  Two-atom systems in small traps are of fundamental interest, first of all for
understanding the role of interactions in degenerate cold gases and for the
creation of quantum gates in quantum information processing with single-atom
traps. One of the key quantities is the inelastic relaxation (decay) time when
one of the atoms or both are in a higher hyperfine state. Here we measure this
quantity in a heteronuclear system of $^{87}$Rb and $^{85}$Rb in a micro
optical trap and demonstrate experimentally and theoretically the presence of
both fast and slow relaxation processes, depending on the choice of the initial
hyperfine states. The developed experimental method allows us to single out a
particular relaxation process and, in this sense, our experiment is a
"superclean platform" for collisional physics studies. Our results have also
implications for engineering of quantum states via controlled collisions and
creation of two-qubit quantum gates.
