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An individual atomic ion localized in the center of a small Paul RF quadrupole trap has potential as an ultimate laser frequency standard because the ion may be brought to `a state of complete rest in free space' by sideband cooling. As a consequence, all Doppler shifts vanish, `Free Space' is approximated in so far as the electric trapping field vanishes in the center of the trap and there is no Stark effect. Neither need there be a Zeeman effect as magnetic fields may be controlled down to the microgauss range. There is also no transit time broadening. Minute laser powers provided by harmonic generators suffice for saturation of optical transitions as strongly focused beams may be used. A million-fold atomic amplification of the single-ion fluorescence from a metastable level may bring resolutions of 1 part in 10 18 within reach.
Hans Dehmelt (Tue,) studied this question.