Microwave spectroscopy of Andreev bound states in topological insulator-based Josephson junctions*

An interest in topological superconductivity is fueled by the promise to realize non-Abelian excitations, a pursuit which has both an intellectual and a practical appeal.  One of the first proposals to realize a topological superconductor is a heterostructured material that combines a topological insulator (TI) and a conventional superconductor (SC). Despite a decade of intense studies, supercurrent dynamics in TI/SC Josephson junctions is poorly understood. One of the hallmarks of a topological Josephson junction is the 4$pi$ periodicity of the energy spectrum of in-gap Majorana excitations, which distinguishes these modes from 2$pi$ periodic Andreev states. The difference is most apparent when a phase difference of $pi$ is applied across the junction, at this phase bias Majorana modes cross at zero energy, while Andreev states remain gapped. We report investigation of low energy spectrum of in-gap excitation in TI/SC Josephson junctions using microwave spectroscopy. The junctions are fabricated from topological insulator BiSbTeSe2 nanoribbons with Ti/Al superconducting electrodes. The junctions are incorporated into an rf-SQUID coupled inductively to a high-Q microwave resonator circuit. Using the microwave photons as a probe, in-gap excitations can be coherently coupled to microwave photons and transition energy between states can be readout by the response of the coupled microwave resonator.