Solid-state sources for bright single photons with orbital angular momentum

Photons with a helical phase front, that is, twisted photons can carry a discrete, in principle, unlimited, but quantized amount of orbital angular momentum (OAM). Twisted single photons, hence, constitute a high-dimensional quantum system with information processing abilities beyond the two-level single-photon qubits. To date, the generation of single photons carrying OAM relies on the nonlinear process in bulk crystals, e.g., spontaneous parametric down-conversion (SPDC), which limits both the efficiency and the scalability of the source. Here, we present a solid-state source of bright single photons in an OAM superposition state with singlephoton purity of g⁽²⁾(0) = 0.115(1) and collection efficiency of 23(4)%. The mode purity of the single-photon OAM states is further examined via projection measurements. Further developments of integrated quantum photonic devices with pure OAM states as an additional degree of freedom may enable high-dimensional quantum information processing.