A room-temperature on-chip orbital angular momentum source that emits well-collimated single photons has been demonstrated.

Radially polarized optical beams are desirable in many applications because of their property to generate longitudinally polarized fields when being strongly focused. Singlephoton sources are however normally limited to the generation of linearly polarized photons due to their dipolar nature. Here, we report on emission of radially polarized single photons from a nitrogen-vacancy (NV) center in a nanodiamond (ND) coupled to a bull's-eye plasmonic antenna. Design optimization and characterization of plasmonic bull's-eye antennas consisting of polymer circular nanoridges deterministically fabricated on a silica protected silver film around an NV-ND emitter are reported. We demonstrate with numerical simulations that the collection efficiency for photons emitted at the design wavelength can exceed 80% with optimized bull's-eye structures. Analysis of the emission angular distribution indicates that, even when limiting the detection to the main lobe by using an 0.2 NA objective, the detection efficiency can overreach 55%. A 3-fold enhancement in the total number of detected radially polarized photons is experimentally observed with an NV-ND single-photon emitter being coupled to a bull's-eye antenna. Generation of radially polarized single photons suggests new interesting possibilities for single-photon imaging and sensing.

Germanium vacancy (GeV) centers in diamonds constitute a promising platform for single-photon sources to be used in quantum information technologies. Emission from these color centers can be enhanced by utilizing a cavity that is resonant at the peak emission wavelength. We investigate circular plasmonic Bragg cavities for enhancing the emission from single GeV centers in nanodiamonds (NDs) at the zero phonon line. Following simulations of the enhancement for different configuration parameters, the appropriately designed Bragg cavities together with out-coupling gratings composed of hydrogen silsesquioxane ridges are fabricated around the NDs containing nitrogen vacancy centers deposited on a silica-coated silver surface. We characterize the fabricated configurations and finely tune the cavity parameters to match the GeV emission. Finally, we fabricate the cavity containing a single GeV-ND and compare the total decay-rate before and after cavity fabrication, finding a decay-rate enhancement of ∼5.5 and thereby experimentally confirming the feasibility of emission enhancement with circular plasmonic cavities.

However, single-photon sources relying on nonlinear processes suffer from probabilistic photon generation and an inherent tradeoff between efficiency and single-photon purity. To realize chip-scale photonic quantum technologies, such as photonic quantum computing, QPCs require deterministic single-photon generation. As such, deterministically positioned solid-state quantum emitters (QEs) (e.g., color centers in nanodiamonds, [6,7] quantum dots, [8,9] and defects in transition metal dichalcogenide monolayers [2,10] ) have been widely used as central blocks for high-quality single-photon sources. Different from free-space QEs that feature many shortcomings in ambient conditions, such as low quantum efficiencies, background emission, lifetime-limited photon rates, omnidirectional emission patterns, and linearly polarized transition dipole, the QEs coupled to dedicated photonic circuits hold the great potential for ideal on-demand solid-state sources that generate a pure stream of single photo ns at high repetition rates with welldefined polarizations and high efficiencies. The spontaneous emission of QEs can efficiently excite the fundamental and high-order modes in nanophotonic waveguides to directionally route the propagating single photons [11] or plasmons. [12] Thus, the radiative channel of emission into free space, in comparison, is small. [13] The single photons or plasmons that are well-confined and routed within the nanophotonic (photonic and plasmonic) waveguides are capable of being functionally modulated by integrating components of resonant cavities, photon detectors, beam splitters, phase shifters, spectral filters, free-space emitters, and mode converters, thereby enabling advanced functional QPCs. [14][15][16][17] Among nanophotonic waveguides, plasmonic waveguides enable extreme confinement of light into a subwavelength regime and allow strong light-matter interaction for efficiently coupling QE radiation. Recently, different types of plasmonic waveguide configurations such as metallic nanowires, wedge/ stripe waveguides, and V-grooves have been investigated for quantum nanophotonics. [7,12,18,19] However, metallic nanowires are originally synthesized from chemical reactions, which suffers from the difficulty of customized design for sophisticated circuits. Lithographically patterned plasmonic wedge/stripe

Impurity-vacancy centers in diamond offer a new class of robust photon sources with versatile quantum properties. While individual color centers commonly act as single-photon sources, their ensembles have been theoretically predicted to have tunable photon-emission statistics. Importantly, the particular type of excitation affects the emission properties of a color center ensemble within a diamond crystal. While optical excitation favors non-synchronized excitation of color centers within an ensemble, electron-beam excitation can synchronize the emitters excitation and thereby provides a control of the second-order correlation function g 2(0). In this letter, we demonstrate experimentally that the photon stream from an ensemble of color centers can exhibit g 2(0) both above and below unity, thereby confirming long standing theoretical predictions by Meuret et al. [S. Meuret, L. H. G. Tizei, T. Cazimajou, et al., “Photon bunching in cathodoluminescence,” Phys. Rev. Lett., vol. 114, no. 19, p. 197401, 2015.]. Such a photon source based on an ensemble of few color centers in a diamond crystal provides a highly tunable platform for informational technologies operating at room temperature.