AN ATOMIC clock so accurate that it should lose only 1 second in 150 million years is being tested in the GPS satellite navigation system. It promises to be the most stable clock ever built and, if it works, will replace the clocks that are now national standards against which a country鈥檚 other clocks are set.
Atomic clocks keep time by tracking the waves atoms emit as they oscillate between different energy levels. Every atom of a particular element emits at exactly the same frequency, so seconds are measured by counting the number of waves.
One of today鈥檚 standard clocks in the US is based on a laser-cooled 鈥渇ountain鈥� of caesium atoms, which emit about 9 billion waves per second. The clock, at the National Institute of Standards and Technology in Boulder, Colorado, is accurate to 1 second in 20 million years. But the fountain is stable for only minutes or hours at a time.
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So researchers have been working on an alternative 鈥� a 鈥渓inear ion trap鈥� based on mercury ions, which emit waves at a much higher frequency. In earlier versions of the clock the mercury ions were housed inside glass cylinders, and collisions with the walls reduced the accuracy of the clock. Now John Prestage and colleagues at NASA鈥檚 Jet Propulsion Laboratory in Pasadena have developed a trap that uses an electric field to keep the ions from hitting the walls.
The clock should be stable for longer periods than caesium clocks 鈥� from many hours to several years, long enough to calibrate the position of satellites.
NASA is now testing Prestage鈥檚 first model in its Deep Space Network, to measure the distances to spacecraft far from Earth. A second version, at the Naval Observatory in Washington DC, is being tested as a master clock for the Pentagon and to calibrate the clocks on GPS satellites. Tests over several days showed they were stable to 3 parts in 1016. 鈥淚t鈥檚 running very well, but we know a lot of ways to keep improving it,鈥� says Prestage.
