The “quantum boomerang” phenomenon, which happens when particles in a chaotic system are thrown out of their positions, was first experimentally observed by physicists at UC Santa Barbara. They spin around and return to where they began, stopping there rather than landing somewhere else as one might anticipate.
“It’s really a fundamentally quantum mechanical effect,” said atomic physicist David Weld, whose lab produced the effect and documented it in a paper published in Physical Review X. “There’s no classical explanation for this phenomenon.”
The boomerang effect is a result of Anderson localization, a disorder-induced behavior that prevents electron transport and was predicted by physicist Philip Anderson over 60 years ago.
The disorder may be caused by flaws, faults, misalignments, or other disruptions in the atomic lattice of a material, according to the paper’s primary author Roshan Sajjad.
“This type of disorder will keep them from basically dispersing anywhere,” Sajjad said.
As a result, what would normally be a conducting material becomes an insulator since the electrons localize as opposed to whizzing over the lattice. The quantum boomerang effect was anticipated to emerge from this somewhat sticky quantum situation a few years ago.
It is highly challenging, if not currently impossible, to launch disordered electrons from their localized position and follow them to examine their behavior, but the Weld Lab has a few tricks up its sleeve.
The researchers were able to create the lattice and the disorder, as well as observe the launch and return of the boomerang, by “kicking” a gas of 100,000 ultracold lithium atoms suspended in a standing wave of light in order to simulate a so-called quantum kicked rotor (similar to a periodically kicked pendulum, according to Weld and Sajjad).
They carried out their study in momentum space, which avoids some practical challenges without altering the fundamental physics of the boomerang effect.
It’s just a really very fundamentally different behavior. 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 localized state.
David Weld
“In normal, position space, if you’re looking for the boomerang effect, you’d give your electron some finite velocity and then look for whether it came back to the same spot,” Sajjad explained. “Because we’re in momentum space, we start with a system that is at zero average momentum, and we look for some departure followed by a return to zero average momentum.”
They pulsed the lattice a few dozen times with their quantum kicked rotor, observing an initial change in average momentum. However, average momentum eventually decreased to zero over time despite numerous kicks.
“It’s just a really very fundamentally different behavior,” Weld said. In a classical system, he explained, a rotor kicked in this way would respond by constantly absorbing energy from the kicks.
“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 localized state.”
This behavior, he said, is due to the wave-like nature of quantum systems.
“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,” Weld explained. “Because of that wave-like nature, it’s subject to interference, and that interference in this system turns out to stabilize a return and dwelling at the origin.”
In their experiment, the researchers demonstrated that the boomerang effect would be produced by periodic kicks with time-reversal symmetry, but that it would be destroyed by randomly timed kicks.
Next up for the Weld Lab: How much more of a party would it be to have several interacting boomerang effects if individual boomerang effects are cool?
“There are a lot of theories and questions about what should happen would interactions destroy the boomerang? Are there interesting many-body effects?” Sajjad said. “The other exciting thing is that we can actually use the system to study the boomerang in higher dimensions.”
Research on this project was also conducted by Jeremy L. Tanlimco, Hector Mas, Eber Nolasco-Martinez and Ethan Q. Simmons at UCSB; Tommaso Macrì at Universidade Federal do Rio Grande do Norte and Patrizia Vignolo at Université Côte d’Azur.
