Can a Bose gas be saturated?

TL;DR

This paper investigates whether Bose-Einstein condensation can be understood as a saturated vapor by studying ultracold atomic gases with tunable interactions, confirming the saturation picture in the non-interacting limit.

Contribution

It demonstrates experimentally that a Bose gas approaches saturation behavior in the non-interacting limit, challenging the common view of partial condensation in interacting gases.

Findings

Partially condensed gases deviate from saturation at typical conditions.

In the non-interacting limit, behavior aligns with the saturation model.

Results are consistent across different atomic species.

Abstract

Bose-Einstein condensation is unique among phase transitions between different states of matter in the sense that it occurs even in the absence of interactions between particles. In Einstein's textbook picture of an ideal gas, purely statistical arguments set an upper bound on the number of particles occupying the excited states of the system, and condensation is driven by this saturation of the quantum vapour. Dilute ultracold atomic gases are celebrated as a realisation of Bose-Einstein condensation in close to its purely statistical form. Here we scrutinise this point of view using an ultracold gas of potassium (39K) atoms, in which the strength of interactions can be tuned via a Feshbach scattering resonance. We first show that under typical experi-mental conditions a partially condensed atomic gas strongly deviates from the textbook concept of a saturated vapour. We then use measurements at a range of interaction strengths and temperatures to extrapolate to the non-interacting limit, and prove that in this limit the behaviour of a Bose gas is consistent with the saturation picture. Finally, we provide evidence for the universality of our observations through additional measurements with a different atomic species, 87Rb. Our results suggest a new way of characterising condensation phenomena in different physical systems.