Narrowband quantum emitters in hexagonal boron nitride with optically addressable spins

Electron spins coupled with optical transitions in solids stand out as a promising platform for developing spin-based quantum technologies. Recently, hexagonal boron nitride has emerged as a promising host for optically addressable spin systems. However, controlled generation of isolated single-photon emitters with predetermined spin transitions has remained elusive. Here we report on a single-step thermal processing of hexagonal boron nitride flakes that produces high-density, narrowband quantum emitters with optically active spin transitions, with over 25% of the emitters exhibiting a clear signature of an optical spin read-out at room temperature. The generated spin defect complexes exhibit both S = 1 and S = 1/2 transitions, which are explained by charge transfer from strongly to weakly coupled spin pairs. Our work advances the understanding of spin complexes in hexagonal boron nitride and paves the way for single spin-photon interfaces in layered materials with applications in quantum sensing and information processing.