A quantum microscope prototype developed by an Australian team has been shown to create high-resolution maps of different physical quantities.
As our knowledge builds of how the quantum world operates, so does our ability to use quantum mechanical principles to develop new and useful technologies. The scientific community is abuzz with excitement over the promise of quantum computers which will be thousands of times faster and more powerful than classical machines.
But quantum technology built on our understanding of matter and energy behaviour on the atomic and subatomic level has other uses.
One such direction for research into quantum technology is quantum microscopy. These microscopes are predicted to be powerful enough to allow us to see electric currents, detect magnetic field fluctuations, and even observe single molecules on a surface.
The quantum microscope developed by Australian researchers is based on atomic impurities – referred to as “spin sensors” because they interact through the quantum phenomenon of spin – within the material. Shining a laser on the impurities, they then emit light that directly relates to interesting physical properties like magnetic field, electric field and the chemistry near the defect.
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The research, led by Professor Igor Aharonovich of the University of Technology Sydney (UTS) and Dr Jean-Philippe Tetienne of RMIT University, is published in Nature Physics.
Microscopes have certainly come a long way.
It’s not exactly know who is responsible for the first microscope, but Dutch spectacle maker Zacharias Janssen is credited with making one of the earliest microscopes which used two lenses, around 1600.
The earliest microscopes could magnify an object up to 20 or 30 times its normal size.
In 1665, Robert Hooke produced the famous Micrographia – a collection of copper-plate illustrations of objects he observed with his own compound microscope.
As technology developed, microscopes became better at magnifying the tiny, but they are limited by their reliance on the wavelength of light used. Objects smaller than a micrometre (a millionth of a metre) are too close in size to the wavelength of visible light to be seen in conventional microscopes.
Enter the electron microscope. Electrons have wavelengths of around one ten-billionth of a metre. The best electron microscopes can show remarkable detail. But, unlike quantum microscopes, they can’t tell us much about the physical properties, like electric and magnetic fields.
Quantum microscopes exist, but they have relied on defects present in bulky, three-dimensional crystals such as diamond. In these materials, the spin sensors are limited in how close they can get to the sample being studied.
Aharonovich says the ingenuity of the new method lies in the use of single atom-thin layers of a crystal called hexagonal boron nitride (hBN) which is known as a van der Waals material.
Van der Waals materials are strongly bonded in two dimensions and bound in the third dimension through weaker forces, meaning individual layers, called graphenes, can be peeled off and used in many different applications. A van der Waals material used every day is the graphite in pencil lead.
“This van der Waals material – that is, made up of strongly bonded two-dimensional layers – can be made to be very thin and can conform to arbitrarily rough surfaces, thus enabling high resolution sensitivity,” says Aharonovich.
“These properties led us to the idea of using ‘quantum-active’ hBN foils to perform quantum microscopy, which essentially is an imaging technique that utilises arrays of quantum sensors to create spatial maps of the quantities they are sensitive to,” adds Tetienne.
“Until now, quantum microscopy has been limited in its spatial resolution and flexibility of application by the interfacing issues inherent in using a bulky three-dimensional sensor. By instead utilising a van der Waals sensor, we hope to extend the utility of quantum microscopy into arenas that were previously inaccessible.”
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The team tested their prototype on a ferromagnetic van der Waals material – a flake of the crystal chromium ditelluride (CrTe2).
The hBN based quantum microscope was able to image the magnetic domains of the ferromagnet, with nanoscale proximity to the sensor at room temperature – something that was believed to be impossible.
The unique properties of the hBN defects allowed the researchers to also record a temperature map. This confirms the microscope can be used to correlate images between the two different physical quantities.
The authors note that the resolution of their quantum microscope is limited by diffraction of light to around 1 micrometre in their results. But they add that this could be sharpened to around 10 nanometres in principle. At this point, it’s no longer microscopy, but nanoscopy we’re talking about.
“There is a huge potential for this new generation of quantum microscopy,” says UTS senior researcher Dr Mehran Kianinia. “Not only can it operate at room temperature and provide simultaneous information on temperature, electric and magnetic fields, it can be seamlessly integrated into nanoscale devices and withstand very harsh environments, as hBN is a very rigid material.
“The main future applications include high resolution MRI (magnetic resonance imaging) and NMR (nuclear magnetic resonance) that can be used to study chemical reactions and identify molecular origins, as well as applications in space, defence and agriculture where remote sensing and imaging are key.”