Exotic atom pairs: Repulsively bound states in an optical lattice

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.