[time-nuts] Updating the unit of,time: the second.

[Ole Petter Rønningen]

As supporting material; BIPM is considering when a redefinition would be appropriate: https://www.bipm.org/utils/common/pdf/CGPM-2018/CGPM-2018-Time-2-LD.pdf

And 
https://www.bipm.org/utils/en/pdf/CCTF-strategy-document.pdf annex 1 (and a few other places)

Extract:
The time for a new definition is right when ...

1. ... at least three different optical clocks (either in different laboratories, or of different species) have demonstrated validated uncertainties of about two orders of magnitude better than the best Cs atomic clocks at that time.

2. ... at least three independent measurements of at least one optical clock of milestone 1 were compared in different institutes (e.g. Df/f < 5 x 10-18) either by transportable clocks, advanced links, or frequency ratio closures.

3. ... there are three independent measurements of the optical frequency standards listed in milestone 1 with three independent Cs primary clocks, where the measurements are limited essentially by the uncertainty of these Cs fountain
clocks (e.g. Df/f< 3 x 10-16).

4. ... optical clocks (secondary representations of the second) contribute regularly to TAI.

5. ... optical frequency ratios between a few (at least 5) other optical frequency standards have been performed; each ratio measured at least twice by independent laboratories and agreement was found (with e.g. Df/f < 5x10-18).

Br,
Ole

> 29. mai 2019 kl. 15:16 skrev Attila Kinali <attila at kinali.ch>:
> 
> On Tue, 28 May 2019 22:56:35 +0200
> Mike Cook <michael.cook at sfr.fr> wrote:
> 
>> a. There is no need for a new definition.
> 
> There is. Current optical clocks deliver a lower uncertainty than
> Cs fountain clocks. Ie the reference we have is less precise than
> the measurement tools we have. Hence a redefinition of the second
> is needed.
> 
>> b. Any new definition would have to be realizable and easily verifiable. 
> 
> That's one of the main concerns and this is also the main reason why
> nobody is actively pursuing a redefinition just yet. But there are people
> out there who are already working on this topic and gathering all the
> requirements to a successful redefinition of the second. My guess,
> based on the current speed of things, is that we will have a new
> definition of the second within 10-15 years. 
> 
>> c. The first commercial cesium clocks were available in 1956, but the second did not get redefined until 1967.  There is no rush.
> 
> Which caesium beam standards were available in 1956? AFAIK the first one
> was the HP5061 and that came much later. Essen and Parry built their
> clock in the 1950s and published the results in 1955. The picture of the
> beam tube is only a small fraction of the clock itself. There are multiple
> racks full of RF equipment not shown. I would be very surprised if there
> was any company that was able to commercialize this contraption within
> only a year. Even in this large size.
> 
>>    I believe that commercial optical clocks are available but:
> 
> No. As far as I am aware of, there are no commercial optical clocks
> available. There are a few optically pumped microwave clocks out there
> (e.g. by Oscilloquartz) and even cold atom clocks (by Muquans and SDI)
> but no optical clocks.
> 
> The main problem with optical clocks is the frequency division of the
> optical signal down to something that can be used in electronics.
> This is usually done using an optical comb. But the commercially
> available ones are big, and according to Michael Wouters also quite
> expensive. There are efforts to use non-linear optical rings to
> generate these combs, but there is no commercial version available
> yet (it's a very new technique, which has been around just a few years)
> 
> The closest I know to a commercial product is what NIST reported
> in Optica just a few days ago[1] (based on two-photon absorption
> in a Rb vapor cell and using two optical combs to divide the
> 778nm down to 22GHz).
> 
>> d. There are too many flavors of optical clocks around on lab benches. So despite their increased precision and stability which flavor would get the vote?
> 
> This is another issue. Of course, a redefinition will use one atomic species
> only (with the others probably becoming secondary definitions). So far
> the jury is still out which of the atoms and which method is the best one.
> As there are not yet enough optical clocks out there, we don't have enough
> data to decide yet. And it doesn't help that an optical atomic clock takes
> several years and a quite large team to build.
> 
>            Attila Kinali
> 
> [1] "Architecture for the photonic integration of an optical atomic clock",
> by Newman et al., 2019
> https://doi.org/10.1364/OPTICA.6.000680
> -- 
> It is upon moral qualities that a society is ultimately founded. All 
> the prosperity and technological sophistication in the world is of no 
> use without that foundation.
>                 -- Miss Matheson, The Diamond Age, Neal Stephenson
> 
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