  Spin defects in hexagonal Boron Nitride (hBN) attract increasing interest for
quantum technology since they represent optically-addressable qubits in a van
der Waals material. In particular, negatively-charged boron vacancy centers
(${V_B}^-$) in hBN have shown promise as sensors of temperature, pressure, and
static magnetic fields. However, the short spin coherence time of this defect
currently limits its scope for quantum technology. Here, we apply dynamical
decoupling techniques to suppress magnetic noise and extend the spin coherence
time by nearly two orders of magnitude, approaching the fundamental $T_1$
relaxation limit. Based on this improvement, we demonstrate advanced spin
control and a set of quantum sensing protocols to detect electromagnetic
signals in the MHz range with sub-Hz resolution. This work lays the foundation
for nanoscale sensing using spin defects in an exfoliable material and opens a
promising path to quantum sensors and quantum networks integrated into
ultra-thin structures.
