  Stimulated emission depletion, or STED microscopy is a well-established
super-resolution technique, but is ultimately limited by the chosen
flourophore. Here we demonstrate STED microscopy with color centers in
nanoscale flakes of hexagonal boron nitride using time-gated continuous wave
STED. For color centers with zero phonon line emission around 580 nm we measure
a STED cross-section of (5.5 $\pm$ 3.2) x $10^{-17} \mbox{cm}^{2}$, achieve a
resolution of $\sim$ 50 nm and resolve two color centers separated by 250 nm,
which is less than the diffraction limit. The achieved resolution is limited by
the numerical aperture of the objective lens (0.8) and the available laser
power, and we predict that a resolution of sub-10 nm can be achieved with an
oil immersion objective lens, similar to state-of-the-art resolution obtained
with nitrogen vacancy centers in diamond.
