  Photons with a helical phase front (twisted photons) can carry a discrete, in
principle, unbounded amount of orbital angular momentum (OAM). Twisted
single-photons have been demonstrated as a high-dimensional quantum system with
information processing ability far beyond the widely used two-level qubits. To
date, the generations of single-photons carrying OAM merely rely on the
non-linear process in bulk crystals, e.g., spontaneous parametric
down-conversion (SPDC), which unavoidably limits both the efficiency and the
scalability of the source. Therefore, an on-demand OAM quantum light source on
a semiconductor chip is yet illusive and highly desirable for integrated
photonic quantum technologies. Here we demonstrate highly-efficient emission of
twisted single-photons from solid-state quantum emitters embedded in a
microring with angular gratings. The cavity QED effect allows the generations
of single-photons and encoding OAM in the same nanostructure and therefore
enables the realization of devices with very small footprints and great
scalability. The OAM states of singe-photons are clearly identified via quantum
interference of single-photons with themselves. Our device may boost the
development of integrated quantum photonic devices with potential applications
towards high-dimensional quantum information processing.
