  Quantum emitters in hexagonal boron nitride (hBN) are emerging as bright and
robust sources of single photons for applications in quantum optics. In this
work we present detailed studies on the limiting factors to achieve Fourier
Transform limited spectral lines. Specifically, we study phonon dephasing and
spectral diffusion of quantum emitters in hBN via resonant excitation
spectroscopy at cryogenic temperatures. We show that the linewidths of hBN
quantum emitters are phonon broadened, even at 5K, with typical values of the
order of one GHz. While spectral diffusion dominates at increasing pump powers,
it can be minimized by working well below saturation excitation power. Our
results are important for future utilization of quantum emitters in hBN for
quantum interference experiments.
