The negatively charged boron vacancy (VB-) in hexagonal boron nitride (hBN) has tremendous potential for quantum sensing applications, since its spin state is optically addressable. It is under intense development worldwide, for quantum sensing of static magnetic fields, temperature, pressure, and nuclear spins. It may also be used in quantum computing, sensing, networks, and communications. Because hBN is a two-dimensional van der Waal material, it has potential advantages over 3D materials such as the NV center in diamond, including better light collection, easier incorporation into heterostructures with other 2D materials, and integrating into nano-photonics.

In this project, VB- defects will be created in hBN single crystals by neutron irradiation, and their optical and spin properties characterized. Neutron irradiation creates boron vacancies by the transmutation of the boron-10 isotope into lithium and alpha particles. By using single crystals, the properties of the point defects may be studied without interference from grain boundaries and dislocations, which are typically found in hBN powders and composites. Furthermore, we will use hBN crystals enriched in boron-10 (>99%), to achieve a higher concentration of boron vacancies, and nitrogen-15 (>99%), to minimize the background noise. This will increase the sensitivity of hBN by increasing the quantum coherence time in comparison to hBN with the natural distribution of boron isotopes.

hBN crystals will be irradiated for four different neutron fluences, ranging from 5.0E15 n-cm^-2 to 5.0E16 n-cm^-2 corresponding to times of 0.5 h, 1.0 h, 2.0 h, and 5.0 h at a thermal neutron flux of 4E12 neutrons·cm-2·s-1. Our previous measurements on neutron irradiated h10B15 established that this fluence range is probably the most optimal. The optical/spin properties of the neutron irradiated hBN will be determined by electron spin resonance (EPR) and optically detected magnetic resonance (ODMR). The optical and magnetic properties and quantum coherence time of the defects will be determined as a function of the neutron dose. The fluence that produces a strong, easily detected signal with a long quantum coherence time is best. Confocal imaging of the ODMR response will also provide spatial resolution of these properties, shedding light on the effects of local defects and microstructures on the spin properties of these materials. The irradiation can be completed in less than two months. The ESR and ODMR measurements can each be completed in six weeks. These experiments will establish control and the properties of the VB- defect, and its suitability for quantum information science applications. Both magnetic resonance techniques can also reveal and track other defects that may arise under the varying neutron fluence conditions. Such defects could potentially play important roles in the hBN sensing behavior. Furthermore, this project will provide material for additional fundamental property measurements, such as the effect of neutron irradiation on hBN’s thermal conductivity and mechanical properties. It will also provide the material needed for quantum sensors based on and incorporating crystalline hBN.

The proposed research will advance the DOE’s Office of Nuclear Energy mission to provide energy that is efficiently produced, reliable, and safe. Establishing the fundamental basic science on the quantum properties of defects in hexagonal boron nitride has the potential to lead to technologies that support this mission. One example is the development of quantum-based, sensitive neutron detectors. These can provide in-pile instrumentation and neutron flux measurement in advanced reactors and monitoring the stockpiles of nuclear materials. A second technology would take advantage of the ability of defects (the boron vacancy) in hBN to be quantum sensors, made possible by the ability to detect their spin state optically. They also present good opportunities for quantum science and engineering, including quantum computing, which is useful for efficient nuclear physics calculations. Defect engineering in hBN may lead to the development of solid-state, quantum devices that can operate at room temperature. In contrast, most quantum information science is currently based on expensive technologies that operate at cryogenic temperatures. This project would be leveraged by on-going research to grow large, high quality, hexagonal boron nitride single crystals, supported by the Office of Naval Research. The unique boron-10 and nitrogen-15 enriched hBN crystals produced under the ONR project will greatly enhance and improve this DOE NSUF proposal by making signals clearer and with finer structure by reducing competing background noise from other types of defects such as grain boundaries.