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The first frequency standard based on laser-cooled atoms is reported. Beryllium atomic ions were stored in a Penning trap and cooled by radiation pressure from a laser. The frequency of the ^9Be^+ (M₈, M₉) = (-32, +12) (-12, +12) ground-state hyperfine transition at its magnetic-field-independent point was determined to be 303 016 377. 265 070 (57) Hz. The accuracy of a frequency standard referenced to this transition was comparable to the best frequency standards, which are based on cesium atomic beams.
Bollinger et al. (1985) studied this question.