  Layered van der Waals materials are emerging as compelling two-dimensional
(2D) platforms for studies of nanophotonics, polaritonics, valleytronics and
spintronics, and have the potential to transform applications in sensing,
imaging and quantum information processing. Amongst these, hexagonal boron
nitride (hBN) is unique in that it hosts ultra-bright, room temperature single
photon emitters (SPEs). However, an outstanding challenge is to locate SPEs in
hBN with high precision, a task which requires breaking the optical diffraction
limit. Here, we report the imaging of SPEs in layered hBN with a spatial
resolution of 63 nm using ground state depletion (GSD) nanoscopy. Furthermore,
we show that SPEs in hBN possess nonlinear photophysical properties which can
be used to realize a new variant of GSD that employs a coincident pair of
doughnut-shaped lasers to reduce the laser power that is needed to achieve a
given resolution target. Our findings expand the current understanding of the
photophysics of quantum emitters in layered hBN and demonstrate the potential
for advanced nanophotonic and bio-imaging applications which require
localization of individual emitters with super-resolution accuracy.
