Efficient single photon emission from a high-purity hexagonal boron   nitride crystal
  Among a variety of layered materials used as building blocks in van der Waals heterostructures, hexagonal boron nitride (hBN) appears as an ideal platform for hosting optically-active defects owing to its large bandgap ($\sim 6$ eV). Here we study the optical response of a high-purity hBN crystal under green laser illumination. By means of photon correlation measurements, we identify individual defects emitting a highly photostable fluorescence under ambient conditions. A detailed analysis of the photophysical properties reveals a high quantum efficiency of the radiative transition, leading to a single photon source with very high brightness. These results illustrate how the wide range of applications offered by hBN could be further extended to photonic-based quantum information science and metrology. 
L. J. Mart\'inez, T. Pelini, V. Waselowski, J. R. Maze, B. Gil, G.   Cassabois, and V. Jacques
cond-mat.mtrl-sci