Shove off.
Like eggs in a carton, atoms nestle into the regular pattern of bright spots in an optical lattice. A new experiment shows that within such a "light crystal," two atoms can get stuck together even though they repel each other.

An Atomic Odd Couple

Staff Writer

Things that repel each other will move apart if you let them, right? Not always. Physicists have shown that within an artificial "crystal of light," two atoms that push each other away can get stuck together as an inseparable pair, like bitter spouses trapped in an unhappy marriage. Similar experiments might help explain high-temperature superconductivity and other phenomena in real crystals, and researchers say the artificial crystals may hold surprises of their own.

The finding is the latest oddity to emerge when physicists chill gases to a millionth of a degree. Thanks to quantum mechanics, ultracold atoms behave weirdly. For example, certain types of atoms can crowd into a single quantum wave in a process called Bose-Einstein condensation that yields matter with properties similar to laser light. Experimenters can also load ultracold atoms into "optical lattices," patterns of laser light that resemble the orderly arrangements of ions in a crystal. The atoms hop from spot to spot much as electrons move from ion to ion within a crystal.

But rubidium atoms in a lattice can get stuck together even though they repel each other, report Klaus Winkler, Johannes Hecker Denschlag, and colleagues at the University of Innsbruck in Austria. The physicists applied a magnetic field that caused the atoms to form two-atom molecules in the bright spots, they report this week in Nature. They then removed the field, and to their surprise the atoms continued to hop from spot to spot in pairs, even though they now pushed strongly against each other.

To figure out what's going on, the experimenters consulted Peter Zoller and other theorists at Innsbruck. They explained that repulsion between paired atoms gives them extra energy, as if they were connected by a compressed spring. If the atoms separated, that energy would go into their motion. But in the lattice, atoms can move with energies only in certain ranges or "bands," just as the electrons in a crystal do. If the spacing and brightness of the laser spots are tuned just right, the separated atoms would have energies that fall between the bands, Denschlag says. That's forbidden, so they remain united.

"If something wants to decay, it needs a state to decay into," explains Wolfgang Ketterle, an experimenter at the Massachusetts Institute of Technology in Cambridge. Lacking such a state, the pairs of atoms have no choice but to stay together. Jason Ho, a theorist at Ohio State University in Columbus, says the result could lead to even stranger things if it's possible to "condense" the pairs into a single quantum wave.

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