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Ultracold gases promise access to many-body quantum phenomena at convenientlength and time scales. However, it is unclear whether the entropy of thesegases is low enough to realize many phenomena relevant to condensed matterphysics, such as quantum magnetism. Here we report reliable single-shottemperature measurements of a degenerate $^{87}$Rb gas by imaging the momentumdistribution of thermalized magnons, which are spin excitations of the atomicgas. We record average temperatures as low as$0.022(1)_\text{stat}(2)_\text{sys}$ times the Bose-Einstein condensationtemperature, indicating an entropy per particle, $S/N\approx0.001\, k_B$ atequilibrium, that is well below the critical entropy for antiferromagneticordering of a Bose-Hubbard system. The magnons themselves can reduce thetemperature of the system by absorbing energy during thermalization and byenhancing evaporative cooling, allowing low-entropy gases to be produced withindeep traps.

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