A theoretically predicted phenomenon known as the quantum boomerang effect has been confirmed by physicists. An experiment divulges that, on average, particles in particular materials come back to their starting points after being nudged, researchers describe in a paper received for publication in Physical Review X.
Particles can boomerang if they are in a disordered material. Instead of an intact material consisting of orderly arranged atoms, the material must contain many defects, such as missing or misaligned atoms, or other types of atoms scattered throughout.
In 1958, physicist Philip Anderson discovered that when there is sufficient disorder, electrons in a material are localized: they become stuck in place, without the ability to travel farther from where they began. The pinned-down electrons restrict the material from conducting electricity, converting it from a metal to an insulator. Such localization is also required for the boomerang effect.
For visualizing the boomerang in action, the University of California, Santa Barbara physicist David Weld imagines diminishing himself down and gliding inside a disordered material. He states that when he attempts to throw away an electron, it will not only take a U-turn and come right back to him, but it will also come straight back to him and stop. (Actually, he means that the electron is more similar to a dog than a boomerang in this regard.) If you don’t catch the boomerang, it will continue to fly past you, but a well-trained dog will be sitting by your side.)
Weld and colleagues illustrated this effect by substituting ultracold lithium atoms for electrons. Rather than looking for atoms coming back to their original positions, the team investigated the analogous situation for momentum, which was relatively easy to be created in the lab. The atoms remained stationary at first, but after being given laser kicks for providing them momenta, the atoms came back to their original standstill states on average, creating a momentum boomerang.
The team also figured out how to break the boomerang. Time-reversal symmetry is required for the boomerang to work, which means that the particles should act the same when time runs forward as they do when time is reversed. The researchers busted time-reversal symmetry by modifying the timing of the first kick from the lasers, causing the kicking pattern to be off-kilter, and the boomerang effect to disappear, as predicted.
I was overjoyed, says study coauthor Patrizia Vignolo. According to Vignolo, a theoretical physicist at Université Côte d’Azur in Valbonne, France, it was in perfect accordance with their theoretical calculations.
Even though Anderson discovered localized particles more than 60 years before, the quantum boomerang effect is a relative newcomer to physics. It appears that no one considered it, probably because it’s very counterintuitive, says physicist Dominique Delande -CNRS and Kastler Brossel Laboratory in Paris, who forecasted the effect with colleagues in 2019.
The strange effect is caused by quantum physics. Quantum particles behave similarly to waves, with ripples that can add and subtract in complex ways (SN: 5/3/19). These waves combine for improving the orbit that returns a particle to its origin and neutralizes other paths going in other directions. Delande explains that since this is a pure quantum effect, it does not contain any equivalent in classical physics.
