Physicists Discover a Quantum System That Refuses To Heat Up

Despite being continually kicked and strongly interacting, the atoms no longer absorb energy. The system localizes in momentum space, the momentum distribution literally freezes, a remarkable phenomenon termed many-body dynamical localization (MBDL). Credit: Universität Innsbruck An experiment with ultracold atoms reveals that a strongly driven quantum system doesn’t always heat up as expected. In daily life, doing work on something over and over usually makes it warmer. You can feel it when you rub your hands together, and you can see it when metal heats up under repeated hammer blows. Even without formal equations, experience teaches the same lesson: if you keep pushing, stirring, pressing, or striking a system, its temperature tends to rise. Physicists generally expect a similar outcome in the quantum world. When a many particle system is repeatedly driven, especially when the particles interact strongly with one another, it should take in energy and gradually heat up. But does that rule always hold, particularly for quantum matter? A recent experiment from Hanns-Christoph Nägerl’s group at the Department of Experimental Physics of the University of Innsbruck suggests the answer is no. Localized in momentum space In the study, the team prepared a one dimensional quantum fluid made of strongly interacting atoms, cooled to only a few nanokelvin above absolute zero. They then subjected the atoms to a lattice potential that switched on in rapid, regular bursts, a periodically “kicked” landscape created with laser light. Under this kind of steady driving, the atoms would normally be expected to absorb energy together over time, similar to how two children on a trampoline might be jostled by the repeated jumping of a single child. 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 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 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 towards 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. Reference: “Observation of many-body dynamical localization” by Yanliang Guo, Sudipta Dhar, Ang Yang, Zekai Chen, Hepeng Yao, Milena Horvath, Lei Ying, Manuele Landini and Hanns-Christoph Nägerl, 14 August 2025, Science. DOI: 10.1126/science.adn8625 The research was financially supported by the Austrian Science Fund FWF, the Austrian Research Promotion Agency FFG, and the European Union, among others.