Interference between two freely expanding Bose-Einstein condensates has been ob-served. Two condensates separated by ~40 micrometers were created by evaporatively cooling sodium atoms in a double-well potential formed by magnetic and optical forces. High-contrast matter-wave interference fringes with a period of ~15 micrometers were observed after switching off the potential and letting the condensates expand for 40 milliseconds and overlap. This demonstrates that Bose condensed atoms are “laser-like”; that is, they are coherent and show long-range correlations. These results have direct implications for the atom laser and the Josephson effect for atoms.
The realization of Bose-Einstein condensation (BEC) in dilute atomic gases has created great interest in this new form of matter. One of its striking features is a macroscopic population of the quantum-mechanical ground state of the system at finite temperature. The Bose condensate is characterized by the absence of thermal excitation; its kinetic energy is solely the result of zero-point motion in the trapping potential (in general, modified by the repulsive inter-action between atoms). This is the property that has been used to detect and study the Bose condensate in previous experiments. The Bose-Einstein phase transition was observed by the sudden appearance of a “peak” of ultra-cold atoms, either in images of ballistically expanding clouds (time-of-flight pictures) (1–3) or as a dense core inside the magnetic trap (4, 5). The anisotropic expansion of the cloud (1–3) and the appearance of collective excitations at frequencies different from multiples of the trapping frequencies (6, 7) were found to be in quantitative agreement with the predictions of the mean-field theory for a weakly interacting Bose gas (8–11). However........

The experimental setup

Interference between two Bose condensates

Outlook

1.M. H. Anderson, J. R. Ensher, M. R. Matthews, C. E. Wieman, E. A. Cornell, Science 269, 198 (1995).
2.K. B. Davis et al., Phys. Rev. Lett. 75, 3969 (1995).
3.M.-O. Mewes et al., ibid. 77, 416 (1996).
4.M. R. Andrews et al., Science 273, 84 (1996).
5.C. C. Bradley, C. A. Sackett, R. G. Hulet, in preparation; see also C. C. Bradley et al., Phys. Rev. Lett. 75, 1687 (1995).