Keeping time with a mercury atom

The National Institute of Standards and Technology (NIST) has announced that a new experimental atomic clock based on a single mercury atom is now at least five times more precise than NIST-F1, the U.S. standard clock. This mercury atomic clock "would neither gain nor lose a second in about 400 million years."

The National Institute of Standards and Technology (NIST) has announced that a new experimental atomic clock based on a single mercury atom is now at least five times more precise than NIST-F1, the U.S. standard clock. This mercury atomic clock "would neither gain nor lose a second in about 400 million years" while it would take "only" 70 million years to NIST-F1, based on a "fountain" of cesium atoms, to gain or lose a second. But even if this new kind of optical atomic clock is more accurate than cesium microwave clocks, it will take a while before such a design can be accepted as an international standard. Update (July 18, 2006): Windell Oskay, one of the authors of the paper I mentioned below, wrote me to tell me I made a mistake by telling you that the top picture represented this new mercury atomic clock. It is not (see below for Oskay's correction).

An experimental atomic clock based on a single mercury atom is now at least five times more precise than the national standard clock based on a "fountain" of cesium atoms.
The experimental clock, which measures the oscillations of a mercury ion (an electrically charged atom) held in an ultra-cold electromagnetic trap, produces "ticks" at optical frequencies. Optical frequencies are much higher than the microwave frequencies measured in cesium atoms in NIST-F1, the national standard and one of the world's most accurate clocks. Higher frequencies allow time to be divided into smaller units, which increases precision.

The NIST Time and Frequency Division built a first prototype of such a mercury optical clock six years ago. But now, this single atom based optical clock is the world's most precise one.

The current version of NIST-F1 -- if it were operated continuously -- would neither gain nor lose a second in about 70 million years. The latest version of the mercury clock would neither gain nor lose a second in about 400 million years.

Here is a picture of this experimental optical clock (Credit: NIST). This image comes from the Spring 2006 issue of JILA Light & Matter, a publication from NIST (PDF format, 8 pages, 1.52 MB). This clock is described on page 6 in "Partnership in Time."

The mercury atomic clock from NIST

Jim Bergquist's optical clock team produced the first optical atomic clock based on a transition of a single mercury (Hg+) ion in 2001. The newest single-ion clocks, using either Hg+ or aluminum (Al+) ions, show the best time-keeping performance ever measured, neither gaining nor losing a second in a billion years.

In fact, Windell Oskay explains that "this is a picture of a strontium atomic clock at JILA. (NIST and JILA are related but not identical. The clock projects are led by different scientists, in different labs, on different campuses.) The strontium clock is a newer technology that is at a much earlier stage of development, and correspondingly, has much lower performance than the mercury clock. The JILA document that you link to also explains this -- they talk about how the accuracy of the strontium clock is about a factor of 1000 less accurate than the mercury clock. It may catch up someday, but it is unlikely to ever be more accurate than the mercury clock."

And below is a picture from Jim Bergquist holding "a portable keyboard used to set up the world's most accurate clock. The silver cylinder in the foreground is a magnetic shield that surrounds a cryogenic vacuum system, which in turn holds the heart of the clock, a single mercury ion (electrically charged atom). The ion is brought to rest by laser-cooling it to near absolute zero (Credit: Geoffrey Wheeler, NIST)

Setting up the mercury atomic clock from NIST

For more information about this optical atomic clock, please read another NIST document, How the Mercury Clock Works.

The latest research work on this clock has been published by Physical Review Letters under the title "Single-Atom Optical Clock with High Accuracy" (Volume 97, Number 2, Article #020801, July 14, 2006). Here is a link to the abstract.

Finally, you might want to know if such an accurate clock is useful for. After all, there is only a very small chance that you're there in 400 million years. Let's return to the NIST news release for a conclusion.

Ultra-precise clocks can be used to improve synchronization in navigation and positioning systems, telecommunications networks, and wireless and deep-space communications. Better frequency standards can be used to improve probes of magnetic and gravitational fields for security and medical applications, and to measure whether "fundamental constants" used in scientific research might be varying over time -- a question that has enormous implications for understanding the origins and ultimate fate of the universe.

So is there a chance for this single atom clock to replace NIST-F1 anytime soon?

Optical clocks based on mercury, strontium or other atoms remain a long way from being accepted as standards. Research groups around the world would first need to agree on an atom and clock design to be used internationally.

So it seems that we'll have to live for a while in a world where the most accurate clock will gain or lose a second within the next 70 million years...

Sources: National Institute of Standards and Technology news release, via EurekAlert!, July 14, 2006; and various web sites

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