  Throughout physics, stable composite objects are usually formed via
attractive forces, which allow the constituents to lower their energy by
binding together. Repulsive forces separate particles in free space. However,
in a structured environment such as a periodic potential and in the absence of
dissipation, stable composite objects can exist even for repulsive
interactions. Here we report on the first observation of such an exotic bound
state, comprised of a pair of ultracold atoms in an optical lattice. Consistent
with our theoretical analysis, these repulsively bound pairs exhibit long
lifetimes, even under collisions with one another. Signatures of the pairs are
also recognised in the characteristic momentum distribution and through
spectroscopic measurements. There is no analogue in traditional condensed
matter systems of such repulsively bound pairs, due to the presence of strong
decay channels. These results exemplify on a new level the strong
correspondence between the optical lattice physics of ultracold bosonic atoms
and the Bose-Hubbard model, a correspondence which is vital for future
applications of these systems to the study of strongly correlated condensed
matter systems and to quantum information.
