Experimental demonstration of cooling of trapped ions by atoms of higher mass

Buffer gas cooling is one of the most widespread methods to cool ions trapped in a Paul trap. The method relies on elastic collisions of the ions with pre-cooled atoms. However, cooling by a uniformly distributed buffer gas has a limitation -- the ion cooling occurs only if the coolant atoms are lower in mass compared to the trapped ion. We surpass this limitation by using a localized ensemble of ultracold atoms. We experimentally demonstrate, for the first time, that cooling of low-mass trapped ions by atoms of higher mass is possible. In particular, we show that trapped $^{\mathrm{39}}$K$^{\mathrm{+}}$ ions are cooled by magneto-optically trapped ultracold $^{\mathrm{85}}$Rb atoms. The atom:ion mass ratio ($=$ 2.18) is well beyond any theoretical predictions for uniform buffer gas cooling. The result opens up the possibility of cooling trapped H$_{\mathrm{2}}^{\mathrm{+}}$ using ultracold $^{\mathrm{6}}$Li atoms.