  One-dimensional (1D) topological superconductivity is a state of matter that
is not found in nature. However, it can be realised, for example, by inducing
superconductivity into the quantum spin Hall edge state of a two-dimensional
topological insulator. Because topological superconductors are proposed to host
Majorana zero modes, they have been suggested as a platform for topological
quantum computing. Yet, conclusive proof of 1D topological superconductivity
has remained elusive. Here, we employ low-temperature scanning tunnelling
microscopy to show 1D topological superconductivity in a van der Waals
heterostructure by directly probing its superconducting properties, instead of
relying on the observation of Majorana zero modes at its boundary. We realise
this by placing the two-dimensional topological insulator monolayer WTe$_2$ on
the superconductor NbSe$_2$. We find that the superconducting topological edge
state is robust against magnetic fields, a hallmark of its triplet pairing. Its
topological protection is underpinned by a lateral self-proximity effect, which
is resilient against disorder in the monolayer edge. By creating this exotic
state in a van der Waals heterostructure, we provide an adaptable platform for
the future realization of Majorana bound states. Finally, our results more
generally demonstrate the power of Abrikosov vortices as effective experimental
probes for superconductivity in nanostructures.
