Quantum emission from coupled spin pairs in hexagonal boron nitride.

Optically addressable defect qubits in wide band gap materials are favorable candidates for room-temperature quantum information processing. Two-dimensional (2D) hexagonal boron nitride (hBN) is an attractive solid-state platform with great potential for hosting bright quantum emitters and quantum memories, leveraging the advantages of 2D materials for scalable preparation of defect qubits. Although room-temperature bright defect qubits have been recently reported in hBN, their microscopic origin, the nature of the optical transition, and the optically detected magnetic resonance (ODMR) have remained elusive. Here, we connect the variance in the optical spectra, optical lifetimes, and spectral stability of quantum emitters to donor-acceptor pairs (DAPs) in hBN through ab initio calculations. We find that DAPs can exhibit ODMR signals for the acceptor counterpart of the defect pair with an S = 1/2 ground state at non-zero magnetic fields, depending on the donor partner and dominantly mediated by the hyperfine interaction. The donor-acceptor pair model and its transition mechanisms provide a recipe for defect qubit identification and performance optimization in hBN for quantum applications.