The oscillator is a light wave produced by a laser that oscillates at one million billion times per second. The point of reference is a single mercury atom, which responds to one specific frequency of light. The counting is done by a femtosecond laser mechanism that can generate hyperfine time.
The creation of the femtosecond laser counter is one of the elements that makes the NIST's optical clock a technological breakthrough. Electronics cannot count fast enough to keep up with an optical oscillator.
"We have a practical and robust clockwork that is able to divide down [the oscillations] to something that can be counted," said Diddams.
In describing the operation, Diddams said the femtosecond laser mechanism (technically called a comb) could be pictured as a set of gears. The laser that is referenced to the mercury atom in the NIST's clock is a tiny gear that spins very fast. Engaged to a huge reduction gear, that tiny gear turns approximately 500,000 times to turn the big gear oncean oscillation equivalent to a microwave frequency, which electronics is able to count.
"What the comb provides is a method of 'downshifting' the optical frequency to a countable radio frequency without impairing the purity of the optical frequency," said Stephen Webster, a researcher at the National Physical Laboratory in the United Kingdom.
Universal Standard Time
Optical clocks are likely to be the next-generation keepers of standard time. For this reason, variations of themeach based on a different atomare under development in research laboratories around the world. While the optical clock at NIST uses a single, cooled mercury ion, researchers at Webster's lab, for example, are developing an optical clock based on an ytterbium ion.
"It is not yet clear which will be the best candidate, with much work yet to be done on improving, testing, and comparing clocks," Webster said. Scientists say it may take at least 20 more years to develop and determine the best approach.
The stability of an optical clock's ticker makes this kind of clock an exceptional tool for testing scientific conundrums such as whether time is truly constant in an ever-expanding universe.
Although atomic clocks are based on the principle that the energy levels of atoms are constant, some scientists think these levels may change as the universe expands. An optical clock, with its greater stability and potentially improved accuracy, could perhaps be used to observe whether such physical constants evolve in time.
From a technological standpoint, the researchers expect optical clocks of the future to have a variety of applications, like atomic clocks, which the navigation industry used as the basis for building global positioning systems (GPS).
GPS uses signals transmitted from satellites, each housing an atomic clock, to pinpoint a particular location on Earth. Optical clocks could possibly be designed and used similarly for travel into deep space.
"If one is trying to position and communicate with a remotely piloted spacecraft on a distant planet, the better one is able to time signals that go back and forth, the better the ability to put something at a certain position," said Diddams.
A paper reporting on the optical clock research by Diddams and his colleagues appears in the July 13 issue of Science.
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