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Keeping Time with a Mercury Atom
Roland Piquepaille writes "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. A ZDNet summary contains pictures and more details about the world's most precise clock."
the isotope you mention (194) is synthetic anyways
See this page: http://www.sciencemuseum.org.uk/on-line/atomclocks /page6.asp
The same way they've been doing it for many years with current atomic clocks... You don't just have a single clock, you have a BANK of numerous atomic clocks, and use statistical sampling to correct drift. And establish a very, very accturate time base.
Slashdot gets worse every day... Pipedot: News for nerds, without the corporate slant
The 400-million year figure is still limited by technical issues, not fundamental physics. It is expected that once a few more calibration methods are tried out, that it will be able to reach its theoretical limit, which actually does turn out to be pretty close to one second in five billion years. In any case, these millions-of-years figures are not really practical-- they're just the way that clock people phrase things so that they sound good in the popular press. What really matters is that the precision that can be obtained in a much shorter period of time is much higher. Right now the mercury clock has errors at the level of about a second in 400 million years-- but a second is a lot of timing error! Perhaps a more useful (but equivalent) figure would be 2.3 ns per year, or perhaps you would rather use 44 picoseconds per week.
An example of the problem is this: for technical reasons, a small magnetic field is needed inside a cesium clock. Magnetic fields change the spacing between all atomic energy levels to some degree. For cesium, the relevant change is very small, but it is still there. What you need to do is measure the magnetic field, calculate how much it affects the frequency of the atomic transition, and correct your output frequency by the required amount. What ultimately sets the accuracy level of a given clock is how well the magnetic field shift (and dozens of others) can be corrected for.
The same is true for the mercury clock. The difference is that the systematic frequency shifts that can affect accuracy of the clock are now understood, and controllable, at a higher level of precision.