BioEd Online

Scientists have used a single mercury atom to build the world's most precise clock.

The clock is proof that optical clocks, which count miniscule fractions of a second using visible or ultraviolet laser light, can outperform the current generation of atomic timepieces. It could also open the door to a new era of precision measurements of fundamental constants, according to Jim Bergquist, a physicist at the National Institute of Standards and Technology in Boulder, Colorado, who headed the study published in the 14 July issue of Physical Review Letters¹.

Marking time

Time is measured in beats, and the most important part of measuring time accurately is having the fastest-beating pulse possible. The pulse in today's best atomic clocks comes in the form of oscillating laser-light waves, says Bergquist. "Just like the beating of the human heart, the laser is the beat of the atomic clock."

But even the best-built lasers can fall victim to a slight drift in the light's frequency, which would play havoc with a clock based on the light. To keep that from happening, the laser is constantly checked by shining it on an atom that can only be stimulated into an excited state by a very specific frequency. This calibrates the laser and keeps it in check.

Most current atomic clocks work by shining microwave lasers on to clusters of cesium atoms. The clocks count 9,192,631,770 oscillations of the laser for every second, and they are expected to only be out of true by up to 1 second after running for 60 million years.

But now Bergquist and his team have outdone this by building a clock with an ultraviolet laser (which has a higher frequency than both visible light and microwaves) and a single atom of mercury. Their calibrating mercury atom is suspended in a cryogenic trap and cooled further by lasers, to reduce any unwanted wobbling from heat. Thanks to the brightness of mercury's atomic emission upon stimulation by the laser, the calibrator can use a solitary atom rather than a cluster, reducing any odd effects that might come of bouncing a laser off a number of atoms.

This clock ticks five times more per second than the best cesium clock, Bergquist says, making it five times more accurate. And it's more reliable too: it would only be expected to drift out by a second over 400 million years.

The fine precision comes with its own problems, however. The clock is so sensitive to things such as gravitational fields that a clock at height will tick differently from one at sea level. Such differences, Bergquist notes, must be carefully corrected for.

Checking constants

The clock and others like it may also be sensitive to changes in the fundamental constants of nature, Bergquist claims. By comparing a mercury clock with a clock of another atom, such as aluminium, it should be possible to see shifts in natural constants, such as the strength of the electromagnetic force. Any shifts that are theoretically possible in such constants are thought to only shift subtly over millions or billions of years. But the high precision of optical clocks would allow you to see such changes in a few seconds, says Bergquist.

"It's a pretty impressive paper," says Patrick Gill, at the National Physical Laboratory in Middlesex, UK. Gill notes that the team made numerous improvements to existing single-atom setups and carefully measured systematic errors.

Still, he says, it's unlikely that the mercury clock will supersede existing cesium technology, at least for the time being. The second has been defined by 9,192,631,770 oscillations of a cesium clock for years, and any switch in unit would require engineers to re-jig global positioning system (GPS) receivers, spacecraft and other gadgets that depend on precise time measurements. "The standard won't change overnight," he says. "It could be up to another decade before the metrology community redefines the second."

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