  Optically addressable spins in materials are important platforms for quantum
technologies, such as repeaters and sensors. Identification of such systems in
two-dimensional (2d) layered materials offers advantages over their bulk
counterparts, as their reduced dimensionality enables more feasible on-chip
integration into devices. Here, we report optically detected magnetic resonance
(ODMR) from previously identified carbon-related defects in 2d hexagonal boron
nitride (hBN). We show that single-defect ODMR contrast can be as strong as 6%
and displays a magnetic-field dependence with both positive or negative sign
per defect. This bipolarity can shed light into low contrast reported recently
for ensemble ODMR measurements for these defects. Further, the ODMR lineshape
comprises a doublet resonance, suggesting either low zero-field splitting or
hyperfine coupling. Our results offer a promising route towards realising a
room-temperature spin-photon quantum interface in hexagonal boron nitride.
