Repulsively bound atom
pairs in an optical lattice

Institut für Experimentalphysik,
University of Innsbruck, and
Austrian Academy of Sciences,
Innsbruck, Austria

What’s new?

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.

Our findings have been published in Nature, Winkler et al., Nature 2006,
see also cond-mat/0605196.

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.

Our work opens many possibilities for the investigation of repulsively bound objects in these different system, also objects involving more than two atoms. This phenomenon will be quite general in optical lattice physics, and repulsively bound states that are not stable in the equivalent solid state systems should be observable in our experiments.

The Bose-Hubbard model which we use to describe the repulsive pairs is also the theoretical basis for these other applications. In this sense, our work exemplifies the important correspondence between the Bose-Hubbard model and atoms in optical lattices, and helps pave the way for many more interesting developments and applications.


The research teams

This research was done in a collaboration of a theoretical and an experimental physics group at the Universität Innsbruck, Austria.



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,

K. Winkler, G. Thalhammer, F. Lang, R. Grimm, & J. Hecker Denschlag

is from the Institut für Experimentalphysik, Universität Innsbruck and IQOQI.

The theoretical physics team,

A. J. Daley, A. Kantian, H. P. Büchler & P. Zoller

is from the Institut für theoretische Physik, Universität Innsbruck and IQOQI.


For more picture material, click here!



[1] Repulsively bound atom pairs in an optical lattice
K. Winkler, G. Thalhammer, F. Lang, R. Grimm, J. Hecker Denschlag, and
A. J. Daley, A. Kantian, H. P. Büchler, P. Zoller
Nature 441, 853-856, (15 June 2006), doi:10.1038/nature04918
pre-print: cond-mat/0605196

Related Articles

[2] News and Views: Quantum physics: United through repulsion
Leonardo Fallani and Massimo Inguscio
Nature 441, (15 June 2006), doi:10.1038/441820a

[3] ScienceNOW: An Atomic Odd Couple
Adrian Cho
Science, (15 June 2006)



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