  Topological superconductors which support Majorana fermions are thought to be
realized in one-dimensional semiconducting wires coupled to a superconductor.
Such excitations are expected to exhibit non-Abelian statistics and can be used
to realize quantum gates that are topologically protected from local sources of
decoherence. Here we report the observation of the fractional a.c. Josephson
effect in a hybrid semiconductor/superconductor InSb/Nb nanowire junction, a
hallmark of topological matter. When the junction is irradiated with a
radio-frequency f in the absence of an external magnetic field, quantized
voltage steps (Shapiro steps) with a height hf/2e are observed, as is expected
for conventional superconductor junctions, where the supercurrent is carried by
charge-2e Cooper pairs. At high magnetic fields the height of the first Shapiro
step is doubled to hf/e, suggesting that the supercurrent is carried by
charge-e quasiparticles. This is a unique signature of Majorana fermions,
elusive particles predicted ca. 80 years ago.
