Magnetic field induced 4$\pi$ periodic Josephson effect in InAs nanowires

Majorana zero modes (MZM) are leading candidates to implement \emph{topological} quantum computing, owing to their predicted non-Abelian properties and their non-local protection against decoherence. While the observation of a zero-bias conductance peak in tunneling spectroscopy of nanowires strongly suggests the onset of these zero-energy states, additional proof of their existence has remained elusive. Here, we report measurements of Josephson radiation in Josephson junctions formed in a proximity-induced superconducting InAs nanowire with an epitaxially-grown Al shell. The emitted radiation is directly measured \emph{on-chip}, using photon-assisted tunneling across nearby capacitively-coupled superconducting tunnel junctions. The frequency of the detected signal evolves from a 2$\pi$ to a 4$\pi$ periodicity as the magnetic field is increased. The evolution of this transition is studied as a function of chemical potential and of transmission across the nanowire junctions. These results are interpreted using both topological and non-topological models.