Frigid ‘dynamite’ assembles into superatom

Since 1995, studies of a new and strange state of matter called a Bose-Einstein condensate have deepened physicists’ understanding of the quantum nature of atoms. After first creating the state with rubidium (SN: 7/15/95, p. 36), researchers have made these ultracold atomic clouds also with sodium, lithium, and hydrogen (SN: 7/25/98, p. 54).

Now, a French team reports both making the first Bose-Einstein condensate of gaseous helium and doing so in a novel way that may illuminate unexplored features of the condensates. “This represents a whole new kind of condensate that opens up a whole new set of possible experiments,” comments William D. Phillips of the National Institute of Standards and Technology in Gaithersburg, Md.

Physicists describe a Bose-Einstein condensate as a kind of superatom. Because atoms behave as waves as well as particles in a dilute, ultracold gas, they overlap and merge into a single entity with only one quantum state. Until now, however, Bose-Einstein condensates have contained atoms only in their lowest energy state, or ground state.

In contrast, the helium atoms are in a so-called metastable state, in which each atom carries an internal energy of 20 electron volts (eV). That’s huge compared with the tiny kinetic energy of such cold atoms, says Alain Aspect, who led the team that made the condensate at the Centre Nationale de la Recherche Scientifique at the Université Paris-Sud in Orsay. He and his colleagues describe their experiment in Sciencexpress, the online supplement to the March 23 Science.

Each atom’s allotment of 20 eV–10 to 20 times the energy in a typical chemical bond–“is sort of a dynamite charge of internal energy,” says Wolfgang Ketterle of the Massachusetts Institute of Technology. It’s easily enough to blast atoms out of the condensate if collisions between atoms were to let it loose.

Only a week behind Aspect’s group, a competing team at the École Normale Supérieure in Paris, led by Claude Cohen-Tannoudji and Michèle Leduc, also made a helium condensate. A preprint describing their experiment is available on the Internet physics archive at http://xxx.lanl.gov/abs/cond-mat/0103387.

Both groups bottled up their atoms’ energies by manipulating magnetic properties. Each atom acts as a tiny bar magnet. Following an earlier theoretical proposal, the teams aligned all internal magnets of their condensate atoms, preventing energy releases in collisions.

Whereas the Paris group used a standard optical technique to observe the novel condensate, Aspect and his team went a step further. Taking advantage of the atoms’ high energies, they devised a new way to detect the particles.

The researchers allowed atoms from the condensate to drop onto a platelike detector. There, the release of each atom’s stored energy knocked an electron loose from the detector, creating a measurable signal. “That’s the thing I find most exciting–that the metastable atoms are easy to detect, one by one,” says Phillips.

The ability to count every atom in the cloud may permit experiments in which physicists can closely compare the numbers of atoms in certain volumes of space. Those studies should yield “a more mature understanding of quantum mechanics and wave-particle duality,” Phillips predicts. Members of the Paris group, which made a larger condensate than Aspect’s team did, have also suggested that such highly energized condensates may someday prove useful for etching nanometer-scale circuit patterns.

In addition to the French work, two other experiments described this week have also shed light on Bose-Einstein condensates. Also in Sciencexpress, Ketterle and his colleagues report observing an extraordinarily large array of tiny whirlpools in a sodium condensate. This “lattice” of vortices dramatically demonstrates the analogous quantum nature of Bose-Einstein condensates and certain superconductors that have similar patterns of swirls in their magnetic fields, Ketterle says. Superconductors conduct electricity resistance-free.

Meanwhile, in the March 23 Science, Mark A. Kasevich and his coworkers at Yale University unveil the first “squeezing” of a condensate’s quantum state. Optics specialists have long used this method to cope with Heisenberg’s uncertainty principle. They sacrifice precision in measuring one parameter by squeezing, or more precisely gauging, another. The new study of rubidium atoms may lead to improvements in prototype measuring devices that use atom waves.

Says Ketterle, “It’s a good week for Bose-Einstein condensates.”