High polarization efficiency of boron vacancy spin defects in hexagonal boron nitride

The negatively charged boron vacancy defect (VB-) in hexagonal boron nitride (hBN) hosts many attractive properties as a quantum platform. hBN, as a 2D material, enables ease of fabrication, integration in solid-state devices, and nanoscale proximity to targets as a quantum sensor. While VB- ensembles exhibit similar magneto-optical properties to nitrogen vacancy centers in diamond, the optical polarization mechanism is not yet fully understood. In this work, we investigate the polarization cycle of the VB- defect by characterizing the associated transition probabilities based on a rate model of electronic spin states and measurements of electronic state dynamics. Our model indicates a nearly perfect electronic spin polarization efficiency and readout fidelity. In addition, we measure and model the effect of surrounding nuclear spins on the dynamics, finding that electronic polarization timescales are significantly altered near the ground and excited state level anti-crossings due to hyperfine interactions and nuclear spin polarization. Our understanding of the optical polarization cycle of the VB- defect opens a path for applications leveraging its near-perfect initialization and readout fidelity and can be further extended to optimize the performance of VB- in quantum devices.