  Controlling photon emission by single quantum emitters with nanostructures is
crucial for scalable on-chip quantum information processing. Nowadays
nanoresonators can affect the lifetime of emitters and ultimately induce strong
coupling between the emitters and the light field, while nanoantennas can
control the directionality of the emission. Expanding this control to the
manipulation of the emission of orbital angular momentum-changing transitions
would enable coupling between long-lived solid-state qubits and flying qubits.
As these transitions are associated with circular rather than linear dipoles,
such control requires detailed knowledge of the spatially dependent interaction
of a complex dipole with highly structured optical eigenstates containing local
helicity. Using a classical analogue, we experimentally map the coupling of
circular dipoles to photonic modes in a model structure, a photonic crystal
waveguide. We show that depending on the local helicity the dipoles can be made
to couple to modes either propagating to the left or to the right. The maps are
in excellent agreement with calculations. Our measurements, therefore,
demonstrate the coupling of spin to photonic pathway with near-unity (0.8 $\pm$
0.1) efficiency.
