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Beating the Atomic Clock
13 July 2001 7:00 pm
A new clock reliably ticks nearly one million billion times per second and could be the most accurate clock ever made, report physicists in a paper published online by Science on 12 July. But the clockmakers won't know for sure until they complete a second equally precise device to compare it against.
Time is the most precisely measured physical quantity in the universe. Atomic clocks now routinely tick off nanoseconds (one billionths of a second) by tuning microwave lasers to match one frequency of light emitted by a cesium atom. That is good enough for Global Positioning System satellites, which use onboard cesium clocks to triangulate precise positions on Earth. Physicists would like an even faster-ticking clock to answer big nagging questions, like: Is the so-called fine structure constant, which determines the strength of electromagnetic interactions, truly constant? Such clocks should be easy to fashion; optical light "ticks" a thousand times faster than lower frequency microwaves. But the problem is that no device can count that fast.
To overcome that hurdle, a team led by physicist Scott Diddams from the Time and Frequency Division of the National Institute of Standards and Technology (NIST) in Boulder, Colorado, built an "optical light gear." It converts the rapid oscillation of the optical laser light wave into a fluctuation in the laser intensity that is precisely a factor of one million slower. To count clock ticks, a standard detector adds up how many times the laser intensity oscillates in a second and then Diddams's team just multiplies that by a million.
Just how accurate is the new clock? "It is no worse than 10 times less accurate than the best cesium clock," says Diddams, "And it could be a thousand times better." But the only way to measure the accuracy of this clock is to compare it to another one of the same or better accuracy. So the team has to build another clock. "They are going to have to bite the bullet," says physicist Dan Kleppner of the Massachusetts Institute of Technology. "It is no good having only one World's Best Clock." Diddams's team hopes to complete their second clock within a year.