Researchers at TMOS, the ARC Centre of Excellence for Trans­formative Meta-Optical Systems, and their colla­borators at RMIT University have developed a new 2D quantum sensing chip using hexagonal boron nitride (hBN) that can simultaneously detect tempera­ture anomalies and magnetic field in any direction in a new, groundbreaking thin-film format. The sensor is signi­ficantly thinner than current quantum technology for magneto­metry, paving the way for cheaper, more versatile quantum sensors. 

To date, quantum sensing chips have been made from diamond as it’s a very robust platform. The limitations of diamond-based sensors, though, is that they can only detect magnetic fields when aligned in the direction of the field. If unaligned, they have large blind spots. As a result, magneto­meters made of diamond must contain multiple sensors at varying degrees of alignment. This increases the difficulty of operation and, as a result, the versa­tility to use in different applications. In addition, the rigid and three-dimen­sional nature of the quantum sensor means that its ability to get close to samples that aren’t perfectly smooth is restricted.  

Jean-Philippe Tetienne from the RMIT University and Igor Aharonovich from the University of Technology Sydney and their teams are pioneering a new quantum sensing platform using hBN. These hBN crystals are made up of layers of atomically thick sheets and are flexible, which allows the sensing chips to conform to the shape of the sample being studied, getting far closer to the sample than diamond can. Different defects exist in the hBN that produce different optical pheno­mena. A recently discovered carbon-based defect, the atomic structure of which remains unidenti­fied, detects magnetic fields in any direction but until now has not been used for magnetic imaging.

In an effort to determine the structure of the unidenti­fied defect, the team ran a Rabi measurement experiment and compared the results with the well-understood boron vacancy defect that also exists in hBN. This boron vacancy defect can be used to measure temperature at a quantum level. Through this comparison, they discovered the new defect behaves as a spin half system. This half spin nature of the carbon defect is what allows for the sensor to detect magnetic fields in any direction. 

The team determined that this new carbon-based half spin sensor could be controlled through electrical excitation, in the same way that the boron vacancy sensor can, and that they could be tuned to interact with one another. Energised by these discoveries, they set out to demonstrate a hBN sensing chip that could use both spin defects simul­taneously to measure magnetic field and temperature. The researchers provide the first magnetic images ever taken with this unidentified isotropic sensor. Sam Scholten from RMIT Univer­sity says, “Opti­cally addressable spin defects in solids form a vital toolkit in the realm of quantum materials due to their potential to be utilized as nanoscale quantum sensors and more generally as robust room temperate quantum systems. What makes hBN unique and exciting is its 2D form, which allows our sensors to get much closer to the sample.” 

Priya Singh from RMIT University adds, “Diamond spins have been used for over a decade in biological systems as an in-situ probe. I am eager to take our hBN into the continuously moving cellular environment, where the directional inde­pendence of the sensor would be an advantage.” Igor Aharonovich says, “hbN has many advantages over diamond as a quantum light source for communi­cations and sensing. In addition to its ultra-thin form factor, it can also operate [as a quantum light source for communications] at room tempera­ture, where diamond often requires cryogenic cooling. hBN is also much cheaper and more accessible than diamond.”  

enerally, these new low-dimensional materials offer the chance of discovering new physics due to their extreme anisotropy. Potential future appli­cations for this quantum sensing techno­logy include in-field identi­fication of magnetic geological features.  The spin half nature of the defect will also allow for radio spectro­scopy across a wider band than competing techno­logies. 

Jean-Phillipe Tetienne says, “The next step for this research is to identify what the atomic defects in the hBN are. By understanding the composition of these, we can make progress toward engi­neering sensor devices for optimal performance. I am excited about exploring the properties and oppor­tunities of this new optical spin defect. Its spin half nature is novel in our community, and there are many questions to answer.” (Source: TMOS)

Reference: S. C. Scholten et al.: Multi-species optically addressable spin defects in a van der Waals material, Nat. Commun. 15, 6727 (2024); DOI: 10.1038/s41467-024-51129-8

Link: ARC Centre of Excellence for Transformative Meta-Optical Systems, Faculty of Science, University of Technology Sydney, Ultimo, Australia