Spinning single photons

Single photons carrying spin angular momentum (SAM), i.e., circularly polarized single photons generated typically by subjecting a quantum emitter (QE) to a strong magnetic field at low temperatures are at the core of chiral quantum optics enabling non-reciprocal single-photon configurations and deterministic spin-photon interfaces. Here we propose a conceptually new approach to the room-temperature generation of SAM-coded single photons (SSPs) entailing QE non-radiative coupling to surface plasmons that are transformed, by interacting with an optical metasurface, into a collimated stream of SSPs with the designed handedness. We report on the design, fabrication and characterization of SSP sources consisting of dielectric circular nanoridges with azimuthally varying widths deterministically fabricated on a dielectric-protected silver film around a nanodiamond containing a nitrogen-vacancy centre. With properly engineered phases of QE-originated fields scattered by nanoridges, the out-coupled photons feature a well-defined SAM (with the chirality > 0.8) and high directionality (collection efficiency up to 92%).