We have observed a novel kind of stable bound state. Two atoms in an optical lattice can be combined to form a pair by repelling each other! This pair is stable and behaves and looks almost like a normal diatomic molecule—but it is not! It’s binding energy has the ‘wrong’ sign.
What is strange about that?
In free space
repulsive pairs cannot exist. If you bring two objects together which
repel each other they will just accelerate away from each other.
In free space two objects fly apart when they repel each other.
In the special environment of the optical lattice this is no longer possible because the kinetic energy of the atoms is restricted to certain ranges, known as "Bloch Bands". In order for the atoms to separate, they have to get rid of the potential energy from the repulsive interaction. Since this cannot be transferred to kinetic energy, and the system is essentially free from dissipation of energy by other means, the atoms have no choice but to stay together. The counterintuitive situation arises that the binding of the atoms gets stronger with increasing repulsion between them! For our experiments we use a Rubidium Bose-Einstein condensate (BEC) which we load in an optical lattice.
Repulsively bound pairs have a very unusual quantum mechanical wavefunction, which we can experimentally study in quasi-momentum space.
The wavefunction of the repulsive pairs in momentum space.
Relevance for science and technology
Our findings are also relevant for current research
on how to build a quantum computer, and especially in how to use atoms
in optical lattices to model very complicated systems from solid state
physics. For example, atoms in an optical lattice can be made to
behave like electrons in the lattice structure of solid state
materials. In the future, these systems could be used as a quantum
simulator, to model systems such as high temperature superconductors,
and other more exotic materials.
The research teams
from left to right: P. Zoller, R. Grimm, A. J. Daley, G. Thalhammer, J. Hecker Denschlag, F. Lang, K. Winkler & A. Kantian
not in picture: H. P. Büchler
The experimental physics team,
For more picture material, click here!
 Repulsively bound
atom pairs in an optical lattice
 News and Views: Quantum physics: United through repulsion
We are supported by the Austrian Science Fund (Fonds zur Förderung der wissenschaftlichen Forschung, FWF) in the frame of the Spezialforschungsbereich F15 "Control and Measurement of Coherent Quantum Systems" and by the European Union in the frame of the Cold Molecules TMR Network.
last change: 06-06-14 by KW