Flip-Chip-Based Microwave Spectroscopy of Andreev Bound States in a Planar Josephson Junction

We demonstrate a flip-chip-based approach to microwave measurements of Andreev bound states (ABSs) in a gate-tunable planar Josephson junction (JJ) using inductively coupled superconducting lowloss resonators. By means of electrostatic gating, we present control of both the density and transmission of ABSs. Phase biasing of the device shifted the resonator frequency, consistent with the modulation of supercurrent in the junction. Two-tone spectroscopy measurements revealed an isolated ABS consistent with an average induced superconducting gap of 184 mu eV and a gate-tunable transmission approaching 0.98. Our results represent the feasibility of using the flip-chip technique to address and study ABSs in planar JJs, and they constitute a promising path towards microwave applications with superconductor-semiconductor two-dimensional materials.