Some
quantum particles gotta get right back to where they started from.
Physicists
have confirmed a theoretically predicted phenomenon called the quantum
boomerang effect. An experiment reveals that, after being given a nudge,
particles in certain materials return to their starting points, on average, researchers
report in a paper accepted in Physical Review X.
Particles
can boomerang if they’re in a material that has lots of disorder. Instead of a
pristine material made up of orderly arranged atoms, the material must have
many defects, such as atoms that are missing or misaligned, or other types of
atoms sprinkled throughout.
In 1958,
physicist Philip Anderson realized that with enough disorder, electrons in a
material become localized: They get stuck in place, unable to travel very far
from where they started. The pinned-down electrons prevent the material from
conducting electricity, thereby turning what might otherwise be a metal into an
insulator. That localization is also necessary for the boomerang effect.
To picture
the boomerang in action, physicist David Weld of the University of California,
Santa Barbara imagines shrinking himself down and slipping inside a disordered
material. If he tries to fling away an electron, he says, “it will not only
turn around and come straight back to me, it’ll come right back to me and
stop.” (Actually, he says, in this sense the electron is “more like a dog than
a boomerang.” The boomerang will keep going past you if you don’t catch it, but
a well-trained dog will sit by your side.)
Weld and
colleagues demonstrated this effect using ultracold lithium atoms as stand-ins
for the electrons. Instead of looking for atoms returning to their original
position, the team studied the analogous situation for momentum, because that
was relatively straightforward to create in the lab. The atoms were initially
stationary, but after being given kicks from lasers to give them momenta, the
atoms returned, on average, to their original standstill states, making a
momentum boomerang.
The team
also determined what’s needed to break the boomerang. To work, the boomerang
effect requires time-reversal symmetry, meaning that the particles should
behave the same when time runs forward as they would on rewind. By changing the
timing of the first kick from the lasers so that the kicking pattern was
off-kilter, the researchers broke time-reversal symmetry, and the boomerang
effect disappeared, as predicted.
“I was so
happy,” says Patrizia Vignolo, a coauthor of the study. “It was perfect agreement”
with their theoretical calculations, says Vignolo, a theoretical physicist at
Université Côte d’Azur based in Valbonne, France.
Even
though Anderson made his discovery about localized particles more than 60 years
ago, the quantum boomerang effect is a recent newcomer to physics. “Nobody
thought about it, apparently, probably because it’s very counterintuitive,”
says physicist Dominique Delande of CNRS and Kastler Brossel Laboratory in
Paris, who predicted the effect with colleagues in 2019.
The weird
effect is the result of quantum physics. Quantum particles act like waves, with
ripples that can add and subtract in complicated ways (SN: 5/3/19). Those waves
combine to enhance the trajectory that returns a particle to its origin and
cancel out paths that go off in other directions. “This is a pure quantum
effect,” Delande says, “so it has no equivalent in classical physics.”
Reference:
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