  Optically addressable spins in widebandgap semiconductors have become one of
the most prominent platforms for exploring fundamental quantum phenomena. While
several candidates in 3D crystals including diamond and silicon carbide have
been extensively studied, the identification of spindependent processes in
atomically thin 2D materials has remained elusive. Although optically
accessible spin states in hBN are theoretically predicted, they have not yet
been observed experimentally. Here, employing rigorous electron paramagnetic
resonance techniques and photoluminescence spectroscopy, we identify
fluorescence lines in hexagonal boron nitride associated with a particular
defect, the negatively charged boron vacancy and determine the parameters of
its spin Hamiltonian. We show that the defect has a triplet ground state with a
zero field splitting of 3.5 GHz and establish that the centre exhibits
optically detected magnetic resonance at room temperature. We also demonstrate
the spin polarization of this centre under optical pumping, which leads to
optically induced population inversion of the spin ground state a prerequisite
for coherent spin manipulation schemes. Our results constitute a leap forward
in establishing two dimensional hBN as a prime platform for scalable quantum
technologies, with extended potential for spin based quantum information and
sensing applications, as our ODMR studies on hBN NV diamonds hybrid structures
show.
