Fig. 1. Single photon sources in quantum dots. (a) High-resolution transmission electron microscope image of PbS/CdS quantum dot and its PL spectrum at room temperature^[25]; (b) scanning electron microscope image of Ⅲ-Nitride nanowire quantum dot^[26]; (c) schematic diagram of single nanowire quantum dot; (d) PL spectrum of single nanowire quantum dot at room temperature^[26]; (e) transmission electron microscope image and (f) PL spectra of CdSe nanocrystals^[27]; (g) atomic force microscope image of InAs self-assembled quantum dot not covered by GaAs^[28]; (h) radiation spectra of InGaAs self-assembled quantum dots under non-resonant excitation (left) and resonant excitation (right)^[28]

Fig. 2. Common types of defects in diamonds. (a) Atomic structure schematic diagram and fluorescence scanning image of NV color center^[34]; (b) energy level structure of NV color center^[34]; (c) atomic structure schematic diagram and PL spectra at room temperature of SiV color center^[10]; (d) bandgap energy level comparison between NV and SiV color centers, the quantum-compatible charge states of each defect type are highlighted by colored regions^[49]

Fig. 3. Single-photon sources in TMDCs. (a) PL intensity map of randomly appearing localized exciton emissions on monolayer WSe₂ sample^[54]; (b) high-resolution PL spectrum on monolayer WSe₂ sample^[54]; (c) schematic diagram of hybrid 2D semiconductor piezoelectric actuator integrated with monolayer WSe₂^[55]; (d) PL spectral profile of single-photon source in a monolayer WSe₂^[55]; (e) deterministically induced defect emitters in atomically thin MoS₂ achieved by focused helium ion^[56]; (f) optical image of monolayer MoTe₂ flake on a nanopillar array^[57]

Fig. 4. h-BN defect single-photon sources. (a) Several types of defects in h-BN^[58]; (b) energy distribution of 90 h-BN defects^[61]; (c) fluorescence spectrum of the h-BN defect single-photon source^[64]; (d) linear polarization relation of the dipole^[64]

Fig. 5. Si defect single-photon sources. (a) Schematic diagram of the G Center defect structure^[65]; (b) PL spectra of the G Center under different fabrication parameters^[66]; comparison of single-photon sources for the (c) G^* Center and (d) G Center in Si^[67]; (e) dipole orientation^[67]; (f) lifetime of the single-photon source^[67]

Fig. 6. Optical properties characterization of the single-photon source in SiC. (a) Schematic diagram of confocal system for single-photon source characterization^[68]; (b) electroluminescence map of edge region for device^[68]; (c) second-order correlation after background correction^[68]; (d) fluorescence spectrum of single-photon source at room temperature^[69]; (e) power dependence curve of single-photon source fluorescence intensity^[69]; (f) test result of fluorescence stability^[69]

Fig. 7. Strain manipulation of single photon sources. (a) Simplified energy level diagram influenced by strain^[76]; (b) schematic diagram of applying stress to h-BN thin slices through polycarbonate^[76]; (c) variation in energy shift with strain, inset shows the secondary energy shift caused by strain^[76]; (d) spectral shifts of single-photon sources under different strains^[76]; (e) schematic diagram of point-like elastic strain perturbation^[88]; (f) schematic diagram of nanopatterned^[89]; (g) schematic diagram of single-photon source consisting of monolayer WSe₂ and dielectric rod structure with nanoscale gap^[90]; (h) monolayer MoSe₂ covered by indentations^[91]

Fig. 8. Temperature control of single photon sources. (a) Second-order correlation function of WSe₂ single-photon source coupled chemical vapor transport (CVT)-growth; second-order correlation functions of WSe₂ single-photon sources coupled flux-growth under (b) 4 K and 160 K, and (c) 140 K and 180 K^[92]; temperature dependence of (d) spectral linewidth, (e) emission energy, and (f) exciton emission intensity^[92]; (g) temperature-dependent spectra under quasi-resonant excitation^[93]; (h) effect of thermal tunning on cavity resonance wavelength from the G Center ZPL^[94]

Fig. 9. Changing the voltage to modulate the spectral properties of h-BN and single molecule. (a) Variation in ZPL spectrum of the h-BN single-photon source with the direction of electric field^[97]; (b) angle-resolved Stark shift and optical polarization data^[97]; (c) schematic diagram of the integrator device for single molecule integrated with two-dimensional materials^[98]; (d) variation in Stark shift of the single-molecule assembly with voltage underneath bilayer graphene^[98]

Fig. 10. Manipulating single photon sources by Stark effect. (a) Schematic diagram of the electric field tuning structure ^[77]; (b) offset of the central wavelength for the single-photon source radiation spectrum with electric field tuning under different temperatures^[77]; (c) fluorescence spectrum of the single-photon source measured at different temperatures, inset shows the purity of the single-photon source^[77]; (d) offset of the FWHM for the single-photon source radiation spectrum with electric field tuning under different temperatures^[77]; (e) second-order correlation function measured at low temperature^[77]; (f) confocal PL map of integrated WSe₂^[99]; (g) gate-dependent PL spectra acquired at the edge^[99]; (h) gate-dependent PL spectra acquired at the charge neutrality point^[99]

Fig. 11. Near-field modulation of different polarization single-photon sources by surface plasmon probe. (a) Schematic diagram of the model^[101]; Purcell-factor corresponding to (b) out-of-plane-oriented and (c) in-plane-oriented dipole varying with the radius of the silver nanowires and near-field distance^[101]; (d) theoretical modulated range for dipoles with polarization direction parallel to the plane^[101]; (e) time-resolved PL delay curves of single quantum dot before and after near-field interaction; time-resolved PL delay curves before and after moving the single-photon source of the hBN under (f) the center of the probe and (g) the edge of the probe^[101]; (h) decay curves of emission photons from single quantum dot^[100]; (i) statistical distribution of the lifetimes for single quantum dots scattered on a quartz substrate^[100]; (j) variation in simulated Purcell-factor with the distance between the AgNW and the dipole when a dipole aligned parallel and perpendicular to the substrate^[100]

Fig. 12. On-chip integration of single photon sources. (a) Schematic diagram of assemble^[102]; (b) schematic diagram of the waveguide and guided mode^[102]; second-order correlation functions of photons (c) excitation and detection via the microscope objective, (d) excitation via the waveguide and detection via the microscope objective, and (e) excitation via the objective and detection via the two output ports of the waveguide^[102]; (f) schematic diagram of InAs quantum dot embedded in GaAs photonic crystal waveguide^[103]; (g) β factors calculated from different group velocities and quantum dot locations^[103]; (h) schematic diagram of single-photon source in a monolithic h-BN waveguide^[104]; (i) schematic diagrams and comparison table of different coupled methods^[104]; (j) collection images when the laser spot is fixed on the emitter, inset is the corresponding schematic for excitation and collection^[104]; (k) scanning electron microscope image of the waveguide, magnified views of the gap waveguide, and grating coupler assembly, and wide field of view image for the laser coupled and guided through the waveguide^[104]

Fig. 13. Single photon sources in quantum information technology. (a) Post-selected high-fidelity quantum controlled-NOT gate^[111]; (b) quantum cryptographic protocol^[80]; (c) quantum teleportation^[112]; (d) all-optical quantum repeater^[113]