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December 11, 2007

More precise clocks driving many technological advances

Wired magazine reports on atomic clocks, which are the basis of GPS technology and help science become more accurate.

The NIST-F1, this atomic clock is more accurate for prolonged periods than any other clock -- an order of magnitude better than the one it replaced in 1999. When the F2 down the hall goes online next year, it will similarly dwarf the F1. Clocks have been improving by a factor of 10 every decade.

NIST is working to shrink atomic clocks to the size of a grain of rice, and testing new breeds of clocks precise enough to detect relativistic fluctuations in gravity and magnetic fields. Within a decade their work could have a significant impact on areas as diverse as medical imaging and geological survey.

Cesium clocks like NIST-F1 use lasers to slow a cloud of cesium atoms to a measurable state, then tune a microwave signal as close as possible to the cesium's resonant frequency of 9,192,631,770 cycles per second. In this manner, the F1 achieves a precision topping 10-15 parts per second.

Cesium, though, is a grandfather clock compared to the 456 trillion cycles per second of calcium, or the 518 trillion provided by an atom of ytterbium. Hollberg's group is dedicated to tuning into these particles, which hold the key to a scary level of precision. Microwaves are too slow for this job -- imagine trying to merge onto the Autobahn in a Model T -- so Hollberg's clocks use colored lasers instead.

"Each atom has its own spectral signature," says Hollberg. Calcium resonates to red, ytterbium to purple. At their most ambitious, NIST scientists hope to wring 10-18 out of a single trapped mercury ion with a chartreuse light -- slicing a second of time into a quadrillion pieces.

At that level, clocks will be precise enough that they'll have to correct for the relativistic effects of the shape of the earth, which changes every day in reaction to environmental factors. (Some of the research clocks already need to account for changes in the NIST building's size on a hot day.) That's where the work at the Time and Frequency Division begins to overlap with cosmology, astrophysics and space-time.

By looking at the things that upset clocks, it's possible to map factors like magnetic fields and gravity variation.

Passing a precise clock over different landscapes yields different gravity offsets, which could be used to map the presence of oil, liquid magma or water underground.

At the University of Pittsburgh last fall, researchers used a NIST-produced atomic clock the size of a grain of rice to map variations in the magnetic field of a mouse's heartbeat. They placed the clock 2 mm away from the mouse's chest, and watched as the mouse's iron-rich blood threw off the clock's ticking with every heartbeat.

Since then, NIST has improved the same clock by an order of magnitude. An array of such clocks, used as magnetometers, could produce completely new kinds of imaging equipment for brains and hearts, packaged as luggable units selling for as little as a few hundred dollars apiece.

Electromagnetic fields are all around us, and change very slightly in response to our movements. A precise enough clock perturbed by these fields can give data on where things are and what's moving. Like the mouse's heart, a closely synced array could build a real-time continuous picture of the surroundings -- an area of research called passive radar. You could passively visualize pedestrians on a sidewalk

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