New Clock Will Lead to More Accurate Measure of Time

John Roach
for National Geographic News
July 13, 2001
Never be late again.

Scientists who specialize in the accuracy of time have created a new kind of clock—an optical atomic clock—that "ticks" one million billion times per second and is at least 20 times more stable than current atomic clocks that are based on microwaves.

The technological breakthrough is like acquiring a fine-grain view of nature, say its creators.

"The analog might be in looking at a biologic sample through a magnifying glass versus looking at it through a microscope," said Scott Diddams, a member of the team conducting optical clock research at the U.S. National Institute of Standards and Technology (NIST) in Boulder, Colorado.

Although it will take several decades for the technology to be tested and accepted by the international timekeeping community, optical atomic clocks have the potential to be 100 to 1,000 times more accurate than current microwave atomic clocks.

NIST and other research laboratories make and maintain the most precise clocks in the world against which less accurate clocks, such as wristwatches and alarm clocks, can be calibrated.

As modern technological demands increase, the master clock that sets the international time standard needs to stay ahead of the curve.

Ticks and Tocks

All clocks consist of two basic components: a device called an oscillator, which produces periodic events, or ticks, and a mechanism for counting and displaying the ticks. In a traditional clock, such as a grandfather clock, a pendulum oscillates back and forth to produce ticks, while a set of gears drives a pair of hands that count and display the ticks.

Atomic clocks add a third component: a point of reference against which to synchronize the clock. Because the energy levels of atoms are thought to be constant, atomic clocks use specific atoms as the point of reference.

In microwave atomic clocks, the well-defined resonance of a cesium atom is the reference point. The oscillator is a microwave source, and high-speed electronics count and display the time.

Because cesium atoms provide a stable reference point no matter what the temperature or air pressure is, atomic clocks have become today's official timekeepers.

The NIST's optical atomic clock works according to the same principle as a microwave atomic clock, but at a much faster rate.

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 once—an 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 them—each based on a different atom—are 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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