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. A feature of ABSs is that they carry supercurrent, which imbues the states with routable long-ranged electrodynamics. ABSs are thus suited to being probed by the well-developed circuit quantum electrodynamics (cQED) toolset, which offers 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. First, I will discuss the use of superconducting resonators for quantitative measurement of microwave response functions and the resulting insights on Coulomb interaction in our ABSs. 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 quantum-coherent supercurrent-carrying spin. Finally, I will argue that these experiments in “fermionic cQED” lay a foundation to explore the exotic physics of Majorana bound states and 2d quantum materials.

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