A new discovery could advance the fabrication of quantum photonic devices

As quantum technology advances at a rapid pace, increasing attention is turning to materials that can generate unique quantum states and keep them stable, even at room temperature. One such material is hexagonal boron nitride (hBN), a two-dimensional crystal similar to the well-known graphene. Remarkably, it retains its structural stability even when it is just a single atom thick.

In recent years, hBN has become a key material for single-photon emitters (SPEs) and quantum defect spins that can operate at room temperature. For a long time, however, it remained unclear exactly which atomic structures – so-called point defects – are responsible for the different colours of light emitted by the crystal, the wide range of optical lifetimes and variations in spectral stability. Another open question was why some defects exhibit optically detected magnetic resonance (ODMR) while others do not.

The research group led by Adam Gali at the Institute for Solid State Physics and Optics of the HUN-REN Wigner Research Centre for Physics has now achieved a breakthrough in this area. In their paper, they developed a new model suggesting that light emission and spin properties are not linked to isolated point defects, but rather to donor–acceptor pairs (DAPs) – in other words, they arise from the interaction of two nearby point defects. Using first-principles (ab initio) calculations based on quantum mechanics, the researchers demonstrated that these defect pairs can explain the observed variations in the colour and intensity of the emitted light. The modelling results are consistent with experimental observations, linking measurements to atomic-level theory.

The paper provides a unified framework for interpreting hBN emitting centres that previously seemed chaotic: many luminescent centres once thought to be different may in fact share the same acceptor defect, forming pairs with different donors at different separations. This insight also offers practical guidance for designing quantum photonic devices: by controlling doping and the distance between defects, the colour, brightness and spin contrast can be tuned.

The point-defect pairs are shown in (a) as a red sphere (an oxygen–vacancy defect) and a brown sphere (a carbon atom on a boron site). The carbon atom is placed progressively further away, with numbers indicating its positions. As a function of this separation, (b) shows the emission energy (the zero-phonon line, or ZPL, which determines the emission colour) and (c) the corresponding electronic structure. The results show that the emission colour depends on the distance between the two point defects.

Identifying the atomic origin of single-photon emitters in two-dimensional crystals is exceptionally difficult, as local mechanical strain, electric fields and encapsulating layers make microscopic observations harder to interpret. By combining detailed electronic-structure theory with results from previously reported magnetic and optical measurements, the authors demonstrate that interactions between point defects – not just isolated defects – play a key role in the quantum optical properties of hBN. This finding could accelerate the targeted creation and performance optimisation of quantum light sources across the wider family of wide-band-gap materials.