This paper first reports single nitrogen-vacancy (NV) centers in diamond as a single-photon source, combining «the robustness of single atoms with the simplicity of experiments with dye molecules».
The main result:
Besides the good antibunching (0.26 at best), with emission rates of the order of thousand counts per second, they highlight the $g^{(2)}$ being larger than one and attribute it to a metastable "shelving state". Shelving refers to the switching-off of the emission of a state due to another, long-lived excited state.[1] They describe this with a rate-equation model for a three-level system:
and find:
and find a bi-exponential decay for $g^{(2)}(\tau)$ which will become popular in the literature to describe such elbows:
They furthermore conduct an analysis of deviations from this due to background emission (Eq. (5)).
They acknowledge the "uncomplicated help" from someone.