Quenched narrow-line cooling of calcium. (News Briefs).

Since their invention, atomic clocks have operated in the microwave portion of the spectrum, where their output frequencies have been amenable to electronic systems. However, the precision of such standards has been limited by their spectral linewidths (relative to the reference frequencies themselves). Substantial progress in this field can be achieved by using very narrow optical transitions instead, in part because the reference frequencies are orders of magnitude higher.

NIST has achieved a technical milestone in its push to develop optical atomic frequency standards and clocks. The objective is to improve upon the accuracy and stability of the calcium optical standard, which is the only optical standard for which high-accuracy international comparisons have been made. The best comparison to date is between NIST and PTB where a difference of only 30 Hz was measured for the 456 THz (657 nm) transition. This was well within the uncertainties of the individual measurements.

The 26 Hz uncertainty in recent measurements at NIST arose from the limitations of the laser-cooling method that was then available. In calcium, normally only Doppler cooling is possible. However, the Doppler limit is large, of the order of 1 mK.

NIST has recently achieved sub-Doppler cooling of [Ca.sup.40] to near the recoil limit using a quenched narrow-line laser-cooling method. A laser operating just below 657 nm (on the clock reference transition) provides a cooling force while a second, co-linear laser (at 552 nm) provides both additional cooling and optical pumping required for the process to work. Using this method, calcium atoms were cooled in one dimension to < 2 [micro]K and in three dimensions to below 10 [micro]K. Further improvements will include further reduction in temperature and increased cooling efficiency.

CONTACT: Chris Oates, [phone]; [e-mail], Anne Curtis, [phone]; [e-mail], or Leo Hollberg, [phone]; [e-mail].