Physicists observe ‘quantum boomerang’ for first time ever

SANTA BARBARA, Calif. — Incredible new research from the University of California, Santa Barbara reports the first experimental observation of a “quantum boomerang.” Like the high-flying tool, this phenomenon has particles within a disordered system kicked out of their locations, and yet somehow turn around and return to their original places.

“It’s really a fundamentally quantum mechanical effect,” says study leader atomic physicist David Weld, in a statement. “There’s no classical explanation for this phenomenon.”

Physicist Philip Anderson first predicted this boomerang effect about 60 years ago. At the time, he described it as a disorder-induced behavior called Anderson localization that stops electrons from moving to another place.  The disorder, lead study author Roshan Sajjad explains, can occur due to “imperfections in a material’s atomic lattice, whether they be impurities, defects, misalignments or other disturbances.”

“This type of disorder will keep them from basically dispersing anywhere,” Sajjad explains. The electrons that initially spread out eventually stay in a particular spot instead of “zipping along the lattice.” As a result, the quantum boomerang acts more like an insulator than conducting material.

The reason this phenomenon was poorly understood (until now) is that tracking every electron is downright impossible. However, researchers were quite creative. They used a gas containing 100,000 ultracold lithium atoms suspended in a standing wave of light and then “kicked” it. The “kick” recreated a quantum kicked rotor (“similar to a periodically kicked pendulum,” study authors say) and created the appropriate lattice and disorder to induce a quantum boomerang effect.

This all took place within “momentum space,” which refers to a method evading certain experimental difficulties without altering the underlying physics of the target (the boomerang effect).

“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 explains. “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.”

Boomerang effect a result of ‘central concept of quantum mechanics’

Through their quantum kicked rotor, researchers pulsed the lattice multiple times. They first observed a shift in average momentum but over time repeated “kicks” returned the momentum back to zero.

“It’s just a really very fundamentally different behavior,” Weld explains.

Within a classical system, Weld says, a rotor kicked in this manner would constantly absorb 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,” he continues.

These observations are likely a byproduct of the wave-like nature observed in 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 adds. “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.”

Ultimately, this experiment strongly shows periodic kicks exhibiting time-reversal symmetry lead to the boomerang effect; but random kicks destroy this symmetry, canceling out the boomerang effect.

Moving forward, researchers want to look deeper into the boomerang effects. “There are a lot of theories and questions about what should happen — would interactions destroy the boomerang? Are there interesting many-body effects?” Sajjad concludes. “The other exciting thing is that we can actually use the system to study the boomerang in higher dimensions.”

The study is published in Physical Review X.

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John Anderer

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