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

[Frank Stellmach]

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