[time-nuts] Exact Rubidium frequency

rlutwak at comcast.net rlutwak at comcast.net
Sat Jun 30 21:16:09 EDT 2007

The Hydrogen ground state hyperfine frequency is actually calculable from first principles, 1420405751.76 Hz.

The cesium ground state hyperfine frequency is defined by international agreement to be 9192631770 Hz (for a single atom, at rest, in free space, at T= 0 K).

To the best of my knowledge, the best measurement of the Rb ground state hyperfine frequency comes from the LPTF group in Paris, 6 834 682 610.904 333 Hz, with an accuracy of a few parts in 10^15, by direct comparison of a Rb fountain to a cesium fountain.

Anything else is a perturbation.  The job of the spectroscopist (and the clockmaker) is to understand, measure, and correct for the perturbations due to the apparatus and interrogation technique. Perturbations may include pressure and collisional shifts, AC and DC magnetic and electric fields, wall shifts, gravitational and relativistic corrections, electronic effects, etc. etc. etc.

Robert Lutwak, Senior Scientist 
Symmetricom - Technology Realization Center 
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Beverly, MA 01915 

RLutwak at Symmetricom.com (Business) 
Lutwak at Alum.MIT.edu (Personal) 

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-------------- Original message -------------- 
From: Peter Vince <pvince at theiet.org> 

> Everyone on the 'net agrees on the frequency of the Caesium 
> oscillation, but the precise frequency quoted for Rubidium varies by 
> a few Hz :-( Now I realise that you can make it anything you like 
> (within reason) by altering the magnetic field, and perhaps different 
> sources quote different figures which are more or less easy to 
> synthesise? 
> On the (USA) NIST web-page 
> (http://tf.nist.gov/general/enc-re.htm#rubidiumoscillator) they quote 
> 6,834,682,608 Hz, but the (German) PTB (on 
> http://www.ptb.de/en/org/4/44/441/info2_e.htm) give 6,384,682,612.8 
> Hz, and the (British) NPL quotes 6,834,682,610.904 324 Hz in a 
> powerpoint presentation 
> http://www.npl.co.uk/time/club/meeting1/secondaryrepsec.pdf 
> (admittedly for a Rubidium fountain). 
> I have also seen the frequency of a Hydrogen Maser given as both 
> 1,420,405,751 Hz and ...752 Hz. I would be very surprised if the 
> frequency was an exact whole number of Hertz different from Caesium, 
> so perhaps this is just rounded for convenience as again it can be 
> steered to anywhere you like? 
> Would anyone care to comment on this differences please? 
> Thank you, 
> Peter Vince (G8ZZR, London) 
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