  The two most widely used ion cooling methods are laser cooling and
sympathetic cooling by elastic collisions (ECs). Here we demonstrate another
method of cooling ions that is based on resonant charge exchange (RCE) between
the trapped ion and the ultracold parent atom. Specifically, trapped Cs$^{+}$
ions are cooled by collisions with co-trapped, ultracold Cs atoms and,
separately, by collisions with co-trapped, ultracold Rb atoms. We observe that
the cooling of Cs$^{+}$ ions by Cs atoms is more efficient than cooling of
Cs$^{+}$ ions by Rb atoms. This signals the presence of a cooling mechanism
apart from the elastic ion-atom collision channel for the Cs-Cs$^{+}$ case,
which is cooling by RCE. The efficiency of cooling by RCE is experimentally
determined and the per-collision cooling is found to be two orders of magnitude
higher than cooling by EC. The result provides the experimental basis for
future studies on charge transport by electron hopping in atom-ion hybrid
systems.
