Single-shot measurement of photonic topological invariant

Topological design enables physicists to engineer robustness into a system. When connected to a topological invariant, the propagation of light remains unchanged in the presence of disorder. However, a general challenge remains to directly characterise the topological properties of systems by experiment. In this work, we demonstrate a novel technique for directly observing a photonic winding number using a single measurement. By propagating light with a sufficiently broad spectrum along a topological photonic crystal fibre, we calculate the winding number invariant from the output intensity pattern. We quantify the limitations of this single-shot method, which works even for surprisingly narrow and asymmetric spectral distributions. Furthermore, we dynamically evaluate the effectiveness of our method by uncovering the loss of the bulk invariant as we twist the fibre. The characterisation method that we present is highly accessible and transferable across topological photonic platforms.