Enables Fiber-Based Quantum Communication Via Stable Defect States in Hexagonal Boron Nitride

The search for reliable single-photon emitters remains a crucial challenge in developing practical quantum technologies, particularly for long-distance quantum communication. Kerem Anar, Berna Akgenc Hanedar, Roya Kavkhani, and Mehmet Cengiz Onbasli, from Koç University and Kırklareli University, have addressed this need by computationally investigating previously overlooked defects in hexagonal boron nitride. Their research focuses on carbon and silicon-based point defects, employing advanced theoretical methods to predict their potential as single-photon emitters. Significantly, the team identified the Si2BVN defect as the first of its kind in monolayer h-BN predicted to emit single photons at the crucial telecom C band wavelength of 1554nm, paving the way for compatibility with existing fibre optic networks and integrated quantum photonic systems. This work offers concrete targets for experimentalists aiming to create stable, spin-active single-photon sources across a broad spectral range.

Scientists employed density functional theory (DFT) calculations, performed using the Vienna ab initio Simulation Package (VASP), to systematically evaluate carbon- and silicon-based point defects. These calculations utilized a plane-wave basis set and a periodic supercell approach, modelling defective h-BN within a 5x5x1 supercell with a vacuum spacing of approximately 20 Å to prevent unwanted interactions. To ensure accuracy, the study meticulously converged all atomic positions, relaxing structures until total energy changes fell below 10−5 eV and residual stresses were less than 1 kbar.

A plane-wave kinetic energy cutoff of 500 eV and a 3x3x1 k-point mesh were implemented for structural optimizations, parameters rigorously tested for convergence of defect formation energies and local geometries. The team classified defects based on dopant species, substitutional site, and vacancy configuration, enabling a systematic comparison of their electronic and spin properties. This work went beyond simple structural analysis, explicitly determining ground-state spin configurations by comparing energetically relevant spin manifolds for each defect. The approach harnessed a configuration-coordinate formalism to evaluate key optical properties, including zero-phonon line (ZPL) energies, electron, phonon coupling strengths, Debye, Waller factors, and radiative lifetimes.

Applying consistent, experimentally motivated screening criteria across these observables, the research identified a subset of defects balancing optical quality, spin functionality, and thermodynamic stability. Notably, the study predicted the Si2BVN defect as the first point defect in monolayer h-BN capable of supporting single-photon emission in the telecom C band at 1554nm, a significant breakthrough for fiber-compatible quantum emission. This prediction, validated through quantitative analysis, directly addresses the longstanding absence of telecom-band emitters identified from first-principles calculations and establishes a robust framework for defect identification in two-dimensional wide-bandgap materials.

H-BN Defects as Single-Photon Sources

Scientists have identified several point defects in monolayer hexagonal boron nitride (h-BN) with the potential to function as single-photon emitters, a crucial component for integrated quantum photonics. The research team employed hybrid density functional theory and a generating-function approach to photoluminescence to screen a range of carbon- and silicon-based defects, confirming their thermodynamic stability with negative formation energies. Five candidates showed moderate electron-phonon coupling, indicated by Huang-Rhys (HR) factors below 5, suggesting narrow emission linewidths. Experiments revealed that these emitters span a broad spectral range, extending from visible light into the near-infrared.

The Si2BVN defect was identified as the first point defect in monolayer h-BN predicted to emit single photons in the telecom C band at 1554nm, directly addressing a critical limitation in fiber-based quantum communication. The team meticulously calculated zero-phonon-line (ZPL) energies, radiative lifetimes, and Huang-Rhys factors to assess the performance of each defect. Further analysis demonstrated that vacancy-containing complexes possess spin-1/2 ground states, a characteristic that enables the creation of spin-photon interfaces compatible with integrated photonic and cavity-based platforms. Measurements confirm that the combined assessment of ZPL energies, electron-phonon coupling, and radiative lifetimes provides a robust framework for identifying spin-active single-photon emitters across the visible and telecom wavelengths, offering concrete guidance for future experimental realization and photonic integration.

Computational methodology involved performing calculations within a first-principles density functional theory framework using the Vienna ab initio Simulation Package. Structural optimizations were conducted using a 5x5x1 supercell, ensuring sufficient spatial separation between defects, and a plane-wave kinetic energy cutoff of 500 eV with a 3x3x1 k-point mesh. Finite-temperature ab initio molecular dynamics simulations at 300 K for 5 picoseconds confirmed the kinetic stability of the predicted defects, demonstrating their robustness against thermal fluctuations. Transition dipole moments were evaluated using Vaspkit, enabling the identification of dipole-allowed defect transitions relevant to single-photon emission.

Si2BVN Telecom Emission Prediction via Modelling

This research presents a systematic investigation of carbon- and silicon-based point defects within monolayer hexagonal boron nitride as potential single-photon emitters. Through a combination of hybrid density functional theory, excited-state relaxations, and photoluminescence modelling, the team evaluated key optical properties of various defects, including zero-phonon-line energies, radiative lifetimes, and electron-phonon coupling. This unified computational framework allowed for a comprehensive comparison of candidate defects. Notably, the study identifies the Si2BVN defect as the first point defect in monolayer h-BN predicted to emit single photons in the telecom C band (1554nm), a crucial advancement for fiber-based quantum communication. Several other thermodynamically stable defects were also found to exhibit promising characteristics for narrow-linewidth emission, and vacancy-containing complexes demonstrate spin-1/2 ground states suitable for spin-photon interfaces. This work establishes a first-principles methodology for identifying optically and spin-active defects in two-dimensional materials, offering concrete targets for realising scalable quantum photonic technologies based on hexagonal boron nitride.