  Topological insulators are a newly discovered phase of matter characterized
by a gapped bulk surrounded by novel conducting boundary states. Since their
theoretical discovery, these materials have encouraged intense efforts to study
their properties and capabilities. Among the most striking results of this
activity are proposals to engineer a new variety of superconductor at the
surfaces of topological insulators. These topological superconductors would be
capable of supporting localized Majorana fermions, particles whose braiding
properties have been proposed as the basis of a fault-tolerant quantum
computer. Despite the clear theoretical motivation, a conclusive realization of
topological superconductivity remains an outstanding experimental goal. Here we
present measurements of superconductivity induced in two-dimensional
HgTe/HgCdTe quantum wells, a material which becomes a quantum spin Hall
insulator when the well width exceeds d_{C}=6.3 nm. In wells that are 7.5 nm
wide, we find that supercurrents are confined to the one-dimensional sample
edges as the bulk density is depleted. However, when the well width is
decreased to 4.5 nm the edge supercurrents cannot be distinguished from those
in the bulk. These results provide evidence for superconductivity induced in
the helical edges of the quantum spin Hall effect, a promising step toward the
demonstration of one-dimensional topological superconductivity. Our results
also provide a direct measurement of the widths of these edge channels, which
range from 180 nm to 408 nm.
