The potential of solid-state quantum emitters for applications critically depends on several key figures of merit. One of the most important is the coherence time of the emitted single photons, which can be compromised by fast dephasing and spectral diffusion. In hexagonal boron nitride (hBN), blue-emitting color centers (or B centers) are seen as favorable in this regard, in the light of prior studies mainly based on resonant excitation. Yet, their time-coherence properties in the more accessible regime of nonresonant excitation (or photoluminescence) have not been extensively characterized. Here, we investigate the coherence and spectral diffusion of the photoluminescence from a B center in the continuous-wave regime using photon correlation Fourier spectroscopy. We determine that the emission line shape consists of a homogeneous contribution, whose linewidth increases with the laser power and which is broadened by spectral diffusion at a timescale of 10 to 100 $\mu \mathrm{s}$. At low power and short time, the emission line is only a factor $\approx 2$ above the Fourier limit, while at long times, the inhomogeneous linewidth increases up to more than a gigahertz. Our work deepens the understanding of decoherence processes of this preeminent family of quantum emitters in hBN.

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• Optical coherence
• Photon statistics
• Single photon sources
• Hexagonal boron nitride
• Fluorescence spectroscopy
• Luminescence