Commun. Phys. 1, 24 (2018)

How can you best cool a cloud of atoms to the nanokelvin temperature range required by modern experiments? And what happens at the phase transition between a thermal gas and a Bose–Einstein condensate? I-Kang Liu and co-workers have answered these questions using detailed numerical simulations.

As expected, the non-equilibrium nature of the quenching process creates Kibble–Zurek topological defects, but the numerics were able to track the path of individual defects as they annihilated or were ejected from the condensate. This allowed the authors to follow the build-up of phase coherence and show that the number of atoms in the condensate and the coherence growth were decoupled during re-equilibration. They were also able to investigate the quasi-condensate regime — where defects separate regions with different local phase coherence — and its crossover to the true condensate. They showed that the quasi-condensate is enhanced by the long lifetime of the defects and so is very sensitive to the quench rate.