[time-nuts] The SI second and the ease of realization (was: leap seconds finally being retired?)

[Attila Kinali]

On Wed, 23 Nov 2022 23:50:35 +0100
Magnus Danielson via time-nuts <time-nuts at lists.febo.com> wrote:

> Further, there is additional concerns here. One such concern is one of 
> both political and practical aspect. If we come up with a definition of 
> a single spieces that for it's realization turns out to be hard to 
> realize or duplicate, with the needed repeatability (our ability to 
> build multiple giving the same measure), we can end up in a new kg 
> reference unit situation, in which only one lab (or possibly very few) 
> have access to the actual reference. This creats both practical issues, 
> you can say it is a problem of democratic access even, and there can 
> become political conotations to that which we really want to avoid.

I would like to add here, that we already have this problem.
If you look at the current list of primary standards contributing
to TAI https://webtai.bipm.org/database/show_psfs.html you see that
it's only a few labs. And it was just SYRTE, PTB, NIST and INRIM
20 years ago. Also note the huge gaps most of the primary standards have.
I.e. very few are run once a month, much less continuous. And this is
a technology that's quite mature and well understood.[1]

We currently only have 5 optical standards that make contributions to
TAI regularly (if I'm not mistaken, all of them are lattice clocks)
This is a quite low number, considering how many labs are currently
working on optical atomic clocks. But keep in mind, that until recently,
we didn't have that many labs with primary references either.
All the same, I have no doubt that this number will increase in the
comming years.

I also would like to add that for a (legal) realization of the second
at some NMI there is no need to run a primary frequency standard. All
that is needed are some stable/low noise secondary frequency standards,
e.g. a iMaser3000 or a 5071, a phase/frequency microstepper and a link
to the TAI/UTC network to contribute to EAL. Then, through Circular T,
one gets a frequency reading of the standards and a time offset of
the local UTC(k) and can correct for that. That's actually what most
NMI do, as far as I am aware of. (Please correct me if I am wrong)

This works because even a lowly 5071 has a long term stability down
to 1e-13 (1e-14 with the high perf tube), which is plenty enough
for most commercial applications and because we still have a hard
time to get time diseminated with better than 1ns uncertainty.

Yes, this means that any time-nut with a GPS disciplined Rb
gets to within 1-2 orders of magnitude of an average NMI.
And yes, I find this incredible!
Sure, there is no legal traceability for a time-nuts lab, but
who needs that anyways? 😜

			Attila Kinali


[1] If you wonder why the 6 Rb fountains from USNO are not listed here, then
it's because they are contributing to EAL as secondary frequency standards.
While they could be equally well be run as primary frequency standards
using the secondary representation of the second[2], a decision was made
to let them contribute to EAL instead of TAI.

[2] "Recommended Values of Standard Frequency ... Rb 87 6.835GHz"
https://www.bipm.org/documents/20126/69375083/87Rb_6.835GHz_2021.pdf/70065a76-1e50-254e-09b6-c29187263da0
-- 
In science if you know what you are doing you should not be doing it.
In engineering if you do not know what you are doing you should not be doing it.
        -- Richard W. Hamming, The Art of Doing Science and Engineering