Synopsis: A Single-Photon Cheshire Cat

Researchers detected the polarization of a photon separate from the photon itself, just as the grin of Lewis Carroll’s Cheshire cat can appear apart from the cat’s body.

In a quantum Cheshire cat, one property of a particle is present detached from the particle itself, just as the grin of the feline in Alice in Wonderland can exist in the absence of the whole animal. Now a research team has successfully separated a single photon from its polarization. The effect was previously demonstrated with neutrons and also with light in a classical scenario, but the latest experiment uses single photons.

To produce the Cheshire cat, Maximilian Schlosshauer and his colleagues at the University of Portland in Oregon used an interferometer where light hits a beam splitter, goes in two, opposite directions around a circuit (but along slightly offset routes), and then recombines at the entrance. The incoming polarization of light was horizontal, but this was switched to vertical for the counterclockwise loop. Each path included a device to adjust the polarization further and another to scatter away a small number of photons of a designated polarization, the equivalent of detecting their presence.

The team fired single photons into the interferometer so that only one photon was present at a time, traversing both paths simultaneously. They placed a horizontal polarizer in front of the output photon detector and gradually varied the difference between the two path lengths. This variation generated a sine-wave interference pattern from the exiting photons only when the polarization was tweaked in the counterclockwise path. At the same time, the average number of photons counted decreased only when light was scattered from the clockwise path. Simultaneously demonstrating these different effects for two routes of a single photon constitutes evidence of the Cheshire cat. The effect might someday improve precision experiments if an unwanted property could be moved away from the property being measured.

This research is published in Physical Review A.

–David Ehrenstein

David Ehrenstein is the Focus Editor for Physics.


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