Cooling required a sequence of different methods
In 1995, the Cornell–Wieman and Ketterle groups produced Bose–Einstein condensates in dilute alkali gases. Laser cooling and trapping prepared the sample, and evaporative cooling removed energetic atoms so the remaining gas could reach much higher phase-space density.
Atom number, confinement and losses had to be controlled together. Low temperature alone does not specify whether a trapped gas has crossed its condensation threshold.
Release images separated two components
After release, absorption imaging measured the expanded density. A narrow component emerged within a broader thermal distribution as conditions crossed the transition. The shape and its evolution supplied more information than a single bright spot.
Expansion is a physical process to model
For ballistic noninteracting expansion at long time, position maps to initial momentum. Interactions can instead convert mean-field energy into motion and alter the aspect ratio. Imaging integrates along a line of sight, so a camera image is not automatically a full three-dimensional momentum distribution.
Coherence tests added another kind of evidence
Interference between condensates tested coherence beyond the appearance of a narrow density component. The 2001 award recognized condensation in dilute gases and early studies of condensate properties. Existing occupation and collective-gas work is reused; no duplicate BEC instrument is added here.