  Scattering immune propagation of light in topological photonic systems may
revolutionarize the design of integrated photonic circuits for information
processing and communications. In optics, various photonic topological circuits
have been developed, which were based on classical emulation of either quantum
spin Hall effect or quantum valley Hall effect. On the other hand, the
combination of both the valley and spin degrees of freedom can lead to a new
kind of topological transport phenomenon, dubbed quantum spin valley Hall
effect (QSVH), which can further expand the number of topologically protected
edge channels and would be useful for information multiplexing. However, it is
challenging to realize QSVH in most known material platforms, due to the
requirement of breaking both the (pseudo-)fermionic time-reversal (T) and
parity symmetries (P) individually, but leaving the combined symmetry S=TP
intact. Here, we propose an experimentally feasible platform to realize QSVH
for light, based on coupled ring resonators mediated by optical Kerr
nonlinearity. Thanks to the inherent flexibility of cross-mode modulation
(XMM), the coupling between the probe light can be engineered in a controllable
way such that spin-dependent staggered sublattice potential emerges in the
effective Hamiltonian. With delicate yet experimentally feasible pump
conditions, we show the existence of spin valley Hall induced topological edge
states. We further demonstrate that both degrees of freedom, i.e., spin and
valley, can be manipulated simultaneously in a reconfigurable manner to realize
spin-valley photonics, doubling the degrees of freedom for enhancing the
information capacity in optical communication systems.
