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Mercury Atomic Clock Keeps Time with Record Accuracy

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, according to a paper by physicists at the Commerce Department's National Institute of Standards and Technology (NIST) in the July 14 issue of Physical Review Letters.

Posted by C1ay
14 July, 2006 12:07:27

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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.

A prototype mercury optical clock was originally demonstrated at NIST in 2000. Over the last five years its absolute frequency has been measured repeatedly with respect to NIST-F1. The improved version of the mercury clock is the most accurate to date of any atomic clock, including a variety of experimental optical clocks using different atoms and designs.

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.

"We finally have addressed the issue of systemic perturbations in the mercury clock. They can be controlled, and we know their uncertainties," says NIST physicist Jim Bergquist, the principal investigator. "By measuring its frequency with respect to the primary standard, NIST-F1, we have been able to realize the most accurate absolute measurement of an optical frequency to date. And in the latest measurement, we have also established that the accuracy of the mercury-ion system is at a level superior to that of the best cesium clocks."

Improved time and frequency standards have many applications. For instance, 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.

Scientists have long recognized that optical atomic clocks could be more stable and accurate than cesium microwave clocks, which have kept world time for more than 50 years. Even with the latest results at NIST, however, 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.

In addition, a system of additional optical clocks would be needed to continuously keep time, because primary standard clocks-such as the mercury ion or other future optical standard-are generally operated only periodically for calibrations. NIST-F1, for instance, is operated several times a year for periods of about one month to calibrate the frequencies of several NIST microwave atomic clocks that continuously track current time. These clocks contribute to an international group of atomic clocks that define the official world time.

Funding for the research was provided by NIST and the Office of Naval Research.

As a non-regulatory agency of the Commerce Department's Technology Administration, NIST promotes U.S. innovation and industrial competitiveness by advancing measurement science, standards and technology in ways that enhance economic security and improve our quality of life.

Source: NIST

Comments (9 posted):

Mercedes Benzene on 14 July, 2006 09:24:51

Yay! How exciting!! For all those interested in "time" and "time keeping", I suggest that you check out the "Time" special edition of "scientific american" magazine... It was one of the earlier months of this year I believe.... ..excellent read!

ronthepon on 15 July, 2006 01:53:00

So time keeping can be as accurate as it gets... Such a pity time is not constant everywhere.

CraigD on 15 July, 2006 08:07:16

Such a pity time is not constant everywhere.On the contrary, the more accurate a clock, the easier it is to confirm the predictions of General and Special Relativity. In 1971, the Hafele-Keating experiment (discussed in the 10/2005 thread [post=65491]�this is it�[/post]) attempted such a confirmation using then state-of-the-art cesium beam atomic clocks aboard east and west traveling commercial airliners. Although it did confirm the predictions of Relativity, its margin or error was so great that as to render the confirmation tentative. To this day, Hafele-Keating is respected more for pushing the state of the art of practical atomic clocks technology than as for confirming Relativity. These new clocks could make it possible for a 8th grader with trans-continental vacation travel to show a conclusive Relativity confirmation as a science fair project � especially if they can be miniaturized and made inexpensive enough to put on a PCMCIA card that you can slip into a laptop!

bryankennedy on 15 July, 2006 08:21:55

Cool. We got to talking about sailing at a BBQ last night and wandered off into the world of clocks. Its amazing how many big technological advancements have required accurate time keeping. For the longest time ships knew how to figure out their latitude with a sextant. But finding the longitude was especially hard to do with out accurate clocks. Cool to see the march of time progressing.

Jay-qu on 16 July, 2006 01:04:25

hmm so how would we know if its gained or lost a second after 400 million years if it is the most precise clock? is time not only defined by how accurately we can measure it?

CraigD on 16 July, 2006 02:43:49

hmm so how would we know if its gained or lost a second after 400 million years if it is the most precise clock? is time not only defined by how accurately we can measure it?Atomic clocks � cesium, mercury, or what have you � are so accurate because what they measure, such as perturbations in the magnetic moment of individual atomic nuclei � is a very fundamental interaction � it has very few �moving parts� to introduce inaccuracy. So, the same way you could inspect the working of a mechanical clock and determine that one with crude working would not be accurate, you can conclude from the basic physics of an atomic clock that, provided nothing goes wrong with the complicated machinery it uses to detect and count these fundamental interactions, the clock will be nearly as accurate as the physical universe allows. Practically speaking, things always go wrong with atomic clocks, so getting the most accurate reading from them depends on having many of them, so that you can tell when a particular one is having problems, and ignore its data. The techniques for doing this cross-checking are more complicated than the physics of how each clock operates. Hopefully, as the technology matures, atomic clocks will become so reliable that this won�t be needed, and the �cheap, nearly perfect clock on a chip� will become a reality.

Jay-qu on 17 July, 2006 01:56:54

yeah, I understand that - but what is it that defines the second for these things to be measured to to be able to say that they will loose a second after 400 million years..