Microwave signature of robust topological Andreev-level crossings in superconducting proximitized crystalline Bi nanowires

Demonstrating the topological protection of Andreev states in Josephson junctions is an experimental challenge. In particular the telltale $4\pi$ periodicity expected for the current phase relation of topological Josephson junctions has remained elusive, because of fast parity breaking processes. It was predicted that low temperature ac susceptibility measurements related to the current thermal noise could reveal the topological protection of quantum Spin Hall edge states \cite{Fu2009}, by probing their low energy Andreev spectrum.% of QSH-based Josephson junctions. We have performed such a microwave probing of the Andreev spectrum of a phase biased Josephson junction built around a bismuth nanowire, which was previously shown to host one-dimensional ballistic edge states. We find absorption peaks at the Andreev level crossings, whose temperature and frequency dependences point to protected topological crossings with an accuracy limited by the electronic temperature of our experiment.