Towards microwave spectroscopy of induced superconductivity on high mobility topological insulator Josephson junctions

Andreev bound states (ABSs) are in-gap carriers of supercurrent across the normal region in S-N-S Josephson junctions (JJ). As a fermionic quasiparticle bound state with particle hole symmetry, the isolated ABSs consist of a true two level system with inherited properties from normal materials, such as spin-orbit couplings, geometries, and energy spectrum.

Topological insulators (TIs), with Dirac surface states and gapped bulk, can be an interesting weak link material with exotic boundary conditions, strong spin orbit, and interplay between bulk and surface states. Here, we are demonstrating the microwave spectroscopy of topological insulator Josephson junctions. To optimize the number of modes in the junction, we used exfoliated nanoribbons with width 100 - 400 nm and thickness ~20nm. Electrostatic gate is applied to tune the chemical potential of both surface and bulk states. Employing the toolbox of circuit QED, we incorporate a TI JJ into an RF SQUID and coupled it inductively to a NbTi superconducting microwave resonator. Parametric modulation of the admittance of the RF SQUID by phase biasing the TI JJ results in resonance shift of the cavity. The coherent coupling between the microwave photon and TI ABSs allows a mapping of the TI JJ spectrum within the flux-gate parameter space, opening a possibility to distinguish 4-$pi$ periodic topological modes from 2-$pi$ periodic trivial modes.