Universal coarsening in box-trapped Bose gases far from equilibrium

Coarsening of an isolated far-from-equilibrium quantum system is a paradigmatic many-body phenomenon, relevant from subnuclear to cosmological lengthscales, and predicted to feature universal dynamic scaling. It is hypothesised that the associated scaling exponents would allow for the classification of nonequilibrium phenomena into an out-of-equilibrium analogue of equilibrium universality classes. In this talk, I will present our recent observations of universal scaling in the coarsening of isolated homogenous two- and three-dimensional Bose gases.

We start by preparing a degenerate gas in a far-from-equilibrium state, and then observe the relaxation towards an equilibrium condensate. We reveal universal scaling in the experimentally accessible finite-time dynamics by elucidating and accounting for initial-state-dependent prescaling effects. The observed scaling exponents match analytical predictions, and are independent of both the initial state and the strength of interparticle interactions.

Finally, we also investigate the speed of spreading of quantum coherence during relaxation. We show that the spreading of coherence through the system is initially slower for weaker interactions, and faster for stronger ones, but always eventually reaches the same fundamental limit, where the square of the coherence length grows at a universal rate given by the ratio of Planck's constant and the particle mass. Our results provide benchmarks for theories of universality far from equilibrium, are relevant for quantum technologies that rely on large-scale coherence, and invite similar measurements in other quantum systems