Attractive atoms pick up repulsive habits

Talk about doing as you’re told. Physicists now have made rubidium atoms act contrary to their nature. The atoms intrinsically attract each other, but new experiments near absolute zero have induced the atoms to repel one another instead.

In three-dimensions (top) and projected onto a plane (bottom), a plot shows the density of the condensate after it exploded. Ridges indicate two outgoing sprays of atoms detectable from a spherical burst. Wieman/JILA

In those experiments, Carl E. Wieman, Eric A. Cornell, and their colleagues observed surprising rubidium explosions that have yet to be explained. The small bursts are faintly akin to supernovas, say the researchers, who are at JILA, an institute in Boulder, Colo., jointly run by the University of Colorado and the National Institute of Standards and Technology (NIST).

In 1995, a team headed by Cornell and Wieman made scientific history by creating the first Bose-Einstein condensate—a dilute cloud of ultracold atoms that all share the same quantum state (SN: 7/15/95, p. 36). No property of the condensates surprises Wieman as much as the recently observed explosions.

“We’re into territory that’s totally uncharted,” he says. The new findings are “really exciting,” comments William D. Phillips of NIST headquarters in Gaithersburg, Md. Switching atoms from attractive to repulsive is “something that nature didn’t allow us to do before.”

When Cornell, Wieman, and their coworkers made the first Bose-Einstein condensate, they used rubidium-87, an isotope whose atoms naturally repel each other. To create condensates, physicists generally choose repulsive atoms such as sodium or hydrogen (SN: 7/25/98, p. 54) so the gaseous condensate won’t easily collapse.

However, by using very few atoms, scientists at Rice University in Houston have succeeded in producing condensates of lithium-7 atoms, which attract their fellow atoms.

In the new experiments, which will be described in an upcoming issue of Physical Review Letters, the JILA team tinkered with the naturally attracting atoms of another rubidium isotope, Rb-85.

Using lasers and evaporative methods to cool the atoms to record low temperatures of only a few nanokelvins, the team trapped the atoms in a magnetic field whose strength can be tuned with exceptional precision, Wieman says. By increasing that strength, the JILA researchers lowered the characteristic, discrete energies at which the rubidium atoms bond into molecules.

Earlier studies had indicated that reducing a characteristic energy level far enough would increase the attraction between nearby rubidium atoms. And that ought to boost their chances of forming molecules. Continuing to lower the level, however, should suddenly render the atoms mutually repulsive and then eventually attractive again.

The JILA researchers not only made their Rb-85 atoms repulsive but also were able to fine-tune the magnetic field to keep the atoms in that state, making a stable condensate possible.

The reversible flip from attraction to repulsion stems from quantum interference among condensate atoms, which behave as waves, explains JILA’s Jacob L. Roberts. “We can pretty much dial up what we want”—attractive or repulsive, weak or strong interactions, he adds.

The fireworks began when the scientists raised the magnetic field strength still further. The condensate suddenly reverted to attraction, shrank beyond detection, and exploded, blowing off about two-thirds of its 10,000 or so atoms. So far, “nobody has a convincing explanation” for the tiny burst, Wieman says.

Dying stars, in titanic blasts called supernovas, similarly implode before ejecting material and leaving a dense core behind. Pushing that analogy is probably not good science, Wieman contends. After all, the conditions for the two events are vastly different.

Nonetheless, he’s found an unassailable way to use the analogy: He’s dubbed the eruptions “bosenovas.”

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