(left) Setup for observing wavelike interference in a Bose-Einstein condensate, a state of matter in which a group of atoms acts as a single wave with properties analogous to light waves from a laser. The atoms in a Bose-Einstein condensate are so cold that their normally miniscule wavelengths increase to the point at which their wavelike properties become potentially detectable. A coherent wave contains wavefronts that vary predictably in time and space. A laser light beam is an example of a coherent wave. Light from a lightbulb, in contrast, is incoherent: later wavefronts have no predictable relationship to earlier ones.
Coherence of the Bose-Einstein condensate (BEC) is observed by creating two independent BECs in a special trap which uses laser beams and magnetic fields and has two separated pockets. When the trap is switched off, the BECs fall down, spread out and eventually overlap. In the overlap region, a high-contrast interference pattern was observed with an electronic camera. Such a pattern is possible only if each BEC's atoms cooperated to form a single coherent wave, with the overall atom wave of one BEC interfering with the atom wave of the other condensate to produce a fringe pattern of light and dark fringes. The MIT team determined that the atom wave associated with each BEC had a wavelength of 30 microns, a million times larger than the wavelengths of room-temperature atoms.
(left) The interference experiment was also done with two atom-laser-beams derived from the Bose-Einstein condensate. The observation of an interference pattern showed that the atom laser beams were coherent, too.
This research is described in Science, 31 January 1997.