  The ability to identify and characterize homogeneous and inhomogeneous
dephasing processes is crucial in solid-state quantum optics. In particular,
spectral diffusion leading to line broadening is difficult to evidence when the
associated timescale is shorter than the inverse of the photon detection rate.
Here, we show that a combination of resonant laser excitation and second-order
photon correlations allows to access such fast dynamics. The resonant laser
drive converts spectral diffusion into intensity fluctuations, leaving a
signature in the second-order coherence function $g^{(2)}(\tau)$ of the
scattered light that can be characterized using two-photon coincidences --
which simultaneously provides the homogeneous dephasing time. We experimentally
implement this method to investigate the fast spectral diffusion of a color
center generated by an electron beam in the two-dimensional material hexagonal
boron nitride. The $g^{(2)}(\tau)$ function of the quantum emitter measured
over more than ten orders of magnitude of delay times, at various laser powers,
establishes that the color center experiences spectral diffusion at a
characteristic timescale of a few tens of microseconds, while emitting
Fourier-limited single photons ($T_2/2T_1 \sim 1$) between spectral jumps.
