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 $\mathrm{Ge}$ high-mobility two-dimensional hole gases contacted by superconducting platinum germanosilicide ($\mathrm{Pt}$$\mathrm{Si}$$\mathrm{Ge}$). The JJs and readout circuitry are located on separate chips and inductively coupled via flip-chip bonding. For a device with 350-nm junction length, the spectroscopic signatures were consistent with the short-junction limit, with an induced superconducting gap ${\mathrm{\Delta }}^{\ast }\approx 48\text{μ}\mathrm{eV}$ and transmission $\tau \approx 0.94$. The interaction between the highest-transmission ABS and the resonator was well described by a Jaynes-Cummings model with a vacuum Rabi splitting of approximately 6 MHz. A device with a junction length of $1\text{μ}\mathrm{m}$ showed an ABS spectrum consistent with a long-junction model. Time-resolved monitoring of the readout resonator in the dispersive regime revealed gate-voltage tunable-junction parity fluctuations on the time scale of seconds. Our work indicates a viable path toward hybrid quantum devices based on planar $\mathrm{Ge}$.