‘Quantum Cheshire Cat’ becomes reality

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

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!” The idea of a “quantum Cheshire Cat” was first proposed in 2010 by Dr Jeff Tollaksen from Chapman University, a co-author on this latest paper. 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. 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). Edited from and more here ‘Quantum Cheshire Cat’ becomes reality.

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