  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.
