  The hallmark property of two-dimensional topological materials is the
incredible robustness of the quantized Hall conductivity to disorder. That
robustness arises from the fact that in the topological band gap, transport can
occur only along the edges modes, which are immune to scattering. However, for
sufficiently strong disorder, the band gap closes and the system becomes
topologically trivial as all states become localized, such that all transport
vanishes -- in accordance with Anderson localization. It therefore came as a
surprise when it was suggested that, for a two-dimensional quantum spin-Hall
topological system, the opposite could occur. In so-called topological Anderson
insulators, the emergence of protected edge states and quantized transport is
caused by the introduction of disorder. However, to date, the observation of
the topological Anderson insulator phase has been elusive. In this article, we
report the first experimental demonstration of a topological Anderson
insulator. We do that in a photonic implementation: an array of helical,
evanescently-coupled waveguides in a detuned honeycomb geometry. Under proper
conditions, adding on-site disorder, in the form of random variations in the
refractive index contrast defining the waveguides, drives the system from a
trivial phase into a topological state.
