Hydrogen atoms chill to quantum sameness.

Atoms of the simplest element, hydrogen, have finally yielded to efforts to supercool them into a single quantum-mechanical state. Coaxing hydrogen into a Bose-Einstein condensate--in which millions of ultracold atoms behave as one mega-atom--is a landmark, physicists say, because hydrogen is the lightest, most abundant, and best understood element.

The achievement caps a 20-year effort by physicists Thomas J. Greytak and Daniel Kleppner of the Massachusetts Institute of Technology, who persevered after other hydrogen-research groups gave up. In 1995, a team using the alkali metal rubidium raced past them to make the first sample of the remarkable form of matter (SN: 7/15/95, p. 36), predicted in 1924 by Albert Einstein and Satyendra Nath Bose. Since then, 10 other groups have also made alkali condensates, including sodium and lithium versions (SN: 5/25/96, p. 327).

Word of hydrogen's condensation leaked out of the MIT group soon after Dale G. Fried, Thomas C. Killian, and others working under Greytak and Kleppner spotted a telltale boost in the density of their pooled atoms in June. Kleppner formally described the results early this month at the Summer Course of the Enrico Fermi International School of Physics in Varenna, Italy.

The condensate is made up of an estimated 100 million atoms--the most so far in any Bose-Einstein condensate. Because the coordinated quantum states of condensates allow them to double as atomic lasers (SN: 2/1/97, p. 71), the large population of the hydrogen version could mean brighter, longer-lasting pulses. Such lasers might be used for depositing or etching circuits and other microstructures, researchers speculate.

The new hydrogen condensate may also emit a more focused atomic laser than other condensates, Greytak says, although that has yet to be confirmed by the team.

Physicists particularly welcome the new condensate because hydrogen's minimal one-proton-one-electron structure offers the simplest case to study. "Experiments in this system might give greater understanding about the alkali [condensates]," says Allan Griffin of the University of Toronto. He notes that certain atomic interactions have been calculated exactly for hydrogen but can only be approximated for the more complex alkalis.

Since such interactions are also crucial to atomic flow, the new condensate "might also give us greater understanding of superfluidity," he adds (SN: 2/7/98, p. 95).

In the late 1970s, when chilling atoms to very low temperatures began to become possible, hydrogen was seen as the most promising candidate for a Bose-Einstein condensate. Having the lowest mass of any element, it was expected to condense at the highest temperature.

That prediction has held so far. Hydrogen condensed at 40 microkelvins. By improving upon cooling technology first developed by their hydrogen-research rivals, scientists working on alkali elements were quicker to reach the still lower temperatures--less than 2 microkelvins--needed to make their condensates.