‘Cheshire Cat’ becomes reality in Physics Quantum

Scientists have separated a particle from one of its physical properties for the first time – creating a “quantum Cheshire Cat”.

The phenomenon is named after the curious feline in Alice in Wonderland, who vanishes leaving only its grin.

Researchers took a beam of neutrons and separated them from their magnetic moment, like passengers and their baggage at airport security.

Researchers from Vienna University of Technology say the same separation trick could, in principle, be performed with any property of any quantum object.

Their technique could be useful in metrology – helping filter out disturbances during high-precision measurements of quantum systems.

Schrodinger’s paradox In Lewis Carroll’s classic children’s story, the Cheshire Cat gradually disappears, leaving only its mischievous grin.

This prompts Alice to exclaim: “Well! I’ve often seen a cat without a grin, but a grin without a cat! It’s the most curious thing I ever saw in my life!”

In the world familiar to us, an object and its properties are always bound. A rotating ball, for instance, cannot become separated from its spin.

 But quantum theory predicts that a particle (such as a photon or neutron) can become physically separated from one of its properties – such as its polarisation or magnetic moment (the strength of its coupling to an external magnetic field).

“We find the cat in one place, and its grin in another,” the researchers once said.

The feline analogy is a nod to Schrodinger’s Cat, the infamous thought experiment in which a cat in a box is both alive and dead simultaneously. It illustrates a quantum phenomenon known as superposition.

To prove that the Cheshire Cat is not just a cute theory, the researchers used an experimental set-up known as an interferometer, at the Institute Laue-Langevin (ILL) in Grenoble, France.

A neutron beam is passed through a silicon crystal, sending it down two different paths—like passengers and their luggage at airport security.

By applying filters and a technique known as “post-selection”, they could detect the physical separation of the neutrons from their magnetic moment – as measured by the direction of their spin.

“The system behaves as if the neutrons go through one beam path, while their magnetic moment travels along the other,” the researchers reported.

Glimpsing this Cheshire Cat requires what quantum physicists call “weak measurement,” whereby you interact with a system so gently that you avoid collapsing it from a quantum state to a classical one.