When facing a fork in the road, neutrons appear to go one direction and their spins in the other
Quantum mechanics has reached Alice in Wonderland levels of weirdness.
An experiment tracking subatomic particles maneuvering past a fork in the road seems to reveal that the particles went one way while one of their intrinsic properties, their spin, went in the other. Although the result conflicts with intuition, it agrees with a decade-old prediction. The phenomenon is called the quantum Cheshire Cat, after the Alice feline whose mischievous grin inexplicably remains after its body has disappeared.
“You can separate basically any property of a particle from the particle itself,” says Jeff Tollaksen, a quantum physicist at Chapman University in Orange, Calif., and coauthor of the Cheshire Cat study, which appears July 29 in Nature Communications. While the result defies explanation for now, he says, it could lead to better understanding of the quantum world as well as improvements in measuring particles’ most subtle properties.
Any physicist trying to decode quantum mechanics has to deal with superposition, a phenomenon in which a neutron or other particle takes multiple paths simultaneously, with a probability of existing along each path at a given time. Physicists can use instruments to locate a neutron, but the act of measuring it destroys the superposition — the neutron is now definitely in one path and there is no way to know where it would have gone if left undisturbed.
In 1988, physicist Yakir Aharonov, now at Chapman and Tel Aviv University in Israel, and colleagues put forward a way to peek behind the superposition curtain. They proposed firing particles toward a detector but placing another instrument along the particles’ path. That instrument would make what’s called a weak measurement — a rough estimate of properties that barely disturbs a particle.
Such a weak measurement could be made by an instrument that exposed the particles to subtle effects, such as a weak magnetic field. An individual weak measurement of a single particle is useless because it is imprecise. But by repeating the measurement on thousands or millions of particles, physicists would learn something about how the particles, on average, behaved en route.
That’s the technique Vienna University of Technology quantum physicist Yuji Hasegawa and his team used to search for the quantum Cheshire Cat, the decoupling of a particle and its properties hypothesized by Aharonov and Tollaksen in 2001.
The researchers steered a beam of neutrons emitted by a nuclear reactor at the Institut Laue-Langevin in Grenoble, France, into a crystal, which divided the beam in two. The team manipulated the beams so that all the neutrons in the upper beam had a particular spin (for instance, +½) and neutrons in the lower beam had the opposite spin (-½). The beams were eventually recombined, and a detector at the end of the crystal counted only neutrons with +½ spin. Intuition suggests that the detected neutrons should have all traveled via the upper beam, and that tinkering with the lower beam would have no effect on the detector’s measurement.
To determine whether intuition is wrong, the researchers performed weak measurements before the particles reached the detector. First the researchers placed a neutron-absorbing metal plate into the paths of each beam, one at a time. As expected, when the plate was in the lower path, it had no effect on particle count at the detector. But the plate reduced the neutron count when placed in the upper path.
Then the researchers exposed each of the beams to a magnetic field just powerful enough to alter the particles’ spins. One would expect that, as with the plate, the field would mess up the detector’s reading only when cast upon the upper beam, where all the neutrons (and presumably their spins) were traveling. Yet the opposite happened: The field had an impact only when exposed to the lower beam, suggesting that neutrons — or at least their spins — were using that thoroughfare.
The researchers concluded that the detected neutrons passed through only the upper path, but their spins traversed only the lower path. In Alice terms, the cat took the upper path and its grin took the lower.
“It’s a very beautiful demonstration,” says Aephraim Steinberg, a quantum physicist at the University of Toronto who was not involved in the study. However, he notes that the study does not prove that any single neutron took a different path than its spin; it shows only that the measured neutrons behaved this way on average. Steinberg is optimistic that this experiment and others using weak measurements will provide insights into quantum behavior of particles — he just isn’t sure what those insights are yet.
Regardless of when physicists make sense of the results, the Cheshire Cat technique could become a useful tool. The researchers suggest that scientists can home in on a hard-to-measure property of a particle by removing the influence of another property. For example, some scientists are probing gravity at microscopic scales, where it is dwarfed by other forces such as electromagnetism (SN Online: 2/26/10). The gravity measurement would become a lot easier, Steinberg suggests, if researchers could isolate particles from their electric charge.
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