In Alice in Wonderland, the Cheshire Cat can disappear leaving only its grin behind. Physicists consider something analogous might exist in quantum mechanics when a particle can be separated from its properties, and the two travel on different paths. This quantum Cheshire cat effect explained that and other weird events like particles swapping disembodied properties. New research claims that what is seen is not what’s actually happening. There is no quantum Cheshire cat effect at all.

In quantum mechanics, observations affect the experiment. This has caused plenty of weirdness, with experiments behaving differently whether they are observed or not. Researchers think that the setup of the quantum Cheshire cat experiment produces the apparently paradoxical effect and that this disappears when a different order of measurements is taken.

"Most people know that quantum mechanics is weird, but identifying what causes this weirdness is still an active area of research. It has been slowly formalized into a notion called contextualize – that quantum systems change depending on what measurements you do on them," lead author Jonte Hance, a research fellow at Hiroshima University and the University of Bristol, said in a statement.

Contextuality is crucial in quantum mechanics and leads to quantum systems that might appear wildly different and even mutually incompatible by doing different measurements on them. The cause of contextuality is unknown and the team set out to investigate using the quantum Cheshire cat. In this way, they found that the measurements were key to producing the effect.

"We want to correct this by showing that different results are obtained if a quantum system is measured in different ways and that the original interpretation of the quantum Cheshire cat only comes about if you combine the results of these different measurements in a very specific way, and ignore this measurement-related change," said Holger Hofmann, a professor at Hiroshima University.

The team wants to investigate other effects that appear paradoxical and see if they are manifestations of contextuality. And from there maybe even understand how contextuality emerges in the first place.

"This will not only help us finally explain why quantum mechanics is so counterintuitive but will also help us develop ways to use this weirdness for practical purposes. Given that contextuality is inherently linked to scenarios where there is a quantum advantage over classical solutions to a given problem, only by understanding contextuality will we be able to realize the full potential of, for instance, quantum computing," said Hance.

A paper discussing this research is published in the New Journal of Physics.