A quantum discovery that breaks the rules of heating
When scientists repeatedly drove a strongly interacting quantum system with laser “kicks,” they expected it to heat up and grow chaotic. Instead, the atoms abruptly stopped absorbing energy and locked into a stable pattern of motion. This strange effect arises from quantum coherence, which prevents the system from thermalizing despite constant forcing. The results overturn classical intuition and offer new insight into how quantum systems can resist disorder.
In everyday experience, applying repeated force almost always leads to heating. Rubbing your hands together warms your skin. Striking metal with a hammer makes it hot to the touch. Even without formal physics training, people quickly learn a basic rule: when you keep driving a system by stirring it, pressing it, or hitting it, its temperature rises.
Physicists expect the same behavior at much smaller scales. In quantum systems made up of many interacting particles, continuous excitation is normally assumed to cause steady energy absorption. As energy builds up, the system should heat. But a recent experiment suggests that this intuition does not always apply at the quantum level.
Researchers from Hanns Christoph Nägerl's group at the Department of Experimental Physics at the University of Innsbruck set out to test whether a strongly driven quantum system must inevitably heat up. Their answer was unexpected.
A Quantum Gas That Stops Absorbing Energy
The team created a one dimensional quantum fluid made of strongly interacting atoms cooled to just a few nanokelvin above absolute zero. Using laser light, they subjected the atoms to a lattice potential that switched on and off rapidly and repeatedly. This setup created a regularly pulsed environment that effectively kicked the atoms over and over again.
Under these conditions, the atoms should have absorbed energy continuously, similar to how motion builds on a trampoline when someone keeps jumping. Instead, the researchers saw a surprising change. After a short initial period, the spread of the atoms' momentum came to a halt. The system's kinetic energy stopped increasing and leveled off.
Even though the atoms were still being driven and continued to interact strongly with one another, they no longer absorbed energy. The system had entered a state known as many body dynamical localization (MBDL). In this state, motion becomes locked in momentum space rather than spreading freely.
"In this state, quantum coherence and many-body entanglement prevent the system from thermalizing and from showing diffusive behavior, even under sustained external driving," explains Hanns Christoph Nägerl. "The momentum distribution essentially freezes and retains whatever structure it has."
An Orderly Outcome That Defied Expectations
The result surprised even the scientists involved. Lead author Yanliang Guo admitted the behavior ran counter to what they had predicted. "We had initially expected that the atoms would start flying all around. Instead, they behaved in an amazingly orderly manner."
Lei Ying, a theory collaborator from Zhejing University in Hangzhou, China, shared that reaction. "This is not to our naïve expectation. What's striking is the fact that in a strongly driven and strongly interacting system, many-body coherence can evidently halt energy absorption. This goes against our classical intuition and reveals a remarkable stability rooted in quantum mechanics."