A quantum gas that refuses to heat—physicists observe many-body dynamical localization

In everyday life, continuously doing work on a system is found to heat it up. Rubbing your hands together warms them. Hammering a piece of metal makes it hot. Even without knowing the equations, we learn from experience: driving any system, whether by stirring, pressing, or striking, leads to a rise in the system’s temperature.

The same expectation holds for microscopic quantum systems: when we continuously excite a many-particle system, especially one with strong particle-particle interactions, we expect it to absorb energy and to heat up. But is this always the case, in particular at the quantum level?

No, says an experiment carried out by a team from Hanns-Christoph Nägerl’s group at the Department of Experimental Physics of the University of Innsbruck. The research has been published in Science.

Localized in momentum space

The researchers created a one-dimensional quantum fluid of strongly interacting atoms cooled to just a few nanokelvin above absolute zero temperature. They then exposed the atoms to a rapidly and periodically flashed-on lattice potential—a kind of periodically “kicked” landscape made by laser light.

Under such conditions, one would expect the atoms to collectively absorb energy as time progresses, a bit like a couple of children on a trampoline being moved by only one child jumping. Yet the team observed something different. After a brief period of initial evolution, the atoms’ momentum distribution stopped spreading, and the system’s kinetic energy plateaued.

Despite being continually kicked and strongly interacting, the system no longer absorbed energy. It had localized in momentum space, a remarkable phenomenon termed many-body dynamical localization (MBDL).

“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.”

Stability rooted in quantum mechanics

Yanliang Guo, the lead author of the study, is still puzzled: “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, agrees: “This is not to our naive 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 adds that simulating such a seemingly simple system on a classical computer is a daunting task. “That’s why we need experiments. They go hand in hand with our theory simulations.”

Quantum coherence is crucial

To test the fragility of the MBDL phenomenon, the researchers introduced randomness into the driving sequence. Indeed, a rather small amount of disorder was already enough to destroy the localization effect and to restore diffusion: the momentum distribution became smeared out, the kinetic energy rose sharply, and the system absorbed energy continuously.

“This test highlighted that quantum coherence is crucial for preventing thermalization in such driven many-body systems,” says Hanns-Christoph Nägerl.

The findings on MBDL are not just of fundamental interest. Understanding how quantum systems evade thermalization is a key step on the road toward building better quantum devices, including quantum simulators and computers, for which uncontrolled heating and decoherence are major obstacles.

“This experiment provides a precise and highly tunable way for exploring how quantum systems can resist the pull of chaos,” says Guo. The results open a new window into the physics of driven quantum matter, and challenge long-held assumptions.