[LEAPSECS] The next primary frequency standard?

[Tony Finch]

http://www.wired.com/wiredscience/2010/02/quantum-logic-atomic-clock

The article is misleading in several ways, most particularly about it
being "100 000 times more precise". What they mean is that the frequency
standard (a 1.1PHz UV laser) that is steered by the paired aluminium and
magnesium ions is 100 000 times higher in frequency than the standard
caesium ion frequency (the famous 9 192 631 770 Hz) which is used directly
by caesium clocks.

The important thing is the stability of their clock, which is a bit more
than 10 times better than current caesium fountain standards - it "keeps
time to within a second every 3.7 billion years" compared to "The cesium
clock loses one second every 100 million years". Still a very good
improvement but not 100 000 times.

There are some other misleading statements in that article.

There's no need to change the definition of the second because the latest
frequency standard is based on a different quantum transition. Magnesium,
mercury, and ytterbium have been used in the past, though AFAIK all the
current primary frequency standards are caesium.

The speed of light in a vacuum is a fixed constant that is used to define
the metre in terms of the second, so I don't know what questions about it
need to be resolved.

There's no question that clocks "could show the effects of general
relativity" - GPS has to deal with it as part of its normal operations,
and you can measure the effects with a few commercial frequency standards
and a mountain. http://leapsecond.com/great2005/

The last paragraph shows an enormous ignorance about the techniques used
for global time and frequency transfer and comparison.

Tony.
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