Looped Trajectories Make Quantum Superposition Even Stranger

Interference patterns for sodium light through wider (top) and narrower double slits. A new triple-slit version has added further complications. Pieter Kuiper/Public Domain

For generations, physics students have had their minds blown by the double-slit experiment – proof of how strange the quantum world can be. Some former students, apparently the type of people who play three-dimensional chess because the normal game is too easy, have come up with something even stranger. They have created a triple-slit experiment where there is a significant probability that photons will follow a path like an s-bend.

Two hundred years ago, Thomas Young showed that when light was directed at two narrow slits, it behaved like an ocean wave passing through two breaks in a sea wall. The light that passes through each slit spreads out and interferes with that from the other slit.

Things got stranger with the discovery of quantum mechanics, which explored what happens when the light is turned down so much that only one photon at a time reaches the slits. The photon continues to behave as if it is interacting with light passing through the other slit, which could be interpreted as it was passing through both slits at once. This is an example of the phenomenon of superposition, where something very small, like a subatomic particle or a photon, exists in two states at once – simultaneously a particle and a wave – or two places at once.

Dr Omar S. Magaña-Loaiza of the University of Rochester, New York, and Dr Israel De Leon of Monterrey Institute of Technology and Higher Education, Mexico, decided that this wasn't crazy enough, so they set out to get photons to pass through one slit before bending around and traveling back through another in what they call a looped trajectory. Physicists had speculated on the existence of looped trajectories before, but the probability of this happening is so low it had never been observed experimentally.

With appropriate charge, a small number of photons can be made to travel an s-bend path (red), creating a superposition with the majority that go straight. Magaña-Loaiza et al/Nature Communications

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