Thermometry and cooling of a Bose-Einstein condensate to 0.02 times the critical temperature

TL;DR

This paper demonstrates precise temperature measurement of a Bose-Einstein condensate at extremely low temperatures using magnon imaging, revealing potential for studying quantum phenomena with very low entropy.

Contribution

It introduces a novel single-shot thermometry method based on magnon imaging and shows how magnons can be used to further cool the atomic gas.

Findings

Achieved temperatures as low as 0.022 times the critical temperature.

Measured entropy per particle below the threshold for antiferromagnetic order.

Magnons can facilitate additional cooling of the system.

Abstract

Ultracold gases promise access to many-body quantum phenomena at convenient length and time scales. However, it is unclear whether the entropy of these gases is low enough to realize many phenomena relevant to condensed matter physics, such as quantum magnetism. Here we report reliable single-shot temperature measurements of a degenerate $^{87}$Rb gas by imaging the momentum distribution of thermalized magnons, which are spin excitations of the atomic gas. We record average temperatures as low as $0.022(1)_\text{stat}(2)_\text{sys}$ times the Bose-Einstein condensation temperature, indicating an entropy per particle, $S/N\approx0.001\, k_B$ at equilibrium, that is well below the critical entropy for antiferromagnetic ordering of a Bose-Hubbard system. The magnons themselves can reduce the temperature of the system by absorbing energy during thermalization and by enhancing evaporative cooling, allowing low-entropy gases to be produced within deep traps.