Collaboration helps make JILA strontium atomic clock 'best in class'

In the JILA clock, a few thousand atoms of the alkaline-earth metal strontium are held in a column of about 100 pancake-shaped traps called an "optical lattice." The lattice is formed by standing waves of intense near-infrared laser light. Forming a sort of artificial crystal of light, the lattice constrains atom motion and reduces systematic errors that occur in clocks that use moving balls of atoms, such as NIST-F1. Using thousands of atoms at once also produces stronger signals and eventually may yield more precise results than clocks relying on a single ion, such as mercury. JILA scientists detect strontium's "ticks" (430 trillion per second) by bathing the atoms in very stable red laser light at the exact frequency that prompts jumps between two electronic energy levels. The JILA team recently improved the clock by achieving much better control of the atoms. For example, they can now cancel out the atoms' internal sensitivity to external magnetic fields, which otherwise degrade clock accuracy. They also characterized more precisely the effects of confining atoms in the lattice.

The NIST calcium clock, which was used to evaluate the performance of the new strontium clock, relies on the ticking of clouds of millions of calcium atoms. This clock offers high stability for short times, relatively compact size and simplicity of operation. NIST scientists believe it could be made portable and perhaps transported to other institutions for evaluations of other optical atomic clocks. JILA scientists were able to take advantage of the calcium clock's good short-term stability by making fast measurements of one property in the strontium clock and then quickly switching to a different property to start the comparison over again.

The JILA-NIST collaborations benefit both institutions by enabling scientists not only to compare and measure clock performance, but also to share tools and expertise. Another key element to the latest comparison was the use of two custom-made frequency combs, the most accurate tool for measuring optical frequencies, which helped to maintain stability during signal transfer between the two institutions. (For background, see http://www.nist.gov/public_affairs/newsfromnist_frequency_combs.htm.)

The JILA research is supported by the Office of Naval Research, National Institute of Standards and Technology, National Science Foundation and Defense Advanced Research Projects Agency.

As a non-regulatory agency of the Commerce Department, NIST promotes U.S. innovation and industrial competitiveness by advancing measurement science, standards and technology in ways that enhance economic security and improve our quality of life.

* A.D. Ludlow, T. Zelevinsky, G.K. Campbell, S. Blatt, M.M. Boyd, M.H.G. de Miranda, M.J. Martin, S.M. Foreman, J. Ye, T.M. Fortier, J.E. Stalnaker, S.A. Diddams, Y. Le Coq, Z.W. Barber, N. Poli, N.D. Lemke, K.M. Beck, & C. Oates. 2008. Sr lattice clock at 1x10-16 fractional uncertainty by remote optical evaluation with a Ca clock. Science Express. Posted online Feb. 14.