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

Wed May 29 17:02:40 UTC 2019

Hi

I believe the “guess” is that all will be met within a year or three.

Bob

> On May 29, 2019, at 12:22 PM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
> 
> Ole,
> 
> Is it when all 5 conditions are met, or just any one of them?
> 
> Thanks,
> 
> Dana
> 
> 
> On Wed, May 29, 2019 at 11:00 AM Ole Petter Rønningen <opronningen at gmail.com>
> wrote:
> 
>> 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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