  We review a recent experiment with ultracold atoms in 3D optical lattices
where we have observed a novel kind of bound state of two atoms which is based
on repulsive interactions between the particles. These repulsively bound pairs
exhibit long lifetimes, even under conditions when they collide with one
another. Stable repulsively bound objects should be viewed as a general
phenomenon and their existence will be ubiquitous in cold atoms lattice
physics. Although the experiment described here is based on bosonic Rb atoms,
other composites with fermions or Bose-Fermi mixtures should exist in an
analogous manner. Furthermore, repulsively bound objects could also be formed
with more than two particles. In the following we will first explain the
theoretical background of repulsively atom pairs. Afterwards we will present
the experiments which demonstrate several key properties of the pairs. Finally
we give a short discussion of how these repulsively bound pairs relate to bound
states in some other physical systems.
