Reproducibility of optical frequency studied.

The mercury-ion optical-frequency standard has the potential for an accuracy surpassing that of the cesium-fountain standard by two or more orders of magnitude. With a Q factor greater than [10.sup.14] and a transition that is relatively insensitive to environmental factors, the potential uncertainty for the standard is as small as 1 aHz/Hz (0.001 fHz/Hz, or 1 x [10.sup.-18]).

The reproducibility of this standard relative to the present cesium standard (NIST-F1) was studied over a 2 year period by NIST. Measurements were referenced to NIST-F1 through the intermediary of a hydrogen maser. The short-term stabil-ity of the mercury standard is superior to that of NIST-F1, so the maser played a key role in the comparisons. The variation in the frequency of the S-D optical transition relative to NIST-F1 was found to be less than [+ or -] 10 fHz/Hz over the 2 years.

The uncertainty of the absolute measurement (at this same level) is the most accurate measurement ever made of an optical frequency, and is a very encouraging result, since no significant effort had been made to control systematic frequency shifts. The largest of these shifts is expected to be the atomic quadrupole shift, which depends on the orientation of the applied magnetic field relative to ambient, static electric-field gradients. Concepts for determining and controlling this and other smaller shifts have been developed, but further studies are needed to test them. To evaluate the systematic effects, a second mercury-ion standard has been constructed. Changes in operating parameters on one of these two mercury standards should readily reveal any perturbative shift in the frequency of the optical clock transition when the frequencies of the two standards are compared.

CONTACT: James Bergquist, [phone]; [e-mail].