spaceSpace and Physics

Looped Trajectories Make Quantum Superposition Even Stranger


Stephen Luntz

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer

sodium slits

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

In order to bring these trajectories to life, the pair used three slits cut in a sheet of metal with electromagnetic fields confined to the surface. By controlling the local fields, they increased the probability photons would undergo looped trajectories by a factor of almost 100. Although the higher probability was for the photon to take a direct path, the new probability was sufficient to have a noticeable effect in interference patterns produced when light is shone on one of the slits.


Once the number of looped trajectories is no longer negligible, the models of superposition need to change, Magaña-Loaiza and De Leon argue in Nature Communications. The photons on looped trajectories interfere with those traveling straight through, producing complex patterns. To describe these, researchers need to model the superposition of looped and straight trajectories.

The authors describe their work as the first experimental evidence of looped trajectories. In previous observations, the probability of photons taking a looped path was so small it was less than the uncertainty in measurements, so no one could tell if it was happening at all.

The work is a long way from practical applications, but with superposition crucial for developing fields such as quantum computing and teleportation, a better understanding could prove important in unpredictable ways.



spaceSpace and Physics
  • tag
  • quantum mechanics,

  • superposition,

  • three slit experiment,

  • looped trajectories,

  • interference patterns