Physicists bring a once-theoretical effect of quantum matter into observable reality

 

Physicists from the University of California, Santa Barbara, have become the first to witness a peculiar quantum behaviour: the "quantum boomerang" effect, which occurs when particles in a disordered system are thrown out of their positions. Instead of landing somewhere else, as one might assume, they circle back to where they started and come to a halt.

"It's a fundamentally quantum mechanical event," said David Weld, an atomic physicist whose lab created the effect and recorded it in an article published in Physical Review X. "This occurrence has no classical explanation."

The boomerang effect is based on a disorder-induced behaviour known as Anderson localization, which inhibits electron transport and was predicted by physicist Philip Anderson around 60 years ago. According to the paper's primary author Roshan Sajjad, the disorder might be caused by impurities, flaws, misalignments, or other disruptions in the atomic lattice of a material.

"This kind of chaos will prevent them from dispersing elsewhere," Sajjad explained. As a result, rather than flying around the lattice, the electrons localise, changing what would otherwise be a conducting material into an insulator. The quantum boomerang effect was anticipated a few years ago to emerge from this rather sticky quantum situation.

It's exceedingly difficult, if not impossible, to launch disordered electrons out from their localised point and follow them to watch their behaviour, but the Weld Lab had a few tricks up its sleeve. The researchers were able to create the lattice and disorder, as well as observe the launch and return of the boomerang, using a gas of 100,000 ultracold lithium atoms suspended in a standing wave of light and "kicking" them, emulating a so-called quantum kicked rotor ("similar to a periodically kicked pendulum," both Weld and Sajjad said). They worked in momentum space, a strategy that avoids some experimental challenges while maintaining the boomerang effect's underlying physics.

"If you're looking for the boomerang effect in normal position space, you'd give your electron a finite velocity and see if it comes back to the same point," Sajjad stated. "We start with a system that has zero average momentum and look for some departure followed by a return to zero average momentum because we're in momentum space."

They pulsed the lattice a few dozen times with their quantum kicked rotor, noticing an early shift in average momentum. Despite repeated kicks, average momentum eventually dropped to zero.

Weld explained, "It's just a fundamentally different behaviour." In a traditional system, a rotor kicked in this way would respond by continuously absorbing energy from the kicks, he explained. "Take a quantum version of the identical item, and you'll notice that it starts acquiring energy for a short period of time before abruptly stopping and never absorbing any more. It enters a state known as dynamically localised."

The wave-like nature of quantum systems, he explained, is responsible for this behaviour.

Weld added, "That lump of stuff you're pushing away is not only a particle, but it's also a wave, and that's a basic concept in quantum physics." "It's subject to interference because of that wave-like nature, and that interference in this system turns out to stabilise a return and dwelling at the origin." The researchers demonstrated that the boomerang effect is produced by periodic kicks with time-reversal symmetry, but that randomly timed kicks disrupt both the symmetry and, as a result, the boomerang effect.

The Weld Lab's next project: If individual boomerang effects are fun, imagine how much more fun it would be to have multiple interacting boomerang effects.

"There are many theories and issues regarding what should happen — would the boomerang be destroyed by interactions? Do there seem to be any fascinating many-body effects?" "Sajjad" stated. "Another fascinating aspect of the device is that we can use it to examine the boomerang in three dimensions."

Reference:

1. Roshan Sajjad, Jeremy L. Tanlimco, Hector Mas, Alec Cao, Eber Nolasco-Martinez, Ethan Q. Simmons, Flávio L. N. Santos, Patrizia Vignolo, Tommaso Macrì, David M. Weld. Observation of the Quantum Boomerang Effect. Physical Review X, 2022; 12 (1) DOI: 10.1103/PhysRevX.12.011035