Fountains of Atoms: Exquisite Timekeepers

If beam clocks are the workhorses of the atomic clock world, fountain clocks are the thoroughbreds — magnificent though fragile beasts that can provide near-perfect seconds, as long as they are properly cared for. 

The clocks today’s timekeepers use to realize the official second are cesium fountain clocks. These clocks take advantage of an ingenious design that turns atoms into pirouetting dancers whose almost unimaginably fast oscillations enable some of humanity’s most exquisite measurements.

Chilling Atoms Out

Fountain clocks solve a problem that has bedeviled beam clocks since they were invented in the 1950s: Atoms at room temperature buzz around like a swarm of bees, offering only a short time to measure their internal oscillations. That makes it hard to get a precise read — like trying to measure a bee's size as it zips by at 1,000 miles an hour. Fountain clocks, by contrast, grab hold of atoms, cool them until they’re nearly still and tightly control their every move.

A fountain clock starts with a cloud of around 100,000 cesium atoms floating in a vacuum chamber. Laser beams shine on the atoms from six directions. Similar to what would happen if you bombarded a rolling bowling ball with enough ping-pong balls, the laser light gradually slows the atoms and lowers the cloud’s overall temperature to near absolute zero. (The temperature of a gas to the average speed of the atoms in the gas.) 

Things happen fast in the atomic world: In less than a second, the atoms are at rest and packed into a tight, frigid ball. 

Then comes the fountain. A short laser pulse tosses the atomic ball about a meter upward. As the atoms rise, they are briefly bathed in a pulse of microwaves whose frequency is close to the cesium resonant frequency. 

Cesium atoms, like all atoms, have an inner nucleus surrounded by a cloud of subatomic particles called electrons. In cesium, an electron wanders farther from the nucleus than the rest. Fountains clocks use the fact that electrons are not only electrically charged particles but also tiny magnets. 

Depending on which direction the magnet of cesium's outer electron is pointing, the atom can be in a lower or a higher energy state. The microwave pulse is designed to put the atoms into a special quantum state that combines the two energy states. This special state oscillates in time at exactly the cesium resonant frequency. 

The atoms rise further and start to fall. On the way down, they receive a second microwave pulse. This time, the interaction between the microwaves and the atoms reveals how close the clock’s microwave frequency is to the atoms’ natural resonant frequency.

The closer the two frequencies are to one another, the more atoms end up in the higher energy state. A detector counts these atoms, and the counts are used to tune the microwave frequency toward the atomic resonance. 

An electronic counter then counts 9,192,631,770 wave peaks of the fine-tuned microwaves. The time it takes to count those cycles defines the official international second.

If this process sounds similar to what happens in a beam clock, it is — but with a crucial difference. Around one second passes between the two microwave pulses — compared to mere milliseconds in a beam clock. That long pause between interactions — long by an atom’s standard, at least! — makes the fountain clock much more sensitive to slight differences between the clock frequency and the atomic resonant frequency.

A Revolutionary Design

Although the fountain clock concept emerged in the 1950s, scientists of that era could not cool atoms. Only in the early 1980s did physicists figure out how to laser-cool atoms to near absolute zero. By the end of the decade, scientists had made the first laser-cooled atomic fountain using sodium atoms. 

Nowadays, most fountain clocks use cesium or rubidium atoms. France’s metrology institute, SYRTE, was the first to use a fountain clock for timekeeping. NIST soon followed its lead.

Fountain clocks blew away every previous clock in terms of precision and accuracy. NIST’s first cesium fountain clock, NIST-F1, started operating in 1999 with an accuracy of one second in 20 million years — more than three times better than NIST-7, and on par with SYRTE’s clock. 

Today’s best fountain clocks are accurate to under one second in 100 million years. That means that if one had started running during the time of the dinosaurs, it would have gained or lost less than one second!

Fountain clocks offer not just the most accurate operational definition of the second — they provide by far the most accurate realization of any fundamental physical unit. In fact, because fountain clocks can measure the second so precisely, physicists have redefined most of the other fundamental units in terms of the second!

The downside is that fountain clocks are extremely expensive and complicated instruments. Fewer than 30 exist in the world. Currently, just 10 countries — Canada, China, Germany, France, Italy, Japan, Russia, Switzerland, the U.K. and the United States — send data from fountain clocks to the International Bureau of Weights and Measures, located on international territory outside Paris, to calibrate universal coordinated time. 

And though they’re called clocks, most fountain clocks do not run 24/7 — and do not actually keep time. Instead, national timekeeping labs run fountain clocks periodically and use them to correct the beam clocks and hydrogen masers that tick off second after second and produce time scales we can all refer to. (A time scale is an agreed-upon system for keeping time using data from clocks around the world.)

Think of fountain clocks as the tuning forks that the rest of the world tunes its clocks to — keeping us all in sync and on the beat.