  Quantum emitters in hexagonal Boron Nitride (hBN) were recently reported to
hol a homogeneous linewidth according to the Fourier-Transform limit up to room
temperature. This unusual observation was traced back to decoupling from
in-plane phonon modes which can arise if the emitter is located between two
planes of the hBN host material. In this work, we investigate the origins for
the mechanical decoupling. Improved sample preparation enabled a reduced
background and a 70-fold decrease of spectral diffusion which was so far the
major drawback of defect center in hBN and allowed us to reveal a gap in the
electron-phonon spectral density for low phonon frequencies. This decoupling
from phonons persists at room temperature and explains the observed Fourier
Transform limited lines up to 300K. Furthermore, we investigate the dipole
emission directionality and show a preferred photon emission through the side
of the hBN flakes supporting the claim for an out-of-plane distortion of the
defect center. Our work lays the foundation to a deeper understanding of the
underlying physics for the persistence of Fourier-Transform limit lines up to
room temperature. It furthermore provides a description on how to identify the
mechanically isolated emitter within the large number of defect centers in hBN.
Therefore, it paves the way for quantum optics applications with defect centers
in hBN at room temperature.
