[time-nuts] Primary Time Standards
Tom Van Baak (lab/iPad)
tvb at leapsecond.com
Fri Jul 15 08:24:18 UTC 2011
A primary frequency standard is one that faithfully implements the definition of the SI second. Thus primary standards are based on Cs. But not all Cs-based clocks are primary. CSAC, for example, is not a primary standard. Rubidium, hydrogen, quartz, or pendulum clocks are not primary.
The definition spells out zero magnetic field, zero temperature (zero velocity), and zero altitude on the earth's rotating geoid. There are many other practical physics details that need to be addressed. For a good example of what it takes to make a Cs clock a primary standard see:
So strictly speaking no Cs clock actually runs at exactly 9192.631770 MHz since you need a certain amount of magnetic field to isolate the hyperfine transition, you can't run at absolute zero, no labs are actually at sea level, and atoms are not simple toys, etc.
A lot of work is required to identify, predict, and quantify a host of factors. Again, please read or glance at those papers to appreciate the work that national metrology labs do to "make copies of the SI second" for their country.
Some day the definition of the SI second will change; optical clocks offer much greater promise than microwave clocks. Note the length of a second won't change, it's just that the definition of a second will be more precise.
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