  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.
