Imagine shaking a snow globe again and again. You’d expect the flakes to scatter endlessly. However, a team of researchers discovered that when they repeatedly jolted some of the coldest atoms in the universe, the atoms refused to spread out.
Instead of heating up and falling apart, these atoms seemed to lock themselves into place, defying one of the most basic expectations in physics that all systems must eventually lose their order and thermalise (spread out their energy evenly until they become featureless).
According to the researchers, this discovery challenges a long-standing assumption about how energy flows in nature and reveals strange forms of quantum behaviour that could one day power novel technologies.
Why didn’t the atoms heat up?
For decades, scientists have struggled with a paradox. When you put many interacting particles together, their jumbled motion normally leads to chaos and heat, making order impossible to maintain.
However, theoretical ideas from as far back as the 1950s suggested that quantum effects could sometimes protect a system from this fate, but proving it in real, many-particle experiments has been nearly impossible. Calculations quickly become unmanageable when more than a few atoms are involved.
Some earlier experiments hinted at temporary slowdowns in heating, but sooner or later the atoms always absorbed energy and lost their coherence. The question remained, could a genuinely interacting many-body quantum system really resist thermalisation at all?
To test this mystery, researchers at Austria’s University of Innsbruck built a remarkably delicate setup. They started with about 100,000 cesium atoms, so cold that their temperature was only a few billionths of a degree above absolute zero. At such extreme cold, atoms stop behaving like classical marbles and instead follow the strange rules of quantum mechanics.
The team confined these atoms inside thousands of microscopic tubes, each just one atom thick, effectively turning the system into a one-dimensional gas. Then came the jolts. Using a pulsed sinusoidal laser potential, the researchers periodically delivered kicks of energy to the atoms, sometimes hundreds of times.
These repeated jolts should have made the atoms heat up, fly apart, and smear into a messy distribution of velocities. However, none of this happened. After some initial evolution, the atoms’ momentum distribution stopped spreading, even after hundreds of kicks.
The momentum eventually froze, and instead of thermalising, the whole collection of particles settled into a stable quantum state, moving with nearly identical velocities as if glued together.
The importance of defeating thermalisation
Normally, heating is the enemy of quantum systems. It wipes out coherence and destroys the very properties researchers hope to harness. “Thermalisation is always the kiss of death to quantum effects,” Robert Konik, an expert in quantum physics at the Brookhaven National Laboratory, told New Scientist. He wasn’t involved in the current research.
Finding ways to prevent this quantum meltdown is essential for technologies like quantum sensors, quantum memories, or even certain kinds of quantum computers.
Moreover, the findings from the current study not only show a way to avoid thermalisation, but also remind us that the quantum world doesn’t always play by classical rules. For instance, systems in the regular world tend to follow the laws of entropy, but this research work shows that ultracold atoms have the power to defy both entropy and chaos.
The Innsbruck team now plans to push further by arranging the cesium atoms into thicker tubes and letting them move between tubes, tests that may reveal whether this frozen behaviour is universal or limited to certain conditions.
The study is published in the journal Science.