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

Date:
February 28, 2022
Source:
University of California - Santa Barbara
Summary:
Physicists have experimentally observed a quirky behavior of
the quantum world: a 'quantum boomerang' effect that occurs
when particles in a disordered system are kicked out of their
locations. Instead of landing elsewhere as one might expect,
they turn around and come back to where they started and stop there.



FULL STORY ========================================================================== Physicists at UC Santa Barbara have become the first to experimentally
observe a quirky behavior of the quantum world: a "quantum boomerang"
effect that occurs when particles in a disordered system are kicked out
of their locations.

Instead of landing elsewhere as one might expect, they turn around and
come back to where they started and stop there.


========================================================================== "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 has its roots
in a phenomenon that physicist Philip Anderson predicted roughly 60
years ago, a disorder-induced behavior called Anderson localization
which inhibits transport of electrons. The disorder, according to the
paper's lead author Roshan Sajjad, can be the result of 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 said. As a result, the electrons localize instead of zipping along
the lattice, turning what would otherwise be a conducting material into
an insulator. From this rather sticky quantum condition, the quantum
boomerang effect was predicted a few years ago to arise.

Launching disordered electrons away from their localized position
and following them to observe their behavior is extremely difficult
if not currently impossible, but the Weld Lab had a few tricks up its
sleeve. 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), the researchers were able to create the lattice and the
disorder, and observe the launch and return of the boomerang. They worked
in momentum space, a method that evades some experimental difficulties
without changing the underlying physics of 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 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."
Using their quantum kicked rotor they pulsed the lattice a few dozen
times, noting an initial shift in average momentum. Over time and despite repeated kicks, however, average momentum returned to zero.



========================================================================== "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 showed that periodic kicks exhibiting time-reversal
symmetry would produce the boomerang effect, but randomly timed kicks
would destroy both the symmetry and, as a result, the boomerang effect.

Up next for the Weld Lab: If individual boomerang effects are cool, how
much more of a party would it be to have several interacting 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 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 Macri`
at Universidade Federal do Rio Grande do Norte and Patrizia Vignolo at Universite' Co^te d'Azur.

========================================================================== Story Source: Materials provided by
University_of_California_-_Santa_Barbara. Original written by Sonia
Fernandez. Note: Content may be edited for style and length.


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

Link to news story: https://www.sciencedaily.com/releases/2022/02/220228150641.htm

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