© William P. Reinhardt (left image) and Wolfgang Ketterle (right image).
A BEC may be split into two spatially distinct parts without destroying the phase coherence of the overall macroscopic quantum system. This is accomplished by raising a barrier in the middle of the trap holding the BEC. Once the BEC is so separated, the trap is turned off, and all of the BEC expands. When the expanded separated parts overlap, they show constructive and destructive interference depending on whether the overlapping BECs are in, or out, of phase. The image on the left shows a calculation of the interference pattern, with the x-y plane representing position and the z-axis representing the density of atoms. The right-hand image shows a similar interference pattern in an MIT experiment. The darker areas correspond to areas with a higher density of atoms. In the experiment, two unacquainted condensates (with no prior phase coherence) were allowed to expand and overlap. Interference is still the outcome. (Unit: 6)