spaceSpace and Physics

“Quantum Cheshire Cat” Phenomenon Observed For The First Time


Stephen Luntz

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer

1639 “Quantum Cheshire Cat” Phenomenon Observed For The First Time
John Tenniel. Only in Wonderland can there be a grin without a cat, and only in quantum physics properties without particles

Quantum mechanics is so weird, people often resort to Alice in Wonderland metaphors to explain it. Particle physicists have now gone one better, actually creating particles modeled on Lewis Carroll’s Cheshire Cat.

The famous puss slowly disappeared leaving its grin behind, prompting Alice to say, "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 Nature Communications this week, a team led by Tobias Denkmayr, a PhD student at the Vienna University of Technology announced something even physicists might call "curiouser and curiouser"; success in separating properties from the particles that normally possess them.


In this case the particles were neutrons. The property was magnetic moment, the extent to which an object is susceptible to rotation by an external magnetic field. Although, as their name suggests, neutrons have no net electric charge, they do have a well established magnetic moments of - 0.97x10-26 JT-1, produced by an internal structure of one up and two down quarks. The negative sign is an indication of the neutrons align in the opposite direction to a magnetic field.

In the classical world we are familiar with the idea that a property like magnetic moment cannot be separated from its object – it would be like taking the taste away from a chocolate bar so that the bar produced no sensation on the tongue, but a disembodied taste could be detected somewhere quite distinct.

However, things work differently in the world of the very small. In the 1990s, Professor Yakir Aharonov of Tel Aviv University proposed the properties could indeed be detached from particles (his book explaining it is delightfully subtitled Quantum Theory for the Perplexed). The idea develops on Schrödinger’s famous feline thought-experiment. However, instead of ending up with a live and dead cat, you have a cat without its properties, and properties without the cat. The naming after Carroll’s Cheshire moggy was inevitable.

A schematic of how Cheshire Cat particles might behave.


The idea of Cheshire cat particles has become a topic for an increasing number of papers in the last few years, but these have generally been about theory – just as no one actually puts Felis catus in boxes with poison vial and radioactive sources, for all the papers discussing what would happen if you did.

Denkmayr and his co-authors, however, temporarily removed the magnetic moment from the neutrons using an interferometer. They used a silicon crystal to split a neutron beam and reported, “The experimental results suggest that the system behaves as if the neutrons go through one beam path, while their magnetic moment travels along the other.” The beams were then reunited, leaving no disembodied magnetic moments prowling the universe.

Denkmayr's experimental set up.

The absence of moment from the neutrons was established by testing their spin in a magnetic field using Aharonov's concept of "weak measurement" which allows observations to take place without disturbing the system as usually occurs in quantum circumstances.


The authors note, “The investigation of Schrödinger cats advanced the field of quantum information processing and communication.” They suggest the work could be useful in “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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