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Sr-atoms are of special interest for an optical frequency standard since a large number of atoms can be cooled down efficiently to ultra-low temperatures in a magneto-optical trap (MOT) [1]. The ultra-narrow line on the forbidden 1S0 → 3P0 transition of 87Sr (3P0 natural lifetime is approx. 100 s), and the ability to interrogate Sr atoms in a perturbation free dipole trap [2] promise un-precedented...
An optical frequency standard is operated at PTB where the frequency of a laser is stabilized to the intercombination transition (λ ≈ 657 nm) in laser cooled Ca atoms. Previously the uncertainty resulting from the residual velocity of the cold atoms at 3 mK limited the fractional uncertainty to 2 × 10−14. We now apply a novel method for producing ultra-cold atoms (λ ≈ 10 µK). The frequency of the...
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