This seems like a normal, if blurry photograph of an object — a regular old cat stencil. The catch is, however, that the photons that hit the camera lens couldn't and didn't interact with the stencil in any way. So why can you see it?

In Vienna, a team of scientists led by Gabriela Barreto Lemos spent a good long time multiplying photons. First they created an experimental set-up in which a photon from a laser could take one of two paths; as long as they didn't try to detect which path the photon took, the photon went simultaneously down both paths, making two virtual photons.

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Next, both paths contained a crystal. Each crystal turned the photon that traveled down its path into a pair of entangled photons. When two photons are entangled, a change that one photon undergoes will be reflected in the second photon. One spin changes, and so does the other. The scientists still did not measure which path the original photon went on. So the one photon "split" in two, and went down each path, where it was turned into four photons in two entangled pairs.

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Here's where it gets tricky. On one of the paths, there was an object. In this case, it was a cat stencil whichwas invisible to red light. The entangled pairs of photons were red and yellow.

The yellow and red pair that passed through the cat stencil recombined with the yellow and red pair that did not pass through the stencil. The red photons were directed to the camera, but the yellow photons were not. And on the camera appeared an image of a cat in red light, despite the fact that the single photon of the four "travelers" that could actually interact with the stencil didn't make it to the camera.

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So quantum entanglement gives us a picture made of photons that could never have interacted with the object the picture represents. Just FYI.

[Via Quantum Imaging With Undetected Photons, Entangled Photons Make a Picture From a Paradox]

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