide (PtSiGe). The JJs and readout circuitry are located on separate chips and coupled inductively via flip-chip bonding. For a device with 350 nm junction length, the spectroscopic signatures were consistent with the short-junction limit, with transmission τ ≈ 0.94 and an induced superconducting gap Δ* ≈ 48 μeV. The interaction between the highest-transmission ABS and the resonator was well-described by a Jaynes-Cummings model yielding a vacuum Rabi splitting of approximately 6 MHz. A device with a junction length of 1 μm showed an ABS spectrum consistent with the long-junction model. Time-resolved monitoring of the resonator in the dispersive regime revealed gate-voltage-tunable junction parity fluctuations on the time scale of seconds. Our results indicate a viable path toward hybrid quantum devices based on planar Ge.