  Spin defects in foils of hexagonal boron nitride are an attractive platform
for magnetic field imaging, since the probe can be placed in close proximity to
the target. However, as a III-V material the electron spin coherence is limited
by the nuclear spin environment, with spin echo coherence time of
$\sim100~\mathrm{ns}$ at room temperature accessible magnetic fields. We use a
strong continuous microwave drive with a modulation in order to stabilize a
Rabi oscillation, extending the coherence time up to $\sim4~\mathrm{\mu s}$,
which is close to the 10-$\mathrm{\mu s}$ electron spin lifetime in our sample.
We then define a protected qubit basis, and show full control of the protected
qubit. The coherence times of a superposition of the protected qubit can be as
high as $0.8~\mathrm{\mu s}$. This work establishes that boron vacancies in
hexagonal boron nitride can have electron spin coherence times that are
competitive with typical NV-centers in small nanodiamonds under ambient
conditions.

Materials Science