  We report on controlled doping of an ultracold Rb gas with single neutral Cs
impurity atoms. Elastic two-body collisions lead to a rapid thermalization of
the impurity inside the Rb gas, representing the first realization of an
ultracold gas doped with a precisely known number of impurity atoms interacting
via s-wave collisions. Inelastic interactions are restricted to a single
three-body recombination channel in a highly controlled and pure setting, which
allows to determine the Rb-Rb-Cs three-body loss rate with unprecedented
precision. Our results pave the way for a coherently interacting hybrid system
of individually controllable impurities in a quantum many-body system.
