Ultracold atoms reveal universal rules far from equilibrium

Recent research has revealed that a two-dimensional homogeneous Bose gas, once driven far from equilibrium, displays universal behaviour independent of its starting conditions. This discovery offers new insights into how different complex systems such as ultracold atomic gases and quark-gluon plasmas can evolve according to shared, simple rules.

One of the landmark achievements of twentieth century physics was the realisation that many different materials in thermal equilibrium can be understood in a systematic way, based on the fact that near phase transitions-such as when water turns to ice-they display fundamentally the same behaviour. This is an example of the principle of universality, which says that in certain situations microscopic details become unimportant and are replaced by a simpler set of rules.

In a recent experimental study, the researchers at the Cavendish Lab have demonstrated such universality in the far-from-equilibrium dynamics of a quantum system. In particular, they studied the emergence of order in an ultracold two-dimensional Bose gas—a model system for studying quantum matter at macroscopic scales. They drove the gas far from equilibrium, then allowed it to evolve in isolation. Over time, the microscopic excitations reorganised themselves, revealing a striking pattern: no matter the starting conditions, the system eventually always followed the same universal trajectory.

“Our study is the first observation of universal ordering dynamics in a two-dimensional Bose gas and provides support for theories seeking to unify far-from-equilibrium dynamics across fields as diverse as early-universe cosmology and high-energy physics,” explained Martin Gazo, first author of the paper and PhD student in the Hadzibabic Group (Quantum Gases and Collective Phenomena) at the Cavendish Laboratory.

“It demonstrates that the complex dynamics of quantum matter far from equilibrium can be captured by a simple set of scaling exponents, similar to the universality in equilibrium phase transitions. This allows scientists to understand very different systems, from ultracold atoms to the quark-gluon plasma and the early universe, within the same framework.”

The key to revealing the universal dynamics was to understand how to separate it from the initial system-specific evolution. This insight and the tools developed here will be applicable for any further studies of universal nonequilibrium dynamics in ultracold atoms.

Looking ahead, the researchers at the Cavendish plan to look for universal behaviour in actively driven far-from-equilibrium systems. Each step brings them closer to a unified understanding of the universal laws that are thought to have governed the birth of the universe and can now be studied in the laboratory.

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