The “photo” of quantum entanglement.
Image: Moreau et al (Science Advances (2019))

A new quantum mechanics experiment shows the field’s spookiest concept, entanglement, in a whole new way. But, of course, it’s not as simple as that.

Lots of other news outlets have covered what they’ve described as the first-ever image, or even a “photo,” of entanglement, sometimes called spooky action at a distance. The picture does sort of show entanglement, but it’s not the two lobes of the image that are entangled—it’s the photons in this image and other photons elsewhere. Additionally, the experiment doesn’t overcome some of the loopholes that other scientists have required and gone great lengths to overcome, for tests like these. It’s not that the physicists are doing anything wrong; it’s that entanglement is a hard topic to cover in easy-to-understand terms without handwaving. So, let’s try.

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Before we get to the image, you’ll need a crash course in quantum mechanics. Quantum mechanics are the mathematical rules devised to explain the weird behavior of the smallest particles. These particles’ properties, like their energy, are “quantized,” meaning they can only assume values from a set of options. At the same time, particles can take on a superposition of states, meaning they assume several of these values simultaneously when you’re not looking at them. Those values are accompanied by amplitudes, which you might think of as signal strengths. An equation called the “wave function” describes all the possible values with their amplitudes.

When you observe the particles, the wave function collapses and the particle assumes just one of the values. If two of these particles interact and separate, they become entangled, meaning that one wave function now describes them both, with amplitudes assigned to both particles’ values simultaneously. If you set up this entangled wave function and measure each particle repeatedly, the results would be more correlated than you’d expect from the usual rules that govern flipping coins or rolling dice.

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The problem with this correlation is that it holds even if the particles are on other sides of the universe from each other. Unless information somehow travels faster than the speed of light in a vacuum (which it cannot, according to Einstein’s laws), it’s completely unclear how these particles can still share this “spooky” connection. Scientists originally thought that maybe there were some “local hidden variables” that they weren’t taking into account governing the correlations. So they devised a test to see whether these hidden variables might exist.

Modern experiments usually require measuring correlations between pairs of particles, tossing those correlations into a special equation, and seeing if the equation holds true (local hidden variables exist) or breaks (we don’t understand the universe). They require four sub-experiments, in which one of two measurements is picked randomly per each photon in the pair. But no matter what experiment we throw at the particles, though, they continue to demonstrate the same result: The equation breaks, no hidden variables.

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The image at the top of this article supposedly shows a pair of light particles breaking the equation—a hallmark quantum result before our very eyes, according to the paper published this week in Science Advances. The experimental setup generates a pair of entangled photons, then sends the first through a filter and onto a sensor and the second through a different, switchable filter and onto another sensor that turns on the camera that records the first photon. Essentially, the camera only turns on and records a hit if entangled photons hit each sensor at the right time.

This image shows a split ring and is a composite of thousands of runs of the experiment. The orientation of the split is determined by the filter that the photon that turns on the camera passes through, not the photon that the camera is imaging. Though spooky, this isn’t quite entanglement. The researchers determined that there was “entanglement” in the system by performing a calculation on the combined data from the photons that turned on the camera and the photons that the camera actually took a picture of. They found correlations between some of these pairs of photons’ properties that were stronger than classical probability would predict.

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But, I said earlier that there are supposed to be four sub-experiments, two on each photon. That’s one of the issues with this new work, explained John Donohue, manager of scientific outreach from the University of Waterloo’s Institute for Quantum Computing, who was not involved in the study. All of the actual experimenting comes from the filter that turns on the camera, while the “experiment” performed on the first photon is extracted later from the data. So, this experiment doesn’t include the element of randomness required for it to rule out hidden variable theories that experiments like these are meant to disprove.

So, while these results are interesting, they’re not as rigorous as some scientists would like. Additionally, there are plenty of more compelling, albeit indirect, images and graphs of entanglement included in other scientific papers. This is meant to be a photo of entangled particles in the literal sense of the term, even if it’s more like detected light from one of a pair of entangled photons that requires a physics degree to fully wrap your head around. It’s still cool, though!

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“The analysis and technique is interesting, and I think it will lead to more people thinking about how we do this in a less arguable way,” Donohue said. “And that can always lead to something super neat.”