New Clock Promises to Be World's Most Precise

ByAmanda Onion

July 13, 2001 -- The most accurate clocks today slip by only one second every 30 million years.

For scientists at the National Institute of Standards and Technology in Boulder, Colo., that's just not accurate enough.

Instead, they've built a clock designed to only slip by a second once every 30 billion years. This clock (which would never fit on the wrist, since it takes up the size of a large lab room) records time by counting the rapid-fire oscillations in a laser. The oscillations, in turn, are kept in pace by a single mercury atom that vibrates at a constant cadence.

The result is a clock that counts time by the femtosecond — a millionth-billionth of a second.

"This has the potential for making frequency and time measurements more precise by orders of magnitude than current clock systems," says Jim Bergquist, a physicist at NIST and co-author of a study about the new optical clock in this week's journal Science.

How to Keep Time

All clocks operate using two main ingredients. The first is something that creates a regular, periodic event, and the second is a device that will count, accumulate and display those events.

In a grandfather clock, for example, a pendulum swings once every second and those swings are recorded by metal gears inside the clock. In quartz watches, time is measured by the oscillations of a quartz crystal and usually recorded using digital counters. Digital clocks use either the oscillations on the power line (60 cycles a second in the United States) or the oscillations of a quartz crystal and also count using digital counters.

The most accurate clocks today measure time by locking the frequency of microwave beams to the frequency of vibrating atoms. Rather than swinging once per second, like the grandfather clock, Cesium 133 atoms oscillate at a rate of 9,192,631,770 times a second.

Since 1967, the cesium atomic clock has defined "one second" as the time it takes a cesium atom to vibrate 9,192,631,770 times.

Before the atomic clock, the second was based on less precise measurements of the motion of the Earth.

Now the optical clock promises to beat all previous standards.

Ways to Count Fast

When the laser first appeared around 1960, scientists realized it had potential to generate a higher frequency than, for example, microwaves. But the trick was how to count them.

"Optical frequencies oscillate at a million billion times a second," says Bergquist. "You can't miss any of them or else you lose precision."

To surmount that problem, Bergquist, his colleague Scott Diddoms, also at NIST, and others used a method first developed by German physicists to accurately count each cycle of the laser's rapid oscillations. Rather than directly counting a million billion oscillations a second, a highly accurate device (called a femtosecond pulsed laser) counts every 100,000th pulse of the laser. For every tick of the counter, the scientists know 100,000 laser ticks have gone by.

Next, they had to find an atom — like the cesium atoms in the atomic clock — that would keep the laser's oscillations at a steady pace. As Bergquist explains, if a laser is tuned to the same frequency of an atom "it's like using a tuning fork — the laser and atom will oscillate in time."

The NIST team settled upon a single mercury ion as the "heart" of their optical clock. When a mercury ion is cooled, it vibrates at just the right rate to "tune" the light from a laser oscillating a million billion times a second, and to keep that cadence steady.

The result, after more than 15 years of work and "several" million dollars in spending, is a clock that measures time by intervals 100,000 times shorter than those recorded by the best current clocks.

It's unlikely anyone would worry about being a femtosecond late to work. So why do we need such accurate timekeepers? It turns out they're important not so much for punctuality, but for science.

Time as a Tool

Global Positioning Systems depend on highly accurate cesium clocks in satellites to measure distance. Location is determined by clocking the time it takes for a signal to reach a satellite and return again. More accurate clocks could help pinpoint location even better.

NASA engineers depend on accurate timekeepers to direct spacecraft around the cosmos. Having more accurate clocks in hand could allow NASA engineers to pilot spacecraft at even greater distances.

"In the future, we might want to navigate outside the solar system for long periods of time," says Lute Maleki, a senior scientist at NASA's Jet Propulsion Laboratory. "Then very high quality clocks on spacecraft can be very useful."

Better clocks could even shed light on how our expanding universe may be affecting the forces of nature. While these changes are too minute to measure with current clocks, an optical clock might be able to detect changes in, for example, the resonance of a single atom.

Despite the promises optical clocks hold, don't expect to see them mounted on your town hall any time soon. The definition of time — and the measuring of it — are very hard things to change.

First of all, the optical clock, as it's now designed, still needs refining. Bergquist says scientists may eventually settle on a different kind of atom, like a calcium or iridium atom, as the optical clock pacer. These atoms might vibrate at more reliable frequencies.

Plus, when scientists first created reliable versions of the cesium clock some 50 years ago, it still took about 20 years before the world community agreed to use them as the universal measure of the second.

"This is only the first demonstration," says Diddoms. "To redefine the second, everyone involved in timekeeping around the world has to agree. It's likely it will happen, but it's decades away."

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