Observation of strong backscattering in valley-Hall photonic topological interface modes

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. Since valley-Hall photonic topological interfaces are currently the only viable contenders for backscattering-protected topological waveguides without absorption or out-of-plane losses, our work raises fundamental questions about the real-world value of topological protection in photonics.