There has been significant interest within the scientific community in developing and utilizing novel two-dimensional materials following the success of graphene. Hexagonal boron nitride (hBN), a material similar to graphene, is gaining traction in quantum technologies. With its wide bandgap and unique properties, hBN is becoming a preferred choice for modern quantum emitters and sensing systems.
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Understanding Hexagonal Boron Nitride
Monolayer hBN consists of a single layer of boron and nitrogen atoms arranged in a honeycomb pattern. Recently, defects in hBN crystals have been identified as color centers, with some being spin-active, making them ideal candidates for single-photon emitters (SPEs) and spin qubits.
SPEs are fundamental components for realizing various quantum technologies, including quantum computation, quantum communication, and quantum metrology.1
Unique Characteristics of hBN in Quantum Applications
The nature of the bandgap in hBN has been debated for some time, with early calculations often predicting an indirect bandgap, while some suggested a direct bandgap. Recent studies have shown that both behaviors can be observed, depending primarily on the thickness of the hBN.2
For modern industrial applications, key properties include spin polarization through optical pumping and spin-dependent photoluminescence, which enable optically detected magnetic resonance (ODMR). The reliable detection of ODMR from color centers in 3D solids has been a major research focus over the past two decades, driving advancements in quantum information processing and quantum sensing.3
Several optically active spin defects have been identified in hBN, with the negatively charged boron vacancy (VB⁻) defect being the most studied.4 Recent research highlights its potential as a novel platform for quantum sensing.
These hBN spin defects can detect external disturbances with remarkable sensitivity.5 An advantage of these defects is their high stability, offering high stability within just a few layers of the material. In addition to their utility in quantum sensing, they show promise for high-resolution magnetic field imaging and can detect a wide array of physical quantities.
Quantum Sensing and Imaging Using hBN
Experiments have revealed spin-related defects in hBN sheets, opening the door for quantum sensing to analyze objects in close proximity, especially magnetic 2D materials. Quantum sensing is used to measure magnetic fields, temperature, strain, and pressure by tracking shifts in the ODMR resonance frequency.6 Various protocols have been established for detecting external fields through these shifts.
In Continuous Wave Optically Detected Magnetic Resonance (CW ODMR), the resonant frequencies are recorded, and shifts are observed in the presence of external fields, reflecting the magnitude of external disturbances.6 This approach does not require pulsed laser excitation or precise microwave manipulation.
Alternatively, pulsed measurements can increase sensitivity to specific external perturbations. Pulsed sensing protocols involve microwave pulses when the laser is off during the sensing period.
To enhance the sensitivity and efficiency of the process, techniques like spin-echo and dynamic decoupling sensing modes are also employed.7 These novel methods ensure the effectiveness of hBN and other 2D materials for quantum sensing applications.
hBN Quantum Emitters: A Crucial Development in Quantum Technology
hBN hosts a diverse range of optically active defects that function as bright single-photon emitters (SPEs). “SPE engineering” typically involves either isolating existing emitters, creating new lattice defects, or restructuring defects. SPE arrays in hBN have been produced using techniques such as nano-indentation with AFM tips and ultrashort laser pulses.
The ODMR of the VB⁻ defect was first identified in 2020 in neutron-irradiated hBN, with emission conclusively reported at approximately 800 nm using electron paramagnetic resonance analysis. In addition to neutron irradiation, VB⁻ defects have also been successfully created through ion, electron, proton irradiation, and femtosecond laser writing.8 Quantum emitters and imaging have been demonstrated using dense layers of VB⁻ defects created by ion irradiation in thin hBN flakes.
Recent efforts have focused on improving emitter stability, particularly in monolayers, and understanding their atomic and electronic structures. Researchers in quantum technology are working on engineering stable emitters, enhancing reproducibility, and integrating them with optical cavities. The potential for ultra-bright SPEs, spin defects, and cavity engineering continues to unite different research communities in addressing the major challenges.9
hBN: Stable Photonic Emitters for Quantum Computing Platforms
hBN has emerged as an effective material for developing photonic and optical quantum systems due to its inherent properties. Photonic hBN emitters are distinct from classical emitters, with LP defects in hBN serving as excellent SPEs.
In a recent experiment, multilayer hBN grown via low-pressure chemical vapor deposition (LPCVD) demonstrated stable photonic quantum emitters, with around 200 emitters per 10 × 10 µm² at room temperature. Optical characterization of emitters from CVD-grown hBN, whether multi-layered, single-layered, or exfoliated, revealed a PL spectrum with zero-phonon lines (ZPL) ranging from approximately 580 nm to 623 nm.10
Multilayer hBN quantum emitters exhibited impressive thermal stability and optical stability, even when operated at temperatures as high as 800 K. They also withstand aggressive annealing in oxidizing and reducing environments without altering their spectral properties.
In optical quantum computing, single photons emitted by hBN SPEs are utilized as qubits in modern fast quantum processor fabrication. Quantum gates are typically developed using a set of birefringent wave plates, and the processed qubits are detected and accumulated using single-photon detectors.
In quantum metrology, entangled photons generated by hBN SPEs facilitate highly sensitive measurements of various signals, particularly weak emissions amid strong unwanted background noise. These entangled photons also help in making precise measurements of physical parameters, thereby reducing statistical errors and enhancing the accuracy of the measurements.
While these results affirm hBN’s potential for photonic quantum applications, further focus is needed on fabricating quantum emitters with a wide emission range on a single platform to optimize the performance of quantum photonic devices and expand their practical applications.
To leverage hBN defects for global quantum networks, they can function as both quantum emitters and quantum memory, benefiting from the wide range of selectable hBN defects with the necessary properties.11 Integrating components based on the same quantum system, such as hBN, presents an ideal scenario for global quantum networks, space applications, and instrumentation. This approach reduces operational errors and simplifies experimentation.
Advancements in site-specific defect generation in hBN with nanometric precision and consistent wavelength are opening new application opportunities. With the involvement of novel processes, hBN will play a crucial role in modern devices such as super-radiant lasers and the exploration of multi-emitter cavity-QED regimes.
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References and Further Reading
[1] Prasad, M., et al. (2024). Hexagonal boron nitride based photonic quantum technologies. arXiv preprint arXiv:2407. 11754. Available at: https://doi.org/10.48550/arXiv.2407.11754
[2] Paleari, F., et al. (2018). Excitons in few-layer hexagonal boron nitride: Davydov splitting and surface localization. 2D Materials. https://doi.org/10.1088/2053-1583/aad586
[3] Doherty, M., et al. (2013). The nitrogen-vacancy colour centre in diamond. Physics Reports. https://doi.org/10.1016/j.physrep.2013.02.001
[4] Stern, H., et al. (2022). Room-temperature optically detected magnetic resonance of single defects in hexagonal boron nitride. Nat Commun. https://doi.org/10.1038/s41467-022-28169-z
[5] Song, T., et al. (2021). Direct visualization of magnetic domains and moiré magnetism in twisted 2D magnets. Science. https://doi.org/10.1126/science.abj7478
[6] Gottscholl, A. et al. (2021). Spin defects in hBN as promising temperature, pressure and magnetic field quantum sensors. Nat Commun 12, 4480. Available at: https://doi.org/10.1038/s41467-021-24725-1
[7] Sumukh ,V., et al. (2023) Quantum sensing and imaging with spin defects in hexagonal boron nitride. Advances in Physics: X. https://doi.org/10.1080/23746149.2023.2206049
[8] Guo, N., et al. (2022). Generation of spin defects by ion implantation in hexagonal boron nitride. ACS omega. https://pubs.acs.org/doi/10.1021/acsomega.1c04564
[9] Aharonovich, I., et al. (2022). Quantum emitters in hexagonal boron nitride. Nano Letters. https://doi.org/10.1021/acs.nanolett.2c03743
[10] Shaik, A., et al. (2021). Optical quantum technologies with hexagonal boron nitride single photon sources. Sci Rep. https://doi.org/10.1038/s41598-021-90804-4
[11] Cholsuk, C., et al. (2024). Theoretical investigation of fluorescent defects in hexagonal boron nitride and their applications in quantum technologies. In Quantum Technologies. SPIE. https://doi.org/10.1117/12.3016410
