“Quantum boomerang” effect demonstrated in reality

A group of physicists have confirmed the existence of the “quantum boomerang” effect.

This effect, which was theoretically predicted a couple of years ago, causes particles under certain conditions to return to their original positions if launched in any direction.

Publishing their findings in Physical Review X, the researchers report that they’ve collected real evidence of this quantum boomerang.

The research centres around a phenomenon called Anderson localisation, in which electrons in a disordered system can’t be transported from one place to another.

“This type of disorder will keep them from basically dispersing anywhere,” says lead author Roshan Sajjad, a researcher at the University of California-Santa Barbara, US.

This disorder means that the material containing it becomes an insulator, and electrons within it – theoretically – should return to their original place if launched out of it.

In practice, this has been difficult to show, because it demands that physicists track the path of an individual electron. But Sajjad and colleagues have managed to demonstrate it.

The researchers examined a gas of 100,000 ultra-cold lithium atoms (for reference, this is about 10-18 grams of material) in a standing wave of light. When the atoms were “kicked” using a method involving a quantum kicked rotor, the researchers were able to observe the average momentum of the system returning to zero over time.

According to the researchers, this means that electrons launched out of their system were returning to their original place, like a boomerang.

Read more: Did scientists really quantum entangle tardigrades?

Associate Professor David Weld, who leads Sajjad’s lab and is a senior author on the paper, says that their observations demonstrate “very fundamentally different behaviour” compared to a system in classical physics.

“Take a quantum version of the same thing, and what you see is that it starts gaining energy at short times, but at some point it just stops and it never absorbs any more energy. It becomes what’s called a dynamically localised state.”

This is because of the wave-like behaviour of quantum particles.

“That chunk of stuff that you’re pushing away is not only a particle, but it’s also a wave, and that’s a central concept of quantum mechanics,” says Weld.

“Because of that wave-like nature, it’s subject to interference, and that interference in this system turns out to stabilise a return and dwelling at the origin.”

The researchers are next investigating whether they can get several quantum boomerangs to interact with each other.

“The other exciting thing is that we can actually use the system to study the boomerang in higher dimensions,” says Sajjad.

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