Quantum light sources are critical components in photonic quantum technologies, enabling applications such as quantum communication, cryptography, and computing. Two-dimensional materials, particularly hexagonal boron nitride (hBN) and transition metal dichalcogenides (TMDCs), have emerged as promising platforms for generating quantum light due to their unique electronic and optical properties. These materials host single-photon emitters (SPEs) that can operate at room temperature, a significant advantage over traditional quantum emitters like nitrogen-vacancy centers in diamond or quantum dots, which often require cryogenic conditions.

In hBN, single-photon emission is primarily associated with defects in the crystal lattice. The exact atomic configuration of these defects remains under investigation, but studies suggest that vacancies, impurities, or complex defect complexes are responsible. These defects create localized electronic states within the bandgap, allowing for efficient photon emission when excited optically or electrically. The emission from hBN defects typically occurs in the visible to near-infrared range, with narrow linewidths and high brightness. The defects exhibit photostability and high purity, with measured second-order correlation functions (g²(0)) often below 0.1, confirming single-photon emission. The brightness of these emitters can exceed 10⁶ counts per second under continuous-wave excitation, making them competitive with other solid-state quantum emitters.

TMDCs, such as monolayer MoS₂ or WSe₂, host single-photon emitters through localized excitons. These excitons can be trapped at strain-induced sites, defects, or edges of the material. The confinement of excitons in TMDCs leads to discrete energy levels, enabling single-photon emission. Unlike hBN, the emission from TMDCs is often associated with tightly bound exciton complexes, such as trions or biexcitons, which can be selectively excited to produce single photons. The emission wavelengths in TMDCs are typically in the visible range, with linewidths on the order of a few meV at low temperatures. At room temperature, the linewidths broaden due to phonon interactions, but single-photon emission persists, with g²(0) values comparable to those in hBN.

Fabrication techniques for engineering quantum light sources in 2D materials are diverse and tailored to the specific material system. In hBN, defect creation can be achieved through ion irradiation, annealing, or electron beam exposure. These methods introduce defects in a controlled manner, though the exact nature of the defects remains challenging to predict. Strain engineering is another powerful tool, where mechanical deformation of hBN flakes creates localized strain fields that can activate or enhance single-photon emission. Nanopatterning techniques, such as focused ion beam milling or plasma etching, are used to create arrays of emitters with precise spatial control.

For TMDCs, strain engineering is particularly effective. By transferring monolayers onto nanostructured substrates, such as nanopillars or trenches, localized strain gradients can be created, leading to the formation of exciton trapping sites. Chemical vapor deposition (CVD) growth of TMDCs can also be tuned to introduce defects or edges that serve as single-photon emitters. Additionally, electrostatic gating can modulate the charge state of excitons, enabling dynamic control of the emission properties.

The performance of quantum light sources is evaluated using several key metrics. Purity, quantified by the g²(0) function, measures the probability of multi-photon emission events. Brightness refers to the photon count rate under a given excitation power, while the emission lifetime provides insight into the radiative and non-radiative decay pathways. For hBN and TMDCs, typical lifetimes range from a few nanoseconds to tens of nanoseconds, depending on the local environment and defect type. Spectral stability is another critical parameter, as spectral diffusion—random shifts in emission wavelength—can degrade the performance of quantum light sources. Encapsulation with inert materials like hBN or polymers has been shown to mitigate spectral diffusion in TMDCs.

Integration of 2D material quantum light sources with photonic circuits is essential for scalable quantum technologies. Plasmonic nanostructures, dielectric waveguides, and photonic crystals have been employed to enhance the coupling efficiency between emitters and optical modes. For example, embedding hBN emitters in a waveguide can direct emitted photons with high efficiency, enabling on-chip quantum photonic circuits. TMDC emitters have been coupled to microring resonators to enhance brightness through Purcell effects. However, challenges remain in achieving uniform coupling across multiple emitters and maintaining spectral alignment with photonic components.

Scalability is a major hurdle for the widespread adoption of 2D material quantum light sources. While individual emitters exhibit excellent properties, producing large-scale arrays with consistent performance is difficult. Variability in defect types, strain distributions, and local environments leads to inhomogeneous emission spectra and brightness levels. Advanced nanofabrication techniques, such as atomic precision patterning or templated growth, may offer solutions to this challenge. Spectral uniformity is another issue, as emission wavelengths can vary significantly between emitters due to differences in local strain or defect configurations. Post-fabrication tuning methods, such as electrostatic gating or strain adjustment, are being explored to achieve spectral overlap for multiplexed quantum networks.

Quantum communication systems require indistinguishable photons for high-fidelity entanglement and interference. The inherent spectral variability of 2D material emitters complicates this requirement. Techniques like resonant excitation or feedback stabilization have shown promise in improving photon indistinguishability. Additionally, integrating 2D emitters with cavities or metasurfaces can narrow the emission linewidth and enhance indistinguishability.

Despite these challenges, the advantages of 2D material quantum light sources—room-temperature operation, high brightness, and compatibility with photonic integration—make them compelling candidates for future quantum technologies. Continued research into defect engineering, strain control, and hybrid photonic structures will be crucial for unlocking their full potential. As fabrication techniques advance and our understanding of defect physics deepens, 2D materials may become the foundation for scalable, on-chip quantum photonic systems.

The development of quantum light sources in hBN and TMDCs represents a convergence of materials science, nanophotonics, and quantum optics. By addressing the current limitations in scalability and spectral uniformity, these materials could enable breakthroughs in quantum communication, sensing, and computing. The interplay between defect engineering, strain manipulation, and photonic integration will define the next generation of quantum emitters, paving the way for practical quantum technologies.