Weird quantum boomerang predicted 60 years ago spotted for the first time

For the first time, physicists have confirmed a weird quantum phenomenon in which tiny particles, when nudged out of place, will snap right back to where they came from.

The strange behavior, called the quantum boomerang effect, had been predicted for more than 60 years. Now, a new experiment published Feb. 23 in the journal Physical Review X shows that the effect is real: When particles in disordered systems are kicked out of their locations, they will fly away briefly. But, most of the time, instead of landing somewhere else, they will zip right back to their starting positions.

The strange effect cannot be explained by classical, deterministic physics; instead, it is a consequence of the bizarre rules of quantum mechanics. When atoms exist not just as particles but simultaneously as waves, these waves can interfere with each other, adding together in some places and cancelling out in others to cause all kinds of strange behavior we wouldn't expect to see.

Related: 12 stunning quantum physics experiments 

American theoretical physicist Philip Anderson first laid the groundwork for the prediction of the quantum boomerang effect in 1958. In the quantum world, objects behave both as discrete particles and waves at the same time, with the amplitude of these waves in any given region of space being tied to the probability of finding a particle at that location. 

Anderson realized that disorder, or randomness (like the random defects in a material's structure) can make a particle's probability wave cancel itself out everywhere but one tiny region of space. Rooted in place, and unable to move, change states or share energy with its surroundings, the particle becomes localized.  

Anderson concluded that the electrons of a disordered system would become localized and that this would transform a metal from an electrical conductor to an insulator. (In conductors, charged particles are free to move within the material but are fixed in place in an insulator.)

But what would happen to a particle forced from its frozen position by a sudden jolt? In 2019, physicists suggested an answer: Quantum interference effects would force most dislodged localized particles to hastily return to their starting positions.

To demonstrate this effect experimentally for the first time, the researchers suspended a gas composed of 100,000 lithium atoms in a magnetic trap before using a laser to cool them down to within a few nano fractions of a degree of absolute zero, transforming the atoms into a phase of matter called a Bose-Einstein condensate. 

By cooling the gas to near absolute zero (minus 459.67 degrees Fahrenheit, or minus 273.15 degrees Celsius), the scientists made the atoms lose energy and enter the same energy states. Because the researchers could only distinguish between otherwise identical atoms in a gas cloud by looking at energy levels, this equalizing has a profound effect: The once-disparate cloud of vibrating, jiggling, colliding atoms that make up a warmer gas then become, from a quantum mechanical point of view, perfectly identical. 

This opens the door to some truly weird quantum effects. One key rule of quantum behavior, Heisenberg's uncertainty principle, states that it is impossible to simultaneously pinpoint a particle's position and momentum with absolute accuracy. Yet, now that the Bose-Einstein condensate atoms are no longer moving, all of their momentum is known. This leads the atoms' positions to become so uncertain that the places they could possibly occupy grow to be larger in area than the spaces between the atoms themselves.

Instead of discrete atoms, then, the overlapping atoms in the fuzzy Bose-Einstein condensate ball act as one giant particle. This gives some Bose-Einstein condensates the property of superfluidity, allowing their particles to flow without any friction. In fact, if you could stir a mug filled with a superfluid Bose-Einstein condensate, it would never stop swirling.

This means that when the researchers jolt their condensate with a laser beam, "it's a collective kick to all the atoms," lead author Roshan Sajjad, a physicist at the University of California, Santa Barbara, told Live Science. "Because we have condensate, they all act as one wave — a macroscopic matter wave."

All of the researchers' 100,000 atoms acting as one enabled them to easily track the momentum given to their system. After subjecting the atom condensate to a series of 25 laser jolts, the researchers watched as the initial jolts increased the momentum of the atoms in the system, suggesting that they had been briefly shifted from their positions. But adding further jolts didn't keep increasing this momentum. Rather, it brought the average momentum back down to zero; the atoms had boomeranged back to their starting locations.

That behavior would never occur in a classical system; in that case, a constantly jolted pendulum or rotor would continually absorb the energy from each jolt. 

"Classical particles go and make some random walk in a complicated landscape, but if you wait a sufficiently long time, they will go very far," Dominique Delande, a physicist at the French National Research Centre for Scientific Research who worked on a team that predicted the effect in 2019, told Live Science.

That's not the case for a system dominated by quantum effects. In such a system, "each particle will explore some part of the landscape, and because they're also waves, each will carry its own phase," Delande said. "When these waves interfere, it turns out the interference is essentially destructive at long distance." This larger-scale destructive interference of the particles' probability waves is what causes them to snap back to their starting points.

The scientists also confirmed the conditions under which the quantum boomerang would no longer work — when something called time-reversal symmetry is broken.

Time-reversal symmetry is when the physical laws acting on an object are the same going forward in time as they would be going backward. For the quantum boomerang effect to work, time-reversal symmetry must be strictly obeyed, meaning the particles need to be hit by a regularly timed pulse of laser jolts. After the team changed the regular laser kick pattern to an irregular one, the time symmetry was broken, the quantum mechanical rules that enable the effect were violated and the boomerang behavior disappeared.

Now that the researchers have confirmed that the effect is real, they want to test it further by seeing if it's possible for multiple, interacting quantum boomerang effects to take place at once.

"If we can tune the interaction between the atoms, while doing this experiment, it becomes a study of many body effects, which is something we're pretty excited about," Sajjad told Live Science. "We also want to look at higher dimensional effects, kicking it with multiple frequencies and introducing a second or third time dimension."

Originally published on Live Science. 

Adam Mann contributed reporting to this story on March 18, 2022.