The South Broadway Shootout

In the race to develop the world's best optical atomic clock, accuracy and precision are what count. Accuracy is the degree to which a measurement of time conforms to time's true value. Precision is a gauge of the exactness, or reproducibility, of the measurements. By definition, a high-precision clock must be extremely stable. JILA may well be home to one of the world's most precise (and stable) optical atomic clocks, thanks to the efforts of graduate students Marty Boyd, Andrew Ludlow, Seth Foreman, and Sebastian Blatt; postdoc Tanya Zelevinsky; former postdoc Tetsuya Ido; and Fellow Jun Ye.

There are two key reasons why the Ye group's lattice-based strontium clock is so precise: (1) Its ultrastable clock laser has a short-term laser linewidth of ~0.2 Hz (NIST's experimental mercury ion clock's laser is the only laser in the world that's more stable), and (2) Its optical lattice holds the strontium atoms in place for a relatively long time but doesn't perturb the critical optical atomic clock transitions. Taken together, the ultrastable laser and perturbation-free atomic sample produce the world's highest quality resonance profile, a measure of the clock's precision (Q ~2.5 x 10¹⁴).

Aided by unprecedented spectral resolution, the group's clock can achieve the highest precision ever measured with coherent spectroscopy. Its precision is much greater than the NIST-F1 cesium fountain atomic clock, the nation's primary time and frequency standard. Boyd says that the precision of his group's clock is potentially superior to the mercury ion clock under development by Jim Bergquist's group at NIST. But Boyd and his colleagues must still prove this claim in a direct comparison of the two optical atomic clocks. The mercury ion clock has already proven itself to be at least five times more precise than the NIST-F1.

Then there's the question of accuracy. The current NIST-F1 clock neither gains nor loses a second in about 70 million years. For the mercury ion clock, which is the most accurate clock in the world, the figure is 400 million years. Ye and his group recently showed that the current version of their Sr clock has an accuracy similar to that of the NIST-F1 cesium fountain clock.

The new optical atomic clocks at JILA and NIST perform so well that it's becoming increasingly difficult to assess their precision and accuracy in comparison to the current cesium fountain technology. To test their performance against a much better clock, the clocks must go head-to-head with each other.

The shootout began in November of 2006 when NIST and JILA began transmitting their optical clock signals across the fiber optic link between the two organizations. The clock designers will use NIST researcher Scott Diddams' optical frequency comb as an intermediate clock gear to facilitate the comparison of the mercury ion and lattice-based Sr clocks. When the dust settles, both groups will have the data they need to go back to the lab and improve their clock designs. The measurements will also help them determine the best optical atomic clock to use in the future.   - Julie Phillips