A team of physicists at the University of Cambridge has unveiled a breakthrough in quantum sensing by demonstrating the use of spin defects in Hexagonal Boron Nitride (hBN) as powerful, room-temperature sensors capable of detecting vectorial magnetic field at the nanoscale. The findings, published in Nature Communications, mark a significant step toward more practical and versatile quantum technologies.
"Quantum sensors allow us to detect nanoscale variations of various quantities. In the case of magnetometry, quantum sensors enable nanoscale visualisation of properties like current flow and magnetisation in materials leading to the discovery of new physics and functionality," said Dr Carmem Gilardoni, co-first author of this study at Cambrdge's Cavendish Laboratory. "This work takes that capability to the next level using hBN, a material that's not only compatible with nanoscale applications but also offers new degrees of freedom compared to state-of-the-art nanoscale quantum sensors."
To date, nanoscale quantum magnetometry at ambient conditions is only possible with the nitrogen vacancy (NV) centre defect in diamond. While a powerful technology, these sensors have limitations that result from their fundamental photophysics. In particular, the NV centre is a single-axis sensor, with limited dynamic range for magnetic field detection. In contrast, the hBN sensor development by the team in Cambridge does not share these limitations and instead presents a multi-axis sensor of magnetic field with large dynamic range.
The team's work demonstrates the capabilities of this new sensor, as well as providing a mechanistic understanding of the origin of its advantageous properties for sensing. Importantly, the team uncovered that the low symmetry, and fortuitous excited state optical rates are responsible for the dynamic range and vectorial capabilities.
hBN is a two-dimensional material, similar to graphene, that can be exfoliated to just a few atomic layers thick. Atomic-scale defects in the hBN lattice absorb and emit visible light in a way that is sensitive to local magnetic conditions, making it an ideal candidate for quantum sensing applications.
In this study, the team investigated the response of the hBN defect fluorescence to variations in magnetic field, using a technique known as optically detected magnetic resonance (ODMR). By carefully tracking the spin response and combining this with detailed analysis of the dynamics of photon emission, the team could uncover the underlying optical rates of the system and their connection to the defect symmetry, and how this combination results in a robust and versatile magnetic field sensor.
"ODMR isn't a new technique - but what we have shown is that probes built using the hBN platform would allow this technique to be applied in a variety of new situations. It's exciting because it opens the door to imaging magnetic phenomena and nanomaterials in a way we couldn't before," said Dr Simone Eizagirre Barker, co-first author of the paper.
"This sensor could open the door to studying magnetic phenomena in new material systems, or with higher spatial resolution that done before," said Prof Hannah Stern, who co-led the research with Prof Mete Atatüre at the Cavendish Laboratory. "The 2D nature of the host material also opens exciting new possibilities for using this sensor. For example, the spatial resolution for this technique is determined by the distance between the sample and sensor. With an atomically-thin material, we can potentially realise atomic scale spatial mapping of magnetic field."
