A shared quantum state
Bosonic statistics allow multiple particles to occupy one quantum state. At sufficiently low temperature and suitable density, many particles accumulate in the lowest-energy state. This collective occupation differs from fusing their nuclei together and is not simply another name for all ordinary gas-to-liquid condensation processes.
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From Bose to Einstein
Bose studied the statistics of light particles in 1924, and Einstein extended the approach to suitable atoms, predicting a special low-temperature collective state. Roughly seven decades separated the prediction from dilute-gas realization, reflecting the challenge of preparing extremely cold samples under conditions that remained experimentally accessible.
Dilute-gas experiments in 1995
Cornell, Wieman and colleagues produced a rubidium condensate in 1995, while Ketterle independently performed sodium experiments. Trapping and cooling methods prepared the ultracold samples. The 2001 Nobel Prize recognized the achievement of dilute-gas condensation and early investigations of condensate properties, rather than the earlier prediction alone.
Interference as collective evidence
Allowing two condensates to expand and overlap produces interference patterns that reveal measurable collective coherence. Such experiments make matter-wave physics accessible in controlled many-particle systems. Researchers vary trapping, interactions and motion to compare reproducibly prepared samples with predictions about collective quantum behaviour and dynamics.