Intrinsic high-fidelity spin polarization of charged vacancies in hexagonal boron nitride

The negatively charged boron vacancy (VB-) in hexagonal boron nitride (hBN) has attracted considerable interest among two-dimensional material defects due to its optical polarizability at room temperature. We investigate the polarization mechanism by conducting extensive measurements of ground and excited state spin dynamics. Building on these observations, we develop a semiclassical model that predicts a near-unity degree of spin polarization, outperforming other solid-state spin defects under ambient conditions. Expanding on our model, we incorporate the nuclear spin degrees of freedom surrounding the VB- and employ detailed Lindbladian numerical simulations to explore hyperfine-induced nuclear spin polarization. Our simulations reveal several key features that vary with the external magnetic field, which are confirmed by experimental observations. To further deepen our understanding of the VB- ensemble as spin defects in two-dimensional limit, we also examine the properties of these defects in few-layer thick hBN flakes, focusing on the spin decoherence mechanisms and their dependence on hBN thickness.