The most accurate clock ever made

A strontium clock can tick for 15 billion years and not lose a second. Cathal O’Connell reports.

Building the world's most accurate clock is no simple task. At the clock's ticking heart is a chamber filled with strontium atoms that are excited by laser light. – Ye group and Brad Baxley/JLA

Scientists have succeeded in making a clock so precise it could tick for 15 billion years – longer than the age of the Universe – without gaining or losing a second. The new research, described in Nature Communications in April, sets a world record for timekeeping and is a three-fold improvement over the previous record, set by the same clock in Boulder, Colorado, last year.

On a practical level, the optical lattice atomic clock Jun Ye and his colleagues at the US National Institute of Standards and Technology are developing could replace the caesium atomic clocks used in GPS systems, internet communications and other technologies that rely on accurate time keeping.

But if Ye can continue to improve the clock’s performance, the device will have applications far beyond keeping time. These range from detecting minerals under the ground to probing questions of fundamental physics such as the nature of dark matter.

“We feel we are in the middle of a revolution,” says Ye. “For the next 10 years I am fairly bullish to say we can make another factor of five or 10 improvement.”

All clocks need a ticking mechanism to keep the beat, and the faster they tick the better. “When you divide time in finer intervals, you get a better resolution of timekeeping,” says Jérôme Lodewyck, a researcher in atomic clocks at SYRTE (SYstème de Référence Temps-Espace) located at Paris Observatory. Early mechanical clocks, ticking to a pendulum’s languorous swing, were accurate to within about 15 seconds per day. In the 1920s, scientists realised that quartz crystals, which vibrate thousands of times per second when electricity passes through them, could make clocks accurate to within one second per day.

Optical clocks will soon be so accurate they can detect the subtle changes in gravity caused by different densities of materials lying in the ground.

Since the 1950s, the world’s best clocks have been based on electrons hopping between energy levels within an atom. As Niels Bohr described more than a century ago, excited electrons shed excess energy by giving out a photon of a specific (quantised) frequency. “Now, timekeeping belongs to quantum physicists,” says Lodewyck.

In standard caesium atomic clocks, the beat is nine billion times a second. These clocks won’t gain or lose a second in a few hundred million years. But strontium ticks at 430 trillion times a second – and that’s what Ye’s clock uses.

In the clock a cloud of strontium atoms float in a vacuum chamber. A network of lasers crisscross the chamber and hold the atoms in position. A second laser beam then bathes the atoms in visible light at a frequency of 430 trillion hertz, which sends the electrons in the atoms hopping with excitement. The clock measures the response of the strontium atoms to this laser light to calibrate its ticking. Because they are activated by visible light, strontium clocks are known as optical clocks, as distinct from the microwave frequency photons used by caesium clocks.

Thanks to some delicate fine-tuning of the chamber, Ye’s clock is three times more accurate than it was last year. Quantum levels can shift slightly depending on the atom’s environment – for example, whether it is being jostled by radio waves. To protect the atoms from such distractions, Ye’s team built a radiation shield to surround the atom chamber. They painstakingly measured any shift to the atomic levels caused by heat and figured out how to compensate for it by taking the temperature inside the chamber very exactly. Also improved was the way they trap the atoms to make sure the laser network itself does not disturb the atomic ticking.

“This paper is yet another milestone in the rapid progress of optical clocks,” Lodewyck says. And such improvements will soon lead to applications that seem straight out of science fiction.

As Einstein described 100 years ago in his general theory of relativity linking gravity with space and time, time is not a constant but flows at different rates depending on gravity. It turns out that the closer you are to the centre of the Earth, the slower time moves.

Ye’s group showed this in 2010 by simply placing one of their atomic clocks on a shelf above a second one—the lower clock ran slow just because it was one metre closer to the Earth. “The time for your feet is slower than the time for your brain,” says Ye.

Optical clocks will soon be so accurate they can detect the subtle changes in gravity caused by different densities of materials lying in the ground beneath them. “With further improvement clocks like this can be expected to measure the different gravitational potential due to, for example, a goldmine or oil or a large body of water,” says Ye.

“We still have to win a factor of 100 or so to give this kind of application,” says Lodewyck. At the current rate of improvement, that could happen within a decade.

Other potential applications are in fundamental physics research. There is the possibility of detecting gravitational waves – ripples in time predicted by Einstein, and long-sought by physicists but never directly detected – as they wash over the clock. Dark matter might be detected in a similar way.

Ye’s team is now seeking to make their lasers more stable, so that they are always tuned exactly to the frequency that excites the strontium atoms. “As you're marching towards the next decimal point, sometimes you can predict the effects, sometime you don’t,” says Ye. “There are always surprises, that's part of the fun of this game.”

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Cathal O'Connell is a science writer based in Melbourne.
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