[time-nuts] Yb clock - stability estimation procedure?

[Bob Camp]

Hi

The most common approach is to *assume* that the two devices are not correlated. SInce it's a negative, you really can't prove it. What you can do is to disprove it by finding and documenting  a correlation. 

ADEV it's self has a confidence level based on the number of samples taken. What is normally reported is the calculated number, not the number plus the uncertainty. The same carries over to the square root of 2. It's simply the best estimate of what's going on. As long as they say what the do / do what they say, it's not really a problem. 

The next step is typically to build a couple more devices and start doing a simultaneous  N way comparison. That will let you play with math and better estimate the performance of each of your devices. The best case would be to compare devices made by different labs using different approaches. That usually lets you spot the correlation issues between devices.

Bob

On Aug 25, 2013, at 12:47 PM, Frank Stellmach <frank.stellmach at freenet.de> wrote:

> Hello time-nuts,
> 
> The NIST paper describes the estimation of the stability of one Yb clock by simply comparing two equivalent clocks, and dividing by sqrt(2).
> This is obviously a common Metrological Practise, every time if "something better" is not existent or not available.
> 
> This practise can be found everywhere in metrology: the comparison of Cs clocks of all National Metrological Institutes, the comparison of two Josephson Junctions in situ, claiming 1e-19 stability, the comparison of the old Weston Cells, comparison of the Primary Kilograms and stating a deviation of 1e-8, and so on.
> 
> Those comparisons and stability estimations later become fixed definitions of the new definition of the unit, accompanied by setting the uncertainty to the stability estimation found before.
> 
> That means, the next definition of the second, based on the Yb optical clock would be provided by a new value and definition for the frequency of the optical excitation, with an uncertainty of something like 1e-18, or the Allan deviation given in the paper.
> 
> 
> I wonder, what is the validity of this stability estimation, as the number of the different standards is very limited, and as there's always the probability, that two different clocks /standards may drift in the same direction.
> 
> Also, there are always some physical effects left, which may (in alinear manner) shift the realization of the unit, let it be the magnetic field for a Cs clock, or an electrical filed for optical clocks.
> 
> Does anybody know, where I can find the suitable standardized metrological regulation for that problem, i.e. under which circumstances such a logical step from estimation to specification is valid, and the associated statistical calculation framework?
> 
> 
> 
> I have naively transferred this procedure to my artefact standards, i.e. 5 Vishay precision resistor , and 4 volt references.
> 
> As those groups have very small annual drift and as I don't see a logical difference in comparison to the stability estimations of those quantum references, I also claim the stability of each artefact to be in the order of the found drift within the observed group.
> 
> Now I would like to know, if I have overlooked something, and how to make a serious stability estimation by correct metrological/statistical calculations.
> 
> 
> Thanks Frank
> 
> 
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