Hexagonal Boron Nitride Defects Enable Room-Temperature Quantum Technologies with Addressable Spins

Atomic imperfections within solid materials are increasingly recognised as essential components for next-generation quantum technologies, offering potential for advancements in communication networks and highly sensitive sensors. Researchers, including Tobias Vogl from the Technical University of Munich, Viktor Ivády of Eötvös Loránd University, and Isaac J. Luxmoore from the University of Exeter, are now focusing on hexagonal boron nitride as a promising host material for these crucial defects. Their work reveals that this atomically thin material supports single-light-emitting defects possessing optically addressable electronic and nuclear spins, even at room temperature. This breakthrough significantly expands the range of viable materials and opens the possibility of creating quantum devices that operate under ambient conditions, representing a major step towards practical quantum applications, alongside Hannah L. Stern.

Defects in hBN and Diamond for Quantum Sensing

A comprehensive body of research focuses on atomic defects within hexagonal boron nitride (hBN) and diamond, exploring their potential as quantum sensors and light emitters. Investigations reveal that these defects, carefully characterized and controlled, offer unique opportunities for advancing quantum technologies, with a growing emphasis on hBN as a particularly promising material. Research centers on identifying the specific structures of these defects, predicting their behavior using computational modeling, and experimentally verifying these predictions through advanced spectroscopic and microscopic techniques. These defects are utilized as quantum sensors, capable of measuring magnetic, electric, temperature, and pressure fields with exceptional sensitivity and spatial resolution.

Simultaneously, researchers are exploring their use as single-photon emitters, crucial for secure quantum communication, powerful quantum computing, and high-resolution quantum imaging. Controlling defect creation is a central challenge, with scientists employing techniques like edge creation, ion implantation, controlled annealing, and heterostructure engineering to tailor defect environments. Theoretical work provides fundamental understanding of defect structures and their electronic properties, while experimental groups focus on characterizing defects in both diamond and hBN, and demonstrating their potential in quantum devices. The research highlights a growing emphasis on hBN, offering advantages like a wider range of tunable defects and compatibility with two-dimensional materials.

Establishing a clear link between defect structure and optical properties is a major theme, requiring a combination of computational modeling and advanced experimental techniques. Understanding hyperfine interactions is crucial for controlling and reading out the spin states of defects, and the creation of defect databases accelerates the development of quantum technologies. Combining hBN with other two-dimensional materials allows for the creation of novel defect environments and enhanced quantum properties, and researchers are exploring methods for optically addressing spin defects in hBN, a crucial step towards building scalable quantum devices.

Single-Photon Emission From Boron Nitride Defects

Researchers are actively investigating atomic defects within hexagonal boron nitride (hBN) as promising building blocks for quantum technologies, and have developed rigorous methods to characterize their potential as single-photon sources for quantum key distribution (QKD). The study pioneers techniques to assess their light emission properties, achieving values below 0. 015 for green-red emitters through refined experimental setups and background correction. Polarization-resolved measurements further improved this to below 0. 02 into free space, without background correction.

To evaluate efficiency, the team focuses on internal quantum efficiency, reaching 40-80%, and photon out-coupling, approaching 100%, due to hBN’s low refractive index and the dipole orientation of the defects. They assess system efficiency, aiming for values comparable to decoy protocols used in conventional QKD, typically around 0. 4-0. 5. Initial QKD demonstrations with hBN defects revealed lower secret key rates than laser-based systems, prompting the development of resonant cavity systems to enhance performance.

These cavities aim to exceed 50% system efficiency, but researchers acknowledge the potential for non-radiative decay via long-lived dark states, which could temporarily halt photon emission. Scientists are also exploring the impact of emission wavelength on QKD performance, recognizing the importance of the telecom C-band (1550nm) and O-band (1330nm) for fiber-based communication. While most hBN defects currently emit in the visible region, theoretical predictions suggest the possibility of telecom emitters, and future work will focus on detecting near-infrared photons and utilizing ion implantation to create compatible defects. For free-space QKD, the broad emission range of hBN defects aligns with atmospheric transmission windows, and the linear polarization of these defects is directly compatible with polarization-based encoding, eliminating the need for polarization modulators.

Hexagonal Boron Nitride Hosts Room-Temperature Quantum Spins

Recent work demonstrates that hexagonal boron nitride (hBN) hosts atomic defects with optically addressable electronic and nuclear spins at room temperature, opening possibilities for atomically thin quantum devices. Investigations reveal that certain hBN defects exhibit spin relaxation (T1) timescales of approximately 10 microseconds and Hahn Echo spin coherence (T2) timescales of around 100 nanoseconds. The V− B defect, created through irradiation, possesses a radiative ground state spin triplet (S = 1) with ground- and excited-state zero-field splitting parameters of approximately 3. 5GHz and 2.

1GHz, respectively. Researchers have identified several classes of spin-active hBN defects, including carbon-related defects emitting in the green-red spectral range. One group shows no appreciable zero-field splitting, exhibiting a broad linewidth and a g-factor of approximately 2, with spin relaxation times of around 10-15 microseconds. Another group, denoted Cx, is a strongly coupled S = 1 system with a zero-field splitting of approximately 1. 97GHz and high ODMR contrast, reaching up to 90%.

A novel model proposes that some ODMR signatures arise from charge transfer between pairs of defects in the 2D lattice, termed optical spin defect pairs (OSDP). This mechanism suggests that spatial proximity between defects can be engineered to tune ODMR properties, such as zero-field splitting parameters and contrast. Measurements confirm a reverse intersystem crossing time of around 10 nanoseconds, explaining the observed photoluminescence despite the low quantum efficiency of the triplet state. These findings establish hBN as a promising platform for spin-based quantum technologies, including quantum networking and sensing.

Room Temperature Quantum Emission in Hexagonal Boron Nitride

Hexagonal boron nitride (hBN) presents a promising new platform for developing future quantum technologies, offering several advantages over existing materials like diamond. Researchers have identified atomic defects within hBN that emit single photons and possess optically addressable electronic and nuclear spins, crucially operating at room temperature. This discovery opens the possibility of creating atomically thin quantum devices that function under ambient conditions, a significant step towards practical quantum applications. These hBN defects demonstrate potential in several key areas, notably quantum sensing and free-space quantum key distribution (QKD). The material’s compatibility with two-dimensional heterostructures suggests opportunities for in-situ quantum sensing of magnetic phenomena, stress gradients, and charge flow, as well as improved device quality control. Furthermore, the high repetition rates and linear polarisation.