To make two objects bind together, you normally need to make them attract one another. Now, however, Denschlag and colleagues have shown that this is not always necessary and that objects can stick together even when there is a repulsive force between them. This is counterintuitive because in free space repulsive pairs cannot exist: if you bring two repelling objects together they will just accelerate away from each other (figure 1).

Denschlag and colleagues have demonstrated that this problem can be overcome by placing the objects in a 3D optical lattice. This is an artificial "crystal" of light formed by the interference of multiple laser beams. The crystal contains potential wells or "dimples" in which atoms can be trapped (figure 2).

The Austrian team began by preparing a sample of ultracold rubidium-87 molecules from a Bose-Einstein condensate (BEC) of rubidium atoms. A BEC is a collection of particles that has been cooled to such low temperatures that all the particles collapse into the same quantum state.

Next, the physicists loaded the rubidium-87 molecules into the optical lattice. By then splitting the molecules in a very controlled way using an applied magnetic field (with the help of a so-called Fesbach resonance), they obtained pairs of atoms that strongly repelled each other. Each potential well contained either one repulsive pair of atoms or none at all.

Denschlag and co-workers observed that even though the pairs of rubidium atoms repelled each other, they still remained together in the potential wells. Moreover, when the researchers tuned the interactions between the atoms to zero (that is, made them non-interacting), the pairs quickly broke up. When the interaction was made repulsive again, this breaking up was suppressed so that the atoms came together once more.

According to the team, the bound configuration is stable because the total energy of the atoms is smaller when they are close together than when they are separate. Put another way, the kinetic energy of the atoms is restricted to certain ranges in the special environment of the optical lattice. In order for the atoms to separate, they would have to enter an energy state that is "forbidden" by quantum physics. This means that the atoms may move together in pairs from one well to the next, but cannot do so on their own (figure 3).

"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," says Daley, who is a theorist. "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 quantum simulators to model materials such as high temperature superconductors and other "exotic" systems.