Tuning Andreev Bound States Using a Quantum Dot Embedded in a Superconducting Qubit

Hybrid superconducting circuits, which integrate non-superconducting elements into a circuit quantum electrodynamics (cQED) architecture expand the possible applications of cQED. In hybrid superconducting circuits, one can investigate the properties of unconventional Josephson junctions with extremely fine energy resolution or perform readout of quantum dots and spin ensembles. In this talk, I will discuss our work using quantum dots to change the nature of Andreev bound states in the weak-link junctions of superconducting transmon qubits based on Al-proximitized InAs nanowires.<br/>First, I will show our work using a quantum dot to control the transmission of Andreev bound states in the junction of a transmon qubit. By tuning the quantum dot on and off resonance, we changed the charge dispersion of the transmon by over an order of magnitude and achieve near-unity transmission in the transmon qubit junction. We observe a factor of two increase in the transmon coherence times at points of low charge dispersion.<br/>Next, I will discuss our work using a transmon to investigate the ground state of a weak-link junction with a quantum dot embedded in it. In such a device, the competition between the charging energy of the dot and superconductivity stabilizes different spin ground states in the junction. We are able to map out the phase diagram of the composite system by changing several control parameters: plunger and tunnel gate voltages, external flux, and magnetic field applied parallel to the wire. We then investigate the quasiparticle poisoning dynamics of the junction as the ground state is tuned from the singlet to the doublet state. We observe more than a magnitude increase in the quasiparticle lifetime of the Andreev bound states in the junction when tuning the ground state the singlet to the doublet phase.<br/>Finally, I will discuss our work building superconducting devices on Ge-based two-dimensional hole gases and prospects for investigating Andreev bound states in these hole gases.