Editor's Note: The main text of this article, originally published in the March 1998 issue of Scientific American, is being made available in light of the recent nomination of one of the authors, Carl Wieman, as associate director for science in the Office of Science and Technology Policy. Both authors won the Nobel Prize in Physics in 2001 for their discovery of the Bose-Einstein condensate. (Wolfgang Ketterle also shared the prize that year for his contributions.) The issue containing the full article with all artwork is available for purchase (PDF).