LONG BEACH, Calif., May 23 -- Calcium is a very promising atom for next-generation clocks, said Anne Curtis of the National Institute of Standards and Technology (NIST) in Boulder at a presentation Wednesday at CLEO/QELS.
   Atomic clocks keep accurate time by locking themselves to the extremely precise frequencies of the electromagnetic radiation absorbed or released by an atom making a transition from one energy level to another. Next-generation atomic clocks, which lock to frequencies in the visible light range, are showing that they can surpass traditional atomic clock designs based on microwave frequencies.
   With the best short-term stability and one of the most accurately measured frequencies, the calcium-40 atomic clock, being developed at NIST Boulder is leading the way for optical frequency standards based on neutral atoms. The Ca optical clock operates at a frequency of 456 THz (a visible red color) and consists of about 10 million laser-cooled and trapped Ca atoms, which are probed with 456 THz light from a diode laser to excite the "clock" transition.
   At the meeting, Curtis and her colleagues will report that they have used lasers to cool the calcium atoms to temperatures of about 10 millionths of a degree above absolute zero. These slower atoms allow more accurate measurements of the 456 THz frequency, resulting in better timekeeping. They are presently working on a 3-D laser trap, from which they can better load the atoms into another, specialized optical trap that provides further confinement and additional cooling.
   With further developments, these new, higher performance clocks should lead to better navigation/positioning systems and advanced communication systems as well as enable more rigorous tests of fundamental physics, including further probing Einstein's theory of relativity and measurements of possible changes in physical constants over time.