The National Institute of Standards and Technology building in Boulder, Colorado, houses lasers and quantum physics that unlock far more than the passage of time. NIST shares the building with the Telecommunications and Information Administration. *

Photo: Quinn Norton * View Slideshow BOULDER, Colorado – The best timepiece in the world lives deep in a '60s-style concrete government building, where it resembles nothing so much as a teenager's science-fair project: a jumble of polished lenses and mirrors converging on a gleaming silver cylinder, all protected by a tent of clear plastic nailed to a frame of two-by-fours.

Called 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.

"We basically have a Moore's Law in clocks," says Tom O'Brian, chief of the Time and Frequency Division of the National Institute of Standards and Technology, or NIST. "They improve by a factor of 10 every decade."

But that precision has brought the science of time to an existential crisis. Since 1904, when NIST purchased a pendulum clock from a German clockmaker, the institute has been America's official timekeeper, caring for the most accurate time-interval standards in the world. It still serves that role. But the latest generation of atomic clocks here, and at time labs around the world, has reached a level of precision well beyond such parochial applications, and much of the clocks' accuracy is wasted.

As a result, the institute is changing. No longer merely concerned with making sure America knows what time it is, the 400 scientists, engineers and staff at NIST's Time and Frequency Division are increasingly interested in what they can do with a clock. They're 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.

"There's a lot of room here to (do more than) just make better and better clocks," says O'Brian.

How the World's Best Clock Works

"The laser comes in from the next room," says Tom Parker, supervisory physicist for NIST's Atomic Standards Group, gesturing upward toward piping on the ceiling.

A visitor to the lab housing the NIST-F1 might be forgiven for casting an appreciative glance at a sleek refrigerator in the corner of the room, instead of the jumble of mirrors and lenses powering the F1. But like all modern atomic clocks, the NIST-F1 relies on laser light to coax precise time from elements – in this case cesium 133. Once the focused light leaves its piping, it's split into six lasers, all directed into the cylindrical cesium fountain that rises to nearly meet the ceiling.

Inside the vacuum of the fountain, the lasers focus on a gas containing around a million cesium atoms, gently slowing them to near motionlessness and gathering them into a very loose ball. Two of the lasers are oriented vertically, and they toss the ball up through the tube, then let gravity take it down again – a process that takes about a second.

During that second, a microwave signal bombards the cesium ball. When the ball reaches the bottom of the cylinder, a laser and detector examine the state of the atoms. The closer the microwave signal comes to the cesium's resonance frequency, the more the atoms will increase in fluorescence. That allows the machine to continuously adjust its microwave signal to approximate, though never reach, the precise 9,192,631,770 cycles per second of the cesium-133 atoms.

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With its fading beige walls and checkered linoleum floors, NIST's Time and Frequency Division hardly invites a sense of precision. Distracted-looking scientists in slightly rumpled button-downs roam the halls, occasionally sparing a quizzical look for outsiders. Graduate students wander in funny T-shirts, passing offices and labs crammed with manila folders and well-used tools, while cables and pipes zigzag across the ceiling.

But NIST's clocks have long been indispensable to the United States. Invisible to most of us, precision time is the heartbeat of today's digital world. Atomic clocks installed in every cellphone site manage the handoff from one tower to the next. Space-based clocks tell your car's dashboard GPS where you are. Lesser clocks keep your radio tuned, and when stability-control technology on your car kicks in, they keep you on the road and out of accidents. Those clocks are all set – through several layers of indirection – by the cesium clocks ticking in NIST's inner sanctum.

That's the present. Leo Hollberg, supervisory physicist of the Optical Frequency Measurements Group, is more concerned with the future of time. He leads the way through darkened labs glowing with laser lights that wander paths of mirrors and lenses from room to room.

These rooms are where NIST is testing a new way of tapping the precision time built into elements like calcium and ytterbium. 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 (See sidebar: How the World's Best Clock Works). In this manner, the F1 achieves a precision topping 10-15 parts per second.

At least, in theory. To tap the F1's full accuracy, scientists have to know their precise relative position to the clock, and account for weather, altitude and other externalities. An optical cable that links the F1 to a lab at the University of Colorado, for example, can vary in length as much as 10 mm on a hot day – something that researchers need to continually track and take into account. At F1's level of precision, even general relativity introduces problems; when technicians recently moved F1 from the third floor to the second, they had to re-tune the system to compensate for the 11-and-a-half foot drop in altitude.

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. "Environmental conditions can make the ticking rate vary slightly," says O'Brian.

That means 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. NIST, in short, is building the first dowsing rod that works.

On a moving ship, such a clock would change rate with the shape of the ocean floor, and even the density of the earth beneath. On a volcano, it would change with the moving and vibrating of magma within. Scientists using maps of these variations could differentiate salt and freshwater, and perhaps eventually predict eruptions, earthquakes or other natural events from the variations in gravity under the surface of the planet.

How the World's Best Clock Works (continued from page 1)

The F1 is among the most accurate frequency standards in the world, but it is scheduled to be replaced next year by an even more precise clock. "The F2 will run at low temperature instead of the (current) room temperature of the F1," says Parker.

While the atoms of F1 are effectively cooled by the lasers, everything else is somewhere around 60 degrees Fahrenheit, which fouls up the reading in small but important ways. Even worse, some cesium atoms interact with each other as they fall down the tube – which renders those atoms unusable.

The F2 will cleverly get around this problem with multiple, but less-dense, balls of cesium, in which atoms rarely touch. NIST researchers have figured out that by offsetting the lasers by 45 degrees, they can throw up multiple balls and get them to land at once, like a juggler finishing a show. When they land, the laser and detector will have far more good atoms to read – making it more accurate then ever.

Elsewhere in the Time and Frequency Division, scientists are thinking small: working to miniaturize – and commoditize – atomic clocks.

"We're trying to shrink down ... with the whole thing the size of a sugar cube and able to run on AA batteries," says O'Brian. The most obvious application is making GPS receivers much more accurate, but a tiny atomic clock would have other applications as well.

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.

The same technique for looking inward works outward too. 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, O'Brian says, "from the microwaves of the Doppler shift of someone walking."

By the time that's working, O'Brian thinks simple timekeeping will be a small part of what his lab does. What will NIST be looking at? "Probably the interaction of space, time and gravity," he says.

Cosmologists are paying attention. Some models of the early universe suggest that the laws of physics may have changed over time – indeed, might still be changing below our ability to detect. If that's true, the scientists here hope the ultra-precise clocks might provide the first proof that the fabric of space time is in flux.

For all their advances, the scientists at NIST say they're no closer to cracking time's biggest secret, O'Brian explains with a resigned chuckle.

"Time is a total mystery. What exactly is time? I can't tell you," he says. "We're measuring something with extreme accuracy, but who knows what?"

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