When evaluating the frequency uncertainty of a cesium-beam primary frequency standard, accurate measurement and control of the atomic velocity distribution is important. In frequency standards which employ atoms with thermal velocities, the measured atomic resonance frequency differs from the true resonance by several parts in 10/sup 13/ due to the second-order Doppler shift. To achieve a frequency uncertainty for NIST-7 of 5 parts in 10/sup 15/, the uncertainty introduced by the second-order Doppler shift must be no more than one part in 10/sup 15/. In addition, the Doppler shift must remain constant throughout the measurement of the atomic resonance frequency. We have developed a new optical technique for efficiently measuring the atomic velocity in real-time. This technique is compatible with the slow square-wave digital frequency servo used by the U.S. Primary frequency standard NIST-7.
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