Microwave Andreev bound state spectroscopy in a semiconductor-based Planar Josephson junction

By coupling a semiconductor-based planar Josephson junction to a superconducting microwave resonator, we investigate the Andreev bound states in the junction using dispersive readout techniques. Using electrostatic gating to create a narrow constriction in the junction, our measurements unveil a strong coupling interaction between the resonator and the Andreev bound states. This enables the mapping of isolated tunable Andreev bound states that can reach near-unity transparency along with an average induced superconducting gap of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mo>∼</a:mo><a:mn>150</a:mn><a:mspace width="0.16em"/><a:mi>µ</a:mi><a:mi>eV</a:mi></a:mrow></a:math>. Exploring the gate parameter space further elucidates a nonmonotonic evolution of multiple Andreev bound states with varying gate voltage. Complementary tight-binding calculations of an Al-InAs planar Josephson junction with strong Rashba spin-orbit coupling provide insight into possible mechanisms responsible for such behavior. Our findings highlight the subtleties of the Andreev spectrum of Josephson junctions fabricated on superconductor-semiconductor heterostructures, offering potential applications in probing topological states in these hybrid platforms.