Probing Andreev bound states with circuit quantum electrodynamics

Andreev bound states (ABSs), the quantum many-body electronic states that are localized at Josephson weak-links, provide a platform to explore the interplay of superconductivity, spin-orbit interaction, Coulomb interaction, and magnetism, including in topological regimes. ABS carry supercurrent and are thus suited to being probed by the circuit quantum electrodynamics (cQED) toolset, which offers high-resolution, high-bandwidth microwave-domain measurement and manipulation of quantum states. In this talk, I will describe our implementation of cQED to reveal the spectrum, dynamics, and potential applications of quasiparticles trapped in ABSs hosted in a Josephson semiconductor nanowire.

 

After an introduction to ABS, I will describe our use of superconducting resonators for quantitative measurement of microwave response functions and spectroscopy in the presence of non-equilibrium state populations. With this tool, we developed insights on Coulomb interaction in ABSs, which are conventionally regarded as chargeless states. Second, I will describe the influence of spin-orbit interaction on ABSs and how we leveraged that interaction to realize the Andreev spin qubit, a supercurrent-carrying spin degree of freedom. Finally, I will argue that these experiments in “fermionic cQED” lay a foundation to explore the physics and applications of Majorana bound states and superconducting devices built from 2d quantum materials.