Role of molecular defects in single-photon emission in hexagonal boron nitride

Hexagonal boron nitride (hBN) has emerged as a promising material for quantum photonics due to its ability to host bright, stable single-photon emitters (SPEs) that operate at room temperature [1,2]. These emitters exhibit remarkable variability in spectral emission, lifetime, polarization, and spin structure [3]. Despite extensive studies, the microscopic origins of most SPEs remain elusive, limiting our understanding of their emission mechanisms and their integration into quantum technologies.

We present recent advances in elucidating the nature of SPEs in hBN and related nitrides through the combined use of resonant inelastic X-ray scattering (RIXS) and photoluminescence (PL) spectroscopy. RIXS measurements on defective hBN reveal a sequence of harmonic excitations that correlate with the emission energies of individual quantum emitters [4]. These excitations arise from nitrogen π* anti-bonding orbitals and form a well-defined vibronic progression ascribable to molecular N₂-like defects. Complementary low-temperature PL measurements on the same samples confirm multiple, spectrally distinct SPEs. Analysis of these spectra using a donor–acceptor pair (DAP)-like nonlocal recombination model [5] uncovers a correlation between PL emission energies and the vibronic states of molecular defects, underscoring their central role in the emission process. Additional evidence from spectral instabilities supports that SPEs in hBN are governed by nonlocal recombination involving molecular-type centers.

These results establish a microscopic connection between single-photon emission and molecular defects in hBN, providing a predictive framework for spectral variability and tunability, and offering new strategies for engineering quantum light sources and solid-state spin qubits.

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4. Pelliciari, J., et al. Nat. Mater. 23, 1230 (2024)

5. Mejia, E., et al. Opt. Mater. Express 14, 2122 (2024)

6. Mejia, E., et al. J. Phys. Chem. C 29, 2044 (2025)