  On-chip realization of complex photonic functionalities is essential for
further progress in planar integrated nanophotonics, especially when involving
nonclassical light sources such as quantum emitters (QEs). Hybrid plasmonic
nanocircuits integrated with QEs have been attracting considerable attention
due to the prospects of significantly enhancing QE emission rates and
miniaturizing quantum nanophotonic components. Spin-orbit interactions on
subwavelength scales have been increasingly explored in both conventional and
quantum nanophotonics for realization and utilization of the spin-dependent
flow of light. Here, we propose and realize a dielectric-loaded plasmonic
nanocircuit consisting of an achiral spin-orbit coupler for unidirectional
routing of pump radiation into branched QE-integrated waveguides. We
demonstrate experimentally the circular-polarization controlled coupling of
532-nm pump laser light into polymer-loaded branched waveguides followed by the
excitation of spatially separated (by a distance of ~ 10 {\mu}m) QEs,
nanodiamonds, with multiple nitrogen vacancy centres, that are embedded in and
efficiently coupled to the corresponding waveguides. The realization of on-chip
spin-orbit controlled excitation of different QEs coupled to branched
waveguides opens new avenues for designing complex quantum plasmonic
nanocircuits exploiting the spin degree of freedom within chiral quantum
nanophotonics.
