Introduction to Bose-Einstein Condensation
Bose-Einstein condensation (BEC) was predicted by Albert Einstein more than 80 years ago. It is a phase transition, like water freezing to ice, but it is one that is intimately related to the quantum mechanical nature of the world. It is thought to cause effects like superconductivity in metals and superfluidity in liquid helium. The experimental observation of BEC, however, has been a long-sought goal. Efforts were begun in the early 1980s using atomic hydrogen. Although this work came very close to phase transition, it was not until 1995 that true BEC was observed in three different laboratories using three different atoms. It was first seen in Eric Cornell's lab at the University of Colorado in Boulder using rubidium atoms. Within the next few months, our Rice group observed BEC in lithium, and Wolfgang Ketterle's lab at the Massachusetts Institute of Technology saw BEC in sodium. (To be fair, however, our initial evidence for BEC was less direct and more qualitative than that observed by the other two groups.) Since then, many other groups have duplicated the process and succeeded in condensing other atom species.
In our lab we are studying BEC using lithium atoms. In particular, our work focuses on the role played by the interactions between atoms in the condensate, since for lithium, these interactions have a profound effect. It had long been thought that interactions would actually prevent BEC from occuring in gases like lithium, but in fact this was wrong. BEC can occur, but the interactions limit the number of atoms allowed in the condensate to a relatively small value. The images above are data showing two clouds of about 70,000 very cold atoms. The one on the left has a condensate containing about 1,200 atoms, while the one on the right has about 200 atoms. By experimentally observing condensates such as these and verifying their limited number, we have made an important check on our understanding of these systems and have developed a system in which unique aspects of quantum and statistical mechanics can be further studied.