Fig. 3. Single-emitter photoluminescence and photon antibunching.

a Schematic of a cascaded biexciton emission proces. GS, X, and XX denotes the ground-state, exciton, and the biexciton states respectively. Biexciton first decays to an exciton either by emitting a vertically (V) or horizontally (H) polarized photon denoted by red or blue arrows respectively. Subsequently, the exciton decays to ground-state by emitting a second vertically or horizontally polarized photon with polarization correlated to the initial emitted photon, completing the emission cascade. The energy splitting between the different linear polarization arises due to the fine-structure splitting. b PL Spectrum of a single-quantum emitter. Two pairs of correlated doublets are observed. c PL intensity vs. Excitation Power: The localized defect exciton shows distinct behavior: Exciton (X) intensity increases sublinearly and begins to saturate at high powers as expected from a two-level system, whereas the biexciton line (XX) intensity increases superlinearly. Dashed lines are linear fit (log (PL Int) = x log (Excitation Power)) to the data with extracted slopes of 0.96 for exciton (X) and 1.587 for biexciton (XX), respectively. d Second-order correlation measurement. Both X and XX exhibit g⁽²⁾(0) < 0.1, showing high-purity single-photon emission.