  The unique properties of light underpin the visions of photonic quantum
technologies, optical interconnects, and a wide range of novel sensors, but a
key limiting factor today is losses due to either absorption or backscattering
on defects. Recent developments in topological photonics have fostered the
vision of backscattering-protected waveguides made from topological interface
modes, but, surprisingly, measurements of their propagation losses were so far
missing. Here we report on measurements of losses in the slow-light regime of
valley-Hall topological waveguides and find no indications of topological
protection against backscattering on ubiquitous structural defects. We image
the light scattered out from the topological waveguides and find that the
propagation losses are due to Anderson localization. The only photonic
topological waveguides proposed for materials without intrinsic absorption in
the optical domain are quantum spin-Hall and valley-Hall interface states, but
the former exhibits strong out-of-plane losses, and our work therefore raises
fundamental questions about the real-world value of topological protection in
reciprocal photonics.
