A quantum discovery that breaks the rules of heating

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.

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."

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."

Ying also pointed out that recreating this behavior using classical computer simulations is extremely challenging. "That's why we need experiments. They go hand in hand with our theory simulations."

To see how robust this unusual state really was, the researchers altered the experiment by adding randomness to the driving sequence. The effect was immediate. Even a small amount of disorder was enough to destroy the localization.

Once coherence was disrupted, the atoms behaved more conventionally. Their momentum spread out again, kinetic energy increased rapidly, and the system resumed absorbing energy without limit. "This test highlighted that quantum coherence is crucial for preventing thermalization in such driven many-body systems," says Nägerl.

The discovery of MBDL has implications that extend well beyond basic physics. Preventing unwanted heating is one of the biggest challenges facing the development of quantum simulators and quantum computers. These devices rely on maintaining delicate quantum states that can easily be lost through energy buildup and decoherence.

"This experiment provides a precise and highly tunable way for exploring how quantum systems can resist the pull of chaos," says Guo. By showing that heating can be halted entirely under the right conditions, the findings challenge long held assumptions about how driven quantum matter behaves.

The study opens new paths for understanding how quantum systems can remain stable even when pushed far from equilibrium.

The research has been published in Science and received financial support from the Austrian Science Fund FWF, the Austrian Research Promotion Agency FFG, and the European Union, among others.