New Clock Promises to Be World's Most Precise

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.

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