  Quantum computation by non-Abelian Majorana zero modes (MZMs) offers an
approach to achieve fault tolerance by encoding quantum information in the
non-local charge parity states of semiconductor nanowire networks in the
topological superconductor regime. Thus far, experimental studies of MZMs
chiefly relied on single electron tunneling measurements which leads to
decoherence of the quantum information stored in the MZM. As a next step
towards topological quantum computation, charge parity conserving experiments
based on the Josephson effect are required, which can also help exclude
suggested non-topological origins of the zero bias conductance anomaly. Here we
report the direct measurement of the Josephson radiation frequency in InAs
nanowires with epitaxial aluminium shells. For the first time, we observe the
$4\pi$-periodic Josephson effect above a magnetic field of $\approx 200\,$mT,
consistent with the estimated and measured topological phase transition of
similar devices.
