Quantum Mechanics Goes Down The Rabbit Hole

Quantum mechanics already hosts a number of strange quirks and baffling behaviours, wave-particle duality and Schrödinger cats to name just a couple. But the latest phenomenon to be observed is more akin to something from Alice in Wonderland.


The quantum Cheshire Cat takes its name from the Lewis Carroll character, The Cheshire Cat, which is capable of separating its grin from its body. This is a good analogy for the phenomenon, where a quantum particle and one its properties, such as its magnetic moment, separate out from each other before reconverging – like the Cheshire Cat and its grin.

Artistic depiction of the quantum Cheshire Cat


Researchers at the Institut Laue-Langevin in Grenoble, France, demonstrated the quantum Cheshire Cat by firing neutrons through a perfect silicon interferometer. Weak measurements were performed on the beam to probe particle location and magnetic moment, the results of which suggested that the neutrons and their magnetic moments travelled along different paths.


Weak measurements had to be taken so as not to disrupt the system in accordance with the Heisenberg uncertainty principle. Due to the limited nature of a single weak measurement, these had to be repeated several times to produce useful results without disturbing the system.


The interferometer produced two particle beams. In order to determine the neutron locations, absorbers were placed into each of the beams and their intensities compared to without the absorbers. Additional magnetic fields on the beams, causing a small spin rotation, allowed the magnetic moment of the beams to be probed.


The findings are not only fascinating, but could lead to breakthroughs in high precision metrology – in that quantum properties could be separated into regions where they cause no disturbance to the measurement of each property.

More information: Nature

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