A cold atom has a small motional kinetic energy relative to a stated experimental scale. Usually an ensemble is prepared with a narrow velocity distribution through laser cooling or evaporative cooling of atoms; the quoted temperature characterizes motion rather than requiring the atom's internal electronic state to be thermally equilibrated.
Slower motion increases the time available for interrogation in atomic clocks or atom interferometry. In a release experiment, a cloud's growth after the trap is switched off constrains its velocity spread. Distinguishing initial cloud size from expansion avoids mistaking tight confinement for low motional temperature.
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