Boffins spot weirder quantum capers as neutrons take the high road, spin takes the low

Cheshire cat effect see neutrons and their properties walk different paths

The quantum cheshire cat

Imagine this: when Australian cricketer Shane Warne bowled “the ball of the century”, a delivery that drifted one way, then hit the pitch and spun the other, the reason batter Mike Gatting was bamboozled was because the spin took a different path from the ball. That's the phenomenon boffins claim they've observed in experiments involving neutrons and an interferometer.

Before you call Vulture South's boss to have this writer relieved of his tasks or his Tequila, read at least the abstract of their paper at Arxiv, now accepted and published at Nature.

The phenomenon is being described as the “Quantum Cheshire cat”, because ever since Schroedinger proposed his thought experiment, quantum physics has been as edin love with cats as Facebook, but I digress.

The experiment is designed as a test of an idea originally proposed in 2001 by professors Yakir Aharonov and Jeff Tollaksen.

A little background is probably in order; I'll try to keep it short. In the classic “double slit” experiment, light behaves like a wave when passed through two slits (producing interference patterns), but passed through a single slit, it behaves like a particle.

The quantum cheshire cat

Bad artist's impression: the neutron (cat) takes one path, the spin (grin) the other. Image from href=""Arxiv

One of the bits of quantum oddness that the world's tested to death is this: confronted with a double slit, even a single photon will exhibit wave-like behaviour and pass through both slits. In other words, the photon approaching two slits will exist as a superposition, passing through both at once.

Neutrons, by way of a process called neutron interferometry, can be observed exhibiting the same behaviour, existing as a superposition on two alternative paths. And, according to the research team behind the new paper, using neutrons in quantum experiments is attractive because “the use of non-zero mass particles is most appealing, since no classical description is possible”.

What Aharonov and Tollaksen proposed is that a quantum and its property might behave the same way. In other words, a neutron and a property of the neutron, in this case its spin, might be persuaded to take two different paths and recombine at the destination. That's what they've described as the “Cheshire cat” experiment, the Lewis Carroll conceit that Alice had often seen a cat without a grin, but never “a grin without a cat”.

Or, for that matter, a spin without a ball.

(Stick with this. While I won't offer you a guinea pig eating a burrito at the end, like John Oliver, I'll do my best to ease your headache.)

In the case of the neutron, the spin is a magnetic property – the magnetic moment, which can be influenced by external magnetic fields.

And now, we get to the experiment itself. The basic steps are as follows:

  • A neutron beam is generated and split into two parts by a neutron interferometer;
  • Polarised spin is applied to each beam. In the upper beam, spin is in the direction of motion; in the lower, spin is in the opposite direction.
  • The beams are recombined, but only the “upper beam” neutrons are retained; the others are discarded post-recombination.

So how where does the “Cheshire cat” effect come from?

The researchers applied magnetic detection to the split beams.

When the two beams are recombined, they can either amplify or cancel each other (the interference pattern that is produced in light in a double-slit experiment). That effect can be adjusted by the magnets – but it only works if the magnet is applied to the lower beam, not the upper.

However: the “lower path” neutrons were the ones discarded in the experiment. In other words, the system behaves as if the spin of the upper beam sometimes travelled on the lower path.

“Along one of the paths, the particles themselves couple to our measurement device, but only the other path is sensitive to magnetic spin coupling,” researcher Tobias Denkmayr explains. “The system behaves as if the particles were spatially separated from their properties.”

“The system behaves as if” is the key phrase here. If you looked at individual neutrons, the waveform collapses – what you'd see is a neutron on Beam A with the spin you expect, or a neutron on Beam B, also with the spin you expect.

But in the probabilistic world of quantum physics, it seems, you can have a ball without spin, or a spin without ball.

As your treat, Shane Warne shows what happens if the spin and the ball take a different path below. ®

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