  The topological valley Hall effect was predicted as a consequence of the bulk
topology of electronic systems. Now it has been observed in photonic crystals,
showing that both topology and valley are innate to classical as well as
quantum systems.
  The discovery of counterflow in opposite valleys not only demonstrates that
the valley can be a novel carrier of information and energy --- particularly
valuable for systems without charge and spin --- but also exemplifies that
symmetry-protected band topology can be universal to both quantum and classical
systems.
