  We experimentally demonstrate topological slow light waveguides in valley
photonic crystals (VPhCs). We employed a bearded interface formed between two
topologically-distinct VPhCs patterned in an air-bridged silicon slab. The
interface supports both topological and non-topological slow light modes below
the light line. By means of optical microscopy, we observed light propagation
in the topological mode in the slow light regime with a group index $n_{\rm g}$
over $30$. Furthermore, we confirmed light transmission via the slow light mode
even under the presence of sharp waveguide bends. In comparison between the
topological and non-topological modes, we found that the topological mode
exhibits much more efficient waveguiding than the trivial one, elucidating
topological protection in the slow light regime. This work paves the way for
exploring topological slow-light devices compatible with existing photonics
technologies.
