Quantum efficiency of the B‐center in hexagonal boron nitride

B-centers in hexagonal boron nitride (hBN) are gaining significant research interest for quantum photonics applications due to precise emitter positioning and highly reproducible emission wavelengths at 436 nm. Here, we leverage the layered nature of hBN to directly measure the quantum efficiency (QE) of single B-centers. The defects were engineered in a 35 nm flake of hBN using electron beam irradiation, and the local dielectric environment was altered by transferring a 250 nm hBN flake on top of the one containing the emitters. By analyzing the resulting change in measured lifetimes, we determined the QE of B-centers in the thin flake of hBN. Additionally, we propose two approaches to quantify the QE of B-centers in thick flakes of hBN. Our results indicate that B-centers located in thin flakes can exhibit QEs higher than 40 %. Near-unity QEs are achievable under reasonable Purcell enhancement for emitters embedded in thick flakes of hBN, highlighting their promise for quantum photonics applications.