Ultra-Cold Atoms Defy Entropy and Resist Heating Up

Repeated energization of vast collections of atoms should result in the disruption of their established structures, yet quantum effects appear to resist these changes.

The expected outcome for a physical system is “thermalization,” where everything becomes hot and eventually turns into a puddle of water. Intuitively, one might think that continuously throwing rocks at a sculpture would accelerate this process. Hanns-Christoph Negerl and his team at the University of Innsbruck in Germany conducted experiments that mimic this notion using some of the coldest atoms on Earth, but they observed no heating.

“We anticipated witnessing the opposite,” Negerl shares. The researchers utilized roughly 100,000 cesium atoms, cooling them down to billionths of absolute zero through laser and electromagnetic pulses. At this chilling temperature, atomic behavior becomes entirely quantum. They arranged the atoms in numerous single-layer tubes and employed additional laser pulses to “kick” them repeatedly.

These kicks were intended to provide the atoms with extra energy, which should have resulted in heating and varying speeds. However, team member Yanliang Guo reported that they observed no such changes, regardless of the kick intensity or the adjustments made to the interactions between atoms. The atoms continued to display remarkably similar speeds, behaving as if they were “frozen” within a singular quantum state.

The concept of quantum particles generating heat isn’t new, tracing back to the 1950s. The timing of such occurrences has long been a topic of debate among physicists. Team member Manuele Landini noted that while previous experiments revealed mechanisms for heating atoms, this current investigation may have unveiled novel physics by exploring an alternate range of parameters.

The mathematical framework explaining these phenomena is complex and often contradictory. Adam Ranson from the University of Lille in France commented that calculating whether interacting atoms will heat up is quite challenging, often resulting in researchers simplifying equations to two or three atoms. There exists a theory suggesting that the quantum states of highly interactive atoms can align in a manner that prevents energy absorption, but Ranson believes this picture remains incomplete.

Experiments like those conducted recently act as quantum simulators capable of deeper insights, although Rançon emphasized that further exploration of kick strengths and interactions is still needed.

Robert Connick at Brookhaven National Laboratory in New York has been developing mathematical models relevant to such experiments that project the unusual behavior of atoms. He posits that discovering systems resistant to energy absorption could inspire new developments in quantum technologies, offering a stable quantum state for long-term reliable detection or data storage. “Thermalization poses a significant threat to maintaining quantum effects,” he explains.

Researchers are already planning follow-up experiments to align atoms in thicker tubes, manipulate different tubes, and investigate the possibility of “freezing” their speeds.