  Single photon emitters in two-dimensional materials are promising candidates
for future generation of quantum photonic technologies. In this work, we
experimentally determine the quantum efficiency (QE) of single photon emitters
(SPE) in few-layer hexagonal boron nitride (hBN). We employ a metal hemisphere
that is attached to the tip of an atomic force microscope to directly measure
the lifetime variation of the SPEs as the tip approaches the hBN. This
technique enables non-destructive, yet direct and absolute measurement of the
QE of SPEs. We find that the emitters exhibit very high QEs approaching $(87
\pm 7)\,\%$ at wavelengths of $\approx\,580\,\mathrm{nm}$, which is amongst the
highest QEs recorded for a solid state single photon emitter.
