Researchers from the Centre for Quantum Dynamics at Griffith University in Brisbane, Queensland, captured the image using their custom-built ion trap, which was also responsible for capturing the highest-resolution image of a single atom last year.
“The question is, how many atoms do you need to cast a shadow? We found that it only takes one,” said David Kielpinski co-author of the paper, which was published in Nature Communications.
“We know exactly how an atom works in theory, and the shadow that we saw was exactly what it ought to be in practice.”
Ion trapped, cooled, captured
Scientists have long known that imaging the shadow (also known as absorption imaging) can provide more information compared to other forms of optical imaging. That’s because it is easier to calculate how much light (or information) is absent in the presence of an object, than to capture all of the light bouncing off it.
This atomic information is helpful for the construction of a large-scale quantum computer.
A quantum computer would operate by manipulating the quantum states of single atoms to relay information and perform computations – potentially providing a vast advancement on current computer systems.
“Anytime you shine the right wavelength of light past an atom, it’s going to cast a shadow,” said Kielpinski. “It’s just a question of whether the resolution of your imaging apparatus is good enough.”
The researchers trapped a Ytterbium ion (174Yb+) in their ultra-high vacuum ‘ion trap’ and laser-cooled it to only a few ‘millikelvin’ (less than -273°C).
An illumination beam was then focussed to the spot where the ion was trapped and the light that passed by the ion was collected through a phase Fresnel lens (a modification of the common lighthouse lens). What was captured was the first ever absorption image of a single atom isolated in vacuum and the first ever image of an atom shadow.
”It’s the lens that does all the work”
“While the trap is novel in the sense that not many people have one like ours, it’s really the phase Fresnel lens that does all the work. It’s the thing that allows us to get all these high-resolution images and it’s the part that is the newest and nobody has ever done before,” said Ben Norton, a fellow team member and co-author of the paper.
This rare phase Fresnel lens features lines on a flat plate of glass instead of a curved surface, which allows the light to be diverted toward the focus. At the same time some of the light is diverted in the opposite direction or passes straight through the lens.
“Having high-resolution imaging is important to get a clean, crisp image of the shadow, but if you delve into the physics a bit more, it also directly ties into how dark that shadow image is,” he added.
Using quantum mechanics, the researchers were able to accurately predict the darkness and size of the atom’s shadow – a mathematical prediction called the ‘quantum limit’.
“It’s not some complicated theoretical formula, it’s the simplest quantum mechanics that tells you what this image should look like,” said Kielpinski.
“We have set up a situation that is extraordinarily simple. And then by looking at this conceptually simple situation, we can deduce things that reflect back on the very real, complicated world.”
Imaging the shadow of a single atom might also provide answers for the future of biological imaging. Current techniques like biomarkers cause severe damage to biological samples, like DNA and cells. But with the less harmful light used for shadow imaging one might be able to successfully image biological samples in the future without consequently killing them.
“It is similar to stringing Christmas lights onto the DNA strands to watch how particular points are moving around in the dark,” said fellow colleague and co-author of the paper, Erik Streed.
“Absorption imaging of DNA would give information about what the whole strand is doing, including the un-labelled parts,” he said.
However, this leap from fundamental physics to biological imaging would require interdisciplinary focus over many years, added the researchers.
“This is an exciting development where they show the maximum extraction of information from absorption imaging,” said physicist Keith Nugent from the University of Melbourne, commenting on the research.
“This is fundamental work that may open up a way of highly sensitive micro-imaging,” he added