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

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, $S\mathrm{/}N\approx 0.001k_B$ 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.