t e

ta ul to th d pti st u w e a tic o i REVIEWS OF MODERN PHYSICS, VOLUME 74, JULY 2002phase-space density must be greater than approximately unity, in natural units. Another way to express this is The appeal of the exotic behavior of superconductiv-ity and of superfluidity, along with that of laser light, the third common system in which macroscopic quantum behavior is evident, provided much of our motivation in 1990 when we decided to pursue BEC in a gas. These three systems all have fascinating counterintuitive be-havior arising from macroscopic occupation of a single quantum state. Any physicist would consider these phe-nomena among the most remarkable topics in physics. In 1990 we were confident that the addition of a new member to the family would constitute a major contri-bution to physics. (Only after we succeeded did we real-ize that the discovery of each of the original Macro-scopic Three had been recognized with a Nobel Prize, and we are grateful that this trend has continued!) Al-though BEC shares the same underlying mechanism with these other systems, it seemed to us that the prop-erties of BEC in a gas would be quite distinct. It is far more dilute and weakly interacting than liquid-helium superfluids, for example, but far more strongly interact-ing than the noninteracting light in a laser beam. Per