The quantum microscope revolution is here

University of Queensland researchers have built a quantum microscope based on the strange phenomenon Albert Einstein once called “spooky action at a distance”.

This new device takes advantage of quantum entanglement to illuminate living samples safely – unlike conventional microscopes, which use potentially damaging high-intensity light.

Warwick Bowen, a quantum physicist at the University of Queensland, says this is the first entanglement-based sensor that supersedes non-quantum technology.

“This is exciting – it’s the first proof of the paradigm-changing potential of entanglement for sensing,” says Bowen, who is lead author on the new paper published in Nature.

Since their invention in the seventeenth century, traditional light-based microscopes have revolutionised our understanding of life by revealing the microscopic structures and behaviours of living systems. The field of microscopy took a big leap when lasers were introduced to more brightly illuminate samples; some recent technologies have even been able to peer down to resolutions nearly at the scale of atoms.

But the best microscopes are limited by the “noisiness” of photons – the tiny packets of energy that make up light. The random times at which individual photons hit a detector introduces noise, which affects the sensitivity, resolution and speed of microscopes. The noise can be reduced by increasing the intensity of light – which fries cells.

“The best light microscopes use bright lasers that are billions of times brighter than the sun,” Bowen explains. “Fragile biological systems like a human cell can only survive a short time in them.

“We’re hitting the limits of what you can do just by increasing the intensity of your light.”

Bowen and team’s new microscope may just kickstart the next revolution in microscopy, because they’ve evaded these limitations by introducing quantum entanglement.

But how does this device actually work? Well, it’s down to quantum physics, so buckle in.

Quantum entanglement is a strange beast to get your head around. The idea is that two particles can become “entangled”, or linked, and will thereafter always mirror each other’s properties – what happens to one instantly happens to the other, even if they’re light-years apart. This instantaneous coordination seems to rebel against common sense; physicists don’t yet know exactly how this works, only that it does.

And this phenomenon can be harnessed in microscopy.

Physicists have known for a while that quantum correlations can be used to extract information from photons – in fact, these correlations used to improve laser interferometric gravitational wave detectors like LIGO, among many other things. They even suspected that quantum correlations could help improve microscopy, but until now they couldn’t build bright enough light sources with quantum correlations that could be interfaced with a microscope.

“However, all previous experiments used optical intensities more than 12 orders of magnitude lower than those for which biophysical damage typically arises, and far below the intensities typically used in precision microscopes,” the authors explain in their paper.

This new set-up uses a coherent Raman scattering microscope – existing technology that probes the vibrational signals of living molecules, giving specific information about their chemical makeup.

But the team custom-designed the microscope so quantum correlations improved the light source illuminating the sample, making the light extremely “quiet”.

“What entanglement allows us to do is basically train the photons in that light so that they arrive at the detector in a nice uniform sort of way,” Bowen says.

This is achieved using a “non-linear crystal”, which changes the light passing through; instead of a normal laser beam they used “squeezed light”, where the photons are intrinsically correlated. This reduced the amplitude of the light and, in turn, reduced the noise.

For a fixed intensity of light, the set-up results in a higher signal-to-noise ratio and therefore higher contrast in the microscope. They were able to image a cell wall of yeast – around 10 nanometres thick.

“We could resolve a much larger region of that cell wall using quantum correlations than was possible using conventional microscopy, without destroying the cell,” Bowen explains.

The team were able to enhance the signal-to-noise ratio by 35%.”

“This removes a fundamental barrier to advances in coherent Raman microscopy and high-performance microscopy more broadly,” they write in their paper.

Bowen comments: “We’re really excited about it because it shows, for the first time, that it is possible to use quantum light to get an absolute advantage in microscopy – to measure something you could not measure in any other way.”

Sergei Slussarenko, a quantum physicist at Griffith University who was not involved in the study, says this is a great achievement.

“The quantum optics community has worked for quite some time on the ways to improve measurement precision using exotic states of light and it is quite challenging to achieve these improvements in practice,” he comments.

“Having an enhancement due to quantum effects here is quite important because there is essentially no way to improve the measurement sensitivity just by cranking up the power. Going quantum to gain further measurement enhancement is the only method.”

Bowen says that this is just the first step.

A 35% improvement is not a lot – he reckons they can do even better by developing an even brighter source of quantum light.

“One of the huge problems with precision microscopes is that they’re really slow,” Bowen says. “If you want to image a virus, it might take you a minute to get one snapshot.”

Improving the quantum source of light could speed up imaging by a factor of ten – so instead of a minute it might take six seconds.

This is important, because, as Bowen says, “there’s all sorts of biology going on in that virus in the time that you’ve taken the image”. Reducing the imaging to six seconds means those changes can be captured, instead of missed.

The next step is to build these systems in the labs of other biophysicists and biologists to test out what is possible, a process which will take place over the next five years or so. Bowen’s team is also working with the US Air Force Office of Scientific Research to build these set-ups in their labs in San Antonio, Texas.

“The aim is to get to the point where we’re really able to ask new questions in biology,” Bowen says. “There’s so much that’s not known. Every time you improve a microscope, you discover new behaviours and new phenomena, so the idea is to make a contribution towards that.”

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