Quantum physics: the curious case of a neutron and its spin

The weird world of quantum physics has thrown up another strange and paradoxical phenomenon, which scientists call the “quantum Cheshire cat”. It involves separating a particle from one of its physical properties, just as the cat in Alice’s Adventures in Wonderland was separated from its grin.

Researchers from Vienna University of Technology report in the journal Nature Communications that they have achieved this feat for the first time, separating neutrons from their magnetic moment, a fundamental property of subatomic particles. They carried out the experiment – which, like most quantum research, is impossible to explain in simple language – at the Institut Laue-Langevin (ILL), a powerful neutron source near Grenoble in France.

A neutron’s magnetic moment describes the strength of its coupling to an external magnetic field. This has a directional preference, called spin. The researchers used the technique of neutron interferometry, in which a silicon crystal splits a neutron beam into two paths with different spin directions; the upper stream had spin parallel to the particles’ direction of travel, while the spin of the lower stream was in the opposite direction.

By introducing a complex set of quantum filters and detection equipment, the researchers achieved a situation in which one path interacted with the outside world as neutrons without spin and the other path as disembodied spin. “The system behaves as if the particles are spatially separated from their properties,” says Tobias Denkmayr, one of the Vienna scientists.

Of course, there are limits to the Lewis Carroll analogy, as there are to all attempts to make sense of quantum mechanics. “I’ve often seen a cat without a grin,” thought Alice, “but a grin without a cat! It’s the most curious thing I ever saw in all my life.” The ILL experiment does not really achieve a spin without a neutron, but the two appear in different places – which may come to the same thing. Physics is becoming curiouser and curiouser.