Looking back on the first-ever photo of quantum entanglement

This stunning image captured last year by physicists at the University of Glasgow in Scotland is the first-ever photo of quantum entanglement - a phenomenon so strange, physicist Albert Einstein famously described it as 'spooky action at a distance'.

It might not look like much, but just stop and think about it for a second: this fuzzy grey image was the first time we'd seen the particle interaction that underpins the strange science of quantum mechanics and forms the basis of quantum computing.

Quantum entanglement occurs when two particles become inextricably linked, and whatever happens to one immediately affects the other, regardless of how far apart they are. Hence the 'spooky action at a distance' description.

This particular photo shows entanglement between two photons - two light particles. They're interacting and - for a brief moment - sharing physical states.

Paul-Antoine Moreau, first author of the paper wherein the image was unveiled back in July 2019, told the BBC the image was "an elegant demonstration of a fundamental property of nature".

To capture the incredible photo, Moreau and a team of physicists created a system that blasted out streams of entangled photons at what they described as 'non-conventional objects'.

The experiment actually involved capturing four images of the photons under four different phase transitions. You can see the full image below:

(Moreau et al., Science Advances, 2019)

What you're looking at here is actually a composite of multiple images of the photons as they go through a series of four phase transitions.

The physicists split the entangled photons up and ran one beam through a liquid crystal material known as β-barium borate, triggering four phase transitions.

At the same time they captured photos of the entangled pair going through the same phase transitions, even though it hadn't passed through the liquid crystal.

You can see the setup below: The entangled beam of photons comes from the bottom left, one half of the entangled pair splits to the left and passes through the four phase filters. The others that go straight ahead didn't go through the filters, but underwent the same phase changes.

(Moreau et al., Science Advances, 2019)

The camera was able to capture images of these at the same time, showing that they'd both shifted the same way despite being split. In other words, they were entangled.

While Einstein made quantum entanglement famous, the late physicist John Stewart Bell helped define quantum entanglement and established a test known as 'Bell inequality'. Basically, if you can break Bell inequality, you can confirm true quantum entanglement.

"Here, we report an experiment demonstrating the violation of a Bell inequality within observed images," the team wrote in Science Advances.

"This result both opens the way to new quantum imaging schemes ... and suggests promise for quantum information schemes based on spatial variables."

The research was published in Science Advances.

A version of this article was first published in July 2019.

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