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Split Seconds: Building a Better Atomic Clock
30 August 2000 7:00 pm
Alarm clocks lose track of time when sleepyheads fling them off the nightstand. The same thing can happen to high-precision atomic timepieces, called fountain clocks. But in this case the floor doesn't do the damage; the ticking of these tiny clocks is interrupted by collisions between atoms. Now physicists report that replacing cesium atoms with hard-to-hit rubidium atoms prevents such collisions and dramatically improves the accuracy of these atomic Old Faithfuls.
The "ticking" of an atomic fountain clock is the carefully tuned oscillations of microwaves that shine into a special cavity. Physicists check the accuracy of the clock by comparing the microwave frequency to a second clock made of individual atoms. When a ball of about a billion atoms, usually cesium, is launched into the cavity, they absorb radiation from the microwaves as they fly through. The radiation flips the atoms into a particular combination of internal quantum states that oscillate. Like the water droplets in a fountain, the atoms then drop back in and another burst of microwaves stops the internal stopwatch, which provides a benchmark for the microwave ticker.
But there is a catch--one that prevents the atomic clock from maintaining perfect time. The atoms bump into each other as they make the round trip out of and back into the cavity. Each collision slightly changes the atomic spin rate, making it impossible to verify the accuracy of the microwave ticker to more than several nanoseconds. The relatively wide cesium atoms collide particularly often, says Yale University physicist Kurt Gibble.
Gibble and collaborator Chad Fertig, also of Yale, have come up with a better design. They report in the 21 August issue of Physical Review Letters that simply replacing cesium with smaller rubidium atoms would reduce the error in an atomic fountain clock by a factor of 30.
They have convinced Chris Ekstrom, a physicist who builds fountain clocks for the U.S. Naval Observatory. "It will help my group build a better clock," he says.