Class-Selective Spin Activity of Two Spectrally Distinct Color Centers in Oxygen-Annealed Hexagonal Boron Nitride

Hexagonal boron nitride (hBN) hosts a variety of room-temperature single-photon emitters, among which optically addressable spin defects are attractive for quantum sensing and spin–photon interfaces. While the negatively charged boron vacancy is well established as an ensemble spin system, bright and individually addressable spin-active emitters remain scarce, and it is unknown whether spin activity is a property of an entire optical emitter class or only of a subset. We have recently shown that oxygen annealing of hBN activates two spectrally distinct emitter populations with so-far unresolved spin properties. Here, we perform optically detected magnetic resonance (ODMR) screening of 158 individual emitters across both populations. We find that spin activity is carried exclusively by the Type I population: 9 of 100 Type I emitters display ODMR with coexisting S = 1 and S = 1/2 transitions and zero-field splitting (ZFS) parameters of D ≈ 977 MHz and E ≈ 174 MHz, while all 58 Type II emitters remain spin-silent. Pulsed ODMR on all three transition branches reveals spin–lattice relaxation times up to 19.1 μs and spin echo coherence times up to 56.0 ns, with the S = 1/2 branch showing systematically shorter T2 than the two S = 1 branches in every spin-active emitter. The exclusive correlation of spin activity with one optical class and the branch-resolved coherence asymmetry provide experimental constraints for identifying the microscopic origin of carbon- and oxygen-related color centers in hBN.