  Optically addressable spins associated with defects in wide-bandgap
semiconductors are versatile platforms for quantum information processing and
nanoscale sensing, where spin-dependent inter-system crossing (ISC) transitions
facilitate optical spin initialization and readout. Recently, the van der Waals
material hexagonal boron nitride (h-BN) has emerged as a robust host for
quantum emitters (QEs), but spin-related effects have yet to be observed. Here,
we report room-temperature observations of strongly anisotropic
photoluminescence (PL) patterns as a function of applied magnetic field for
select QEs in h-BN. Field-dependent variations in the steady-state PL and
photon emission statistics are consistent with an electronic model featuring a
spin-dependent ISC between triplet and singlet manifolds, indicating that
optically-addressable spin defects are present in h-BN $-$ a versatile
two-dimensional material promising efficient photon extraction, atom-scale
engineering, and the realization of spin-based quantum technologies using van
der Waals heterostructures.
