Quantum emission from coupled spin pairs in hexagonal boron nitride

Introduction

Isolated optically active atomic defects in wide-band-gap materials serve as single-photon emitters (SPEs), which are key components for quantum information technologies^1,2. Hexagonal boron nitride (hBN) is a layered van der Waals (vdW) material and a favorable host for SPEs due to its fabrication versatility and compatibility with lithographic processing^{3, 4, 5, 6–7}. The observed SPEs in hBN feature high brightness, room-temperature stability, sharp zero-phonon-line (ZPL) peaks around 2 eV, and short excited-state lifetimes^{8, 9, 10, 11, 12, 13, 14, 15–16}. Furthermore, coherent control of single electron spins has been recently demonstrated in hBN¹⁷, including systems operating at room temperature^18,19, where electron spin initialization and readout rely on optical excitation and emission of defect spins, a technique known as optically detected magnetic resonance (ODMR).

One major challenge is the identification of the exact defect structures responsible for SPEs and single-spin ODMR centers, which is a prerequisite for realizing deterministic formation and control. The observed photoluminescence (PL) spectra exhibit varying ZPL energies and phonon sidebands (PSBs), and many show similar optical lineshapes⁹. These emissions may originate from various defects, but the similarities in optical lineshape imply the presence of common defect types in diverse crystalline environments^{20, 21–22}.

An S = 1/2 paramagnetic defect with strong hyperfine interaction involving two equivalent nitrogen nuclei has been observed by electron paramagnetic resonance (EPR)²³, and we assigned this signal to the negatively charged O_NV_B defect—i.e., oxygen substituting nitrogen adjacent to a boron vacancy—based on excellent agreement between experimental and simulated EPR spectra²⁴. Notably, the existence of the O_NV_B defect was confirmed by subsequent annular dark-field scanning transmission electron microscopy (ADF-STEM) measurements²⁵. In addition, carbon and oxygen substitutions were simultaneously observed nearby using the same technique. This provides strong evidence that the extra charge on the O_NV_B defect giving rise to the EPR signal could originate from donor-like substitutions of boron by carbon (C_B) or nitrogen by oxygen (O_N)²⁶. In other words, C_B or O_N may form donor-acceptor pairs (DAPs) with O_NV_B, described as ${\mathrm{C}}_{\mathrm{B}}^{+}$–${\mathrm{O}}_{\mathrm{N}}$