Joint time and length standard

A Tabulation of Absolute Laser-Frequency Measurements

“…There is a prospect of making measurements on transitions which can be calculated, as in the hydrogen atom [43], and on systems having much reduced first-and second-order Doppler perturbations through being cooled and held in ion traps [44]. These systems, however, must first be chosen for a strong resonance transition accessible to laser radiation, to permit cooling them by photon absorption and re-emission [45].…”