Broadband dispersionless topological slow light.

We present a scheme to realize topological slow-light state with low group velocity and vanishing group velocity dispersion. By harnessing the strong interactions between two regular co-propagating topological photonic states in a magneto-optical photonic crystal waveguide, the energy flux transport of light exhibits a peculiar eight-shaped flowing loop within each unit cell of the waveguide. This permits the broadband pulse transporting with low group velocity (=13.26), broad bandwidth with a relative bandwidth of 3.08%, large normalized delay-bandwidth product (about 0.409), and vanishing group velocity dispersion. More importantly, they are robust against backscattering from obstacles. Our scheme paves the way to dig into the concept and physics of topology for solving the difficult problems of signal distortion and scattering loss in slow-light systems.