'Quantum Cheshire Cat' becomes reality

Cheshire Cat The Cheshire Cat mysteriously disappeared leaving only his mischievous grin

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Scientists have for the first time separated a particle from one of its physical properties - 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.

They describe their feat in Nature Communications.

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

Start Quote

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!”

End Quote Alice's Adventures in Wonderland

Their technique could have a useful application in metrology - helping to 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 together. A rotating ball, for instance, cannot become separated from its spin.

Cheshire Cat The cat (the neutron) goes via the upper beam path, while its grin (the magnetic moment) goes via the lower

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 its magnetic moment (the strength of its coupling to an external magnetic field).

"We find the cat in one place, and its grin in another," as the researchers once put it.

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 - illustrating 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 was 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 were able to 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.

ILL reactor The high flux neutron source at the ILL made the weak signal of the 'Cheshire Cat' detectable

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.

Their delicate apparatus could have useful applications in high-precision metrology, the researchers say.

"For example, one could imagine a situation in which the magnetic moment of a particle overshadows another of the particle's properties which one wants to measure very precisely.

"The Cheshire Cat effect might lead to a technology which allows one to separate the unwanted magnetic moment to a region where it causes no disturbance to the high-precision measurement of the other property."

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