These atoms are cooled to a temperature of only a few billionths of a degree above absolute zero, the coldest possible temperature, at which all motion stops.

BBC: A Bose-Einstein condensate: It slows down light

Their tool, the Bose-Einstein condensate, is a superchilled soup of matter that can be created only when the temperature is near absolute zero.

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This slows down the movement, and hence the temperature, of its atoms to a fraction of a degree above absolute zero.

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