Single-photon emission from defects in hexagonal boron nitride has emerged as a promising platform for quantum photonics due to its robust room-temperature operation, tunability, and compatibility with nanophotonic integration. The atomic-scale defects in hBN exhibit bright, stable photoluminescence with high photon purity, making them attractive for applications in quantum communication, sensing, and computing.
The origin of single-photon emission in hBN is attributed to point defects, such as vacancies, antisites, or impurity complexes, which create localized electronic states within the bandgap. These defects act as quantum emitters, with transition energies typically in the visible to near-infrared range. The exact atomic configurations of the most commonly observed emitters remain under investigation, but first-principles calculations suggest that nitrogen vacancies, carbon substitutions, or boron dangling bonds may be responsible. The emission spectra often exhibit a zero-phonon line accompanied by phonon sidebands, reflecting interactions with the hBN lattice.
A key advantage of hBN defects is their ability to operate at room temperature. Unlike many solid-state emitters that require cryogenic conditions, hBN hosts defects with large zero-phonon line shifts and weak electron-phonon coupling, reducing thermal broadening. Studies have reported linewidths as narrow as a few meV even at 300 K, enabling high-visibility photon antibunching. The photostability of these emitters is also remarkable, with some defects showing no blinking or bleaching over extended periods under laser excitation.
Spectral tuning of hBN quantum emitters is achievable through several methods. Strain engineering is particularly effective due to hBN’s mechanical flexibility. Applying uniaxial or biaxial strain shifts the defect energy levels, enabling precise control over emission wavelengths. Electric fields can also modulate the transition energies via the Stark effect, with tuning ranges exceeding 10 meV in some cases. Additionally, temperature variations induce spectral shifts, though this method is less practical for dynamic control.
Integration of hBN emitters with photonic cavities enhances their performance by increasing emission rates and directing light into useful modes. Plasmonic nanostructures, such as gold or silver nanoparticles, can strongly couple to hBN defects, leading to Purcell enhancement and improved collection efficiency. Metallic nanoantennas have been shown to boost brightness by over an order of magnitude while preserving single-photon characteristics. Dielectric cavities, including photonic crystal resonators and microrings, offer low-loss confinement and can be fabricated directly on hBN flakes. These structures enable tailored emission profiles and improved photon extraction.
Hybrid systems combining hBN with other 2D materials further expand functionality. For example, coupling hBN emitters to graphene electrodes allows electrical control of charge states, while heterostructures with transition metal dichalcogenides enable energy transfer processes. The van der Waals nature of hBN simplifies integration, as it can be mechanically exfoliated or transferred onto prefabricated photonic circuits without lattice-matching constraints.
Despite these advantages, challenges remain in achieving uniform emitter properties. The defect formation process is not yet fully deterministic, leading to variability in emission wavelengths and brightness. Post-growth treatments, such as annealing or ion irradiation, can increase emitter density but require optimization for reproducibility. Another limitation is the moderate quantum efficiency of some defects, which may necessitate further passivation or encapsulation strategies.
Applications of hBN single-photon sources are progressing rapidly. In quantum key distribution, their room-temperature operation simplifies system design compared to cryogenic alternatives. For sensing, the defects’ sensitivity to local electric fields and strain makes them viable nanoscale probes. In quantum networks, integrating hBN emitters with waveguides and detectors could enable on-chip photon routing. Future developments may focus on scalable fabrication techniques, such as controlled defect implantation or patterned growth, to ensure consistent emitter performance across large areas.
The mechanical and chemical stability of hBN provides additional practical benefits. Unlike some emitters that degrade in air or moisture, hBN defects remain stable under ambient conditions, reducing encapsulation requirements. This robustness facilitates device integration and long-term operation in real-world environments.
Research into hBN quantum emitters continues to uncover new defect types with tailored properties. Recent studies have identified emitters with polarized emission, spin-dependent transitions, and enhanced brightness, expanding the toolkit for quantum photonics. Advances in nanoscale characterization techniques, such as tip-enhanced spectroscopy, are helping to correlate emitter properties with atomic-scale structure.
In summary, single-photon emitters in hexagonal boron nitride offer a versatile platform for quantum technologies, combining room-temperature operation, spectral tunability, and compatibility with nanophotonic integration. While challenges in defect control and uniformity persist, ongoing progress in material engineering and device fabrication is paving the way for practical applications in quantum information science and beyond. The unique properties of hBN position it as a leading candidate for next-generation quantum light sources.