  Pseudo-spin and valley degrees of freedom (DOFs) engineered in photonic
analogues of topological insulators (TI) provide potential approaches to
optical encoding and robust signal transport. Here we observe a ballistic edge
state whose spin-valley indices are locked to the direction of propagation
along the interface between a valley photonic crystal and a metacrystal
emulating the quantum spin Hall effect. We demonstrate the inhibition of
inter-valley scattering at a Y-junction formed at the interfaces between
photonic TIs carrying different spin-valley Chern numbers. These results open
up the possibility of using the valley DOF to control the flow of optical
signals in 2D structures.
