Laser-Cooled-Atomic Frequency Standard

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 ${}_{}{}^9{}_{}{}^{}\mathrm{Be}_{}^{+}{}_{}{}^{}$ $(M_I, M_J)=(-\frac{3}{2}, +\frac{1}{2})\leftrightarrow (-\frac{1}{2}, +\frac{1}{2})$ 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.