Singlet-doublet transitions of a quantum dot Josephson junction revealed in a transmon circuit: ground state spectroscopy*

Probing the mesoscopic physics of hybrid superconductor-semiconductor elements using a circuit quantum electrodynamics (cQED) architecture offers enhanced energy and time resolution compared to DC transport techniques, and allows for additional methods of coherent control. Here we investigate the parity phase diagram of a quantum dot with superconducting leads using a hybrid transmon architecture. Our device is composed of a transmon where the Josephson coupling is determined by a gate-controlled quantum dot defined in an InAs-Al nanowire. We map out the parity phase diagram as a function of several control parameters: plunger and tunnel gate voltages, external flux, and magnetic field applied parallel to the wire. The measured phase diagram boundaries are in excellent agreement with those predicted by a single-impurity Anderson model with superconducting leads.