  On-chip photon sources carrying orbital angular momentum (OAM) are in demand
for high-capacity optical information processing in both classical and quantum
regimes. However, currently-exploited integrated OAM sources have been
primarily limited to the classical regime. Herein, we demonstrate a
room-temperature on-chip integrated OAM source that emits well-collimated
single photons, with a single-photon purity of g(2)(0) = 0.22, carrying
entangled spin and orbital angular momentum states and forming two spatially
separated entangled radiation channels with different polarization properties.
The OAM-encoded single photons are generated by efficiently outcoupling
diverging surface plasmon polaritons excited with a deterministically
positioned quantum emitter via Archimedean spiral gratings. Our OAM
single-photon sources bridge the gap between conventional OAM manipulation and
nonclassical light sources, enabling high-dimensional and large-scale photonic
quantum systems for information processing.
