  We have designed honeycomb lattices for microwave photons with a frequency
imbalance between the two sites in the unit cell. This imbalance is the
equivalent of a mass term that breaks the lattice inversion symmetry. At the
interface between two lattices with opposite imbalance, we observe topological
valley edge states. By imaging the spatial dependence of the modes along the
interface, we obtain their dispersion relation that we compare to the
predictions of an ab initio tight-binding model describing our microwave
photonic lattices.
