Curiouser and curiouser as the Quantum Cheshire Cat appears

In the weird world of quantum mechanics, scientists have managed to separate a particle from its properties. Cathal O'Connell looks into the paradox.

Lewis Carroll’s Cheshire Cat turns out to be a perfect way to describe what physicists have just uncovered in the quantum world – Shutterstock

Well! 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!”

Lewis Carroll’s Cheshire Cat, which vanishes to leave only its smile, is a weird notion but it turns out to be a perfect way to describe what physicists have just uncovered in the quantum world.

New research performed at the Laue-Langevin Institute in Grenoble has shown that matter can be separated from its properties. The matter in question was a collection of neutrons, particles found at the centre of every atom. The property was their direction of spin in a magnetic field. Physicists once considered these things as inseparable as a cat and its grin. No more.

In the study, published in Nature Communications last month, the researchers sent a beam of neutrons one way and their magnetic spin, seemingly, went another. The phenomenon, which was actually predicted by theorists, has been dubbed the “Quantum Cheshire Cat”.

The mind-bending findings could give us a new way to study the properties of subatomic particles. And this may just be the beginning of a major new assault on our notion of reality.

Until recently studying quantum particles was limited by the Heisenberg uncertainty principle – the mere act of measuring particles’ properties affects those properties. But now, by probing particles much more gently than Heisenberg envisioned, physicists appear to have found a way to sneak around the uncertainty barrier. The new technique opens the lid on a whole new class of quantum weirdness and the Quantum Cheshire Cat is just the latest example. According to Howard Wiseman, Director of the Centre for Quantum Dynamics at Griffith University, the new measurements “reveal things about quantum systems that you can't otherwise get at.”

Quantum mechanics, our theory of physical phenomena in the nanoscopic world, is probably the most successful theory in the history of physics. No experiment has ever disagreed with its predictions. Since its formulation at the beginning of the 20th century it has led to many of the defining technologies of the modern age, including the computer chip. But it’s also just plain weird. In the quantum world objects can be in two places at once, things pop in and out of existence from nothingness, and Schrödinger’s cat can be simultaneously alive and dead.

But even by quantum standards the latest discovery is very strange. The experiment, performed by Tobias Denkmayr and colleagues from Vienna Technical University, began inside the core of the High-Flux nuclear reactor which generates one of the most powerful neutron beams in the world. Denkmayr and his team fired the neutron beam at a silicon crystal which, as a prism does to light, split the beam into upper and lower streams. The neutrons that travelled in the upper stream were given a spin of “up” while those in the lower path were given a “down” spin. The split streams were then funnelled into a single beam whose properties were measured by a detector.

Until recently physicists had their hands tied when it came to
measuring the properties of quantum particles.

When the experimenters partially blocked off the upper stream fewer “up” spin neutrons made it to the detector but when they blocked the lower one, the “up” spin signal was unaffected – all as expected.

Then the team repeated the experiment. But this time they gently probed the spin of the neutrons midstream before they hit the detector, using a magnetic field. Amazingly the “up” spin was detected not along the upper path where the particles were known to be, but along the lower beam.

The upper path neutrons had been separated from their spin properties. It’s like a magnet making iron filings line up on a sheet of paper as if it was directly under the paper, when in fact it is in the next room.

The result was a total contradiction – but not unexpected. Jeff Tollaksen of Chapman University in California, who collaborated on the experiment, predicted the phenomenon and named it “The Quantum Cheshire Cat hypothesis” in his 2001 PhD thesis.

Denkmayr was able to prove the hypothesis because the midstream magnetic field measurement used what’s called “weak measurement”. Until recently physicists had their hands tied when it came to measuring the properties of quantum particles because (as per Heisenberg) the very act of observing them disturbs them – indeed they are particularly skittish around particle detectors.

The magnetic probe on the other hand, a so-called “weak measurement”, seems to be like the equivalent of a bird-watcher’s hide, enabling physicists to collect data without disturbing the particles.

First proposed by Yakir Aharonov, now at Chapman University in California, and others in 1988, it ekes out more information from quantum systems than physicists ever thought possible. This is not the first time weak measurements have been used to probe quantum systems but the Cheshire Cat is certainly the strangest quantum phenomenon it has uncovered.

And, according to theory, the effect should not be limited to magnetic spin. “The effect is completely general for every quantum system,” Denkmayr says. “You could take an electron and separate it from its charge.” This could lead to a new type of instrument that might be used to split apart and measure the different properties of fundamental particles, much like a prism separates out the colours of light.

“It's certainly a surprising result,” says Wiseman. “If you had asked experts in quantum physics if it was likely most would probably have said, 'No, that sounds crazy’.”

But for Tollaksen this experiment vindicated a quest to peer over the Heisenberg barrier and reveal the true nature of the quantum world.

“The grandeur of the universe is really beyond anything we can imagine at this point.”

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