First image of the shape of a single photon revealed in light study

The first direct visualization of the shape of a photon has been created. These particles of light are impossible to photograph

By Michael Irving

The first direct visualization of the shape of a photon has been created. These particles of light are impossible to photograph, but physicists at the University of Birmingham have now calculated their wave function to produce an accurate image of a photon as it’s emitted.

Photons are what allow us to see, both with our eyes and with cameras. When they reach our retinas or camera sensors, they carry with them information about the source that emitted them, or objects they’ve bounced off on the way, allowing our brains or cameras to construct an image.

However, one thing photons can never capture images of is other photons. That’s because they don’t interact with each other in any way. But now, Birmingham physicists have created the next best thing: a mathematically accurate visualization of the shape of a photon.

“The visualization is an exact simulation of a photon as it is emitted by an atom sitting on the surface of a nanoparticle,” co-author Ben Yuen told New Atlas. “The shape of the photon is deeply affected by the nanoparticle, making it thousands of times more likely that the photon is emitted, and even allowing it to be reabsorbed by the atom multiple times.”

The “shape” of a photon is a tricky thing to pin down, and it doesn’t quite mean the same thing as illustrating the shape of a regular object. Instead, it’s an intensity distribution – basically, a map of where you could expect to find the photon at a certain point in time. Brighter areas indicate a higher chance of the photon appearing there when its location is measured.

“The visualization is exactly that distribution of a photon a short time after it has been emitted,” Yuen told us. “Because it’s a quantum particle you cannot measure it in one go as the measurement destroys it. However, if you were to repeat the measurement of where a photon was detected many times, you would see exactly this distribution.

“Furthermore, and one of the strangest things about quantum mechanics, is that before the photon is ever detected, all the detail information of this intensity distribution already exists through what we call its ‘wave function,’ which is exactly what we were able to calculate for the first time,” Yuen continues.

So with the long history of studying photons, why haven’t scientists been able to create this kind of image before? It turns out, Yuen and co-author Angela Demetriadou weren’t actively trying to – it came about as a kind of by-product of a more general study.

“We set out to answer something quite fundamental: How are photons really emitted by atoms and molecules, and what effect does their environment have on this?” Yuen told us. “This is something physicists have only be able to accurately model in a perfect vacuum containing just a single atom/molecule, but nothing else around. However, it’s been known for a long time that the environment can have a profound impact on this process, yet no theory has been able to fully capture all its detail.”

To achieve this, the team started by developing a version of quantum field theory that included a silicon nanoparticle interacting with photons. The problem is that there’s essentially infinite possibilities for how the nanoparticle can interact with a continuous spectrum of light. Thankfully, the team found a way to narrow that down.

“We used a branch of mathematics called complex analysis to transform the problem from a continuous set based on the real numbers, into a discrete set based on some distinct complex numbers,” said Yuen. “Whilst it might seem ‘complex’ this simplified the problem massively, allowing us to exactly represent it as an interaction with just a few hundred ‘complex’ light modes.

This work incidentally allowed the team to create the photon visualization above.

“Quite amazingly, when we did this a number of details just started to drop out of our theory such as exactly how light propagates, and exactly what the shape of the photons intensity distribution is expected to be like,” Yuen said.

The researchers say that this work drastically improves our understanding of how light and matter interact, which could have applications in solar cells, quantum computing and sensors.

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