Chris,

As you noticed, earth rotation and leap seconds have been in the news a
bit the past few years, from newspaper articles to clickbait websites.
We tend to steer leap second discussions over to the LEAPSECS [1]
mailing list rather than here on time-nuts [2]. It's been convenient to
operate both lists this way because it keeps the time-nuts list pretty
calm while the leap second list tends to be rather polarized, a two
decades long heated argument on a single topic.

The short answer is that, yes, after many years of consideration and
maneuvering the idea to eliminate leap seconds in UTC is one step closer
to happening (resolution 4). The sides have run out of arguments.

On a lesser note, resolution 3 is the addition of new SI prefixes for a
couple more outrageously large and small units. They have now run out of
letters.

And, yes, resolution 5 paves the way for an optical clock [re]definition
of the SI second, which will be fun to watch. They have now run out of
atoms to try.

Don't worry, leapsecond.com and time-nuts will be around whether leap
seconds exists or not, whether the SI second is based cesium or not. So
far not a single optical clock has appeared on eBay ;-)

/tvb

[1] https://pairlist6.pair.net/mailman/listinfo/leapsecs

[2] http://leapsecond.com/time-nuts.htm

On 11/22/2022 10:21 AM, Chris Caudle via time-nuts wrote:

On Tue, 22 Nov 2022 12:21:07 -0600
Chris Caudle via time-nuts time-nuts@lists.febo.com wrote:

"A global group of scientists and government officials" is kind of an
understatement. It was the BIPM CGPM. The BIPM is the super-governmental
organisation that defines our units and that ensures that all countries
use the same definitions. Not to mention their constant effort to ensure
that the realizations of various units are actually within the specified
uncertainty. Globally! The CGPM is the meeting that the BIPM holds every
four years to revise units and coordinate other metrology related tasks.

It is quite an overstatement that we are now going to abolish leap seconds.
While the |UTC - UT1| bound will be revised and made larger than 1s in the
future (until 2035), it does not mean we are going to abolish leap seconds.
Not yet. It's politically a quite difficult topic which will take a lot
of time to resolve.

I.e. until further notice, we will still continue with the leap seconds.

Let us just hope that we don't get a negative leap second until 2035.

This has been ongoing for almost a decade. There was a presentation
at IFCS this April by Noel Dimarq from BIPM/CCTF on the current state
of affairs. There are still quite a few open problems/tasks that need
to be resolved before we can redefine the second. The biggest are that
the frequency accuracy budgets have not been validated yet and that
we do not have any optical atomic clock regularly contributing to TAI yet.

There is also a discussion going on whether we do want a single definition
as we are used to, or whether we want to have an average over multiple
atomic species.

While I think it is possible that we fulfill all the necessary criteria
until 2026 for a redefinition, given how slow things have been moving
in the past years, I am not very confident they will be ready by then.

		Attila Kinali

--
The driving force behind research is the question: "Why?"
There are things we don't understand and things we always
wonder about. And that's why we do research.
-- Kobayashi Makoto

Tom Van Baak via time-nuts writes:

Not quite: µ set a precedent for using greek letters as prefixes, so there's room for another dozen prefixes.

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
FreeBSD committer      | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

Hi Chris,

On 2022-11-22 19:21, Chris Caudle via time-nuts wrote:

Do notice that it acknowledge that this could either be a single spieces
or an ensemble definition.

No, at the best I could be there as a visitor, and that would be great
fun for sure. In those meetings I will have no official say in anything.

I have however provided input to the work of CCTF in proposing a palette
of different proposals, including an ensamble definition for a future SI
second. The ensemble definition has an interesting elegance to it, but
breaks the comfort zone.

When the can of worm was opppend in 2015 of a redefinition of the SI
second in terms of an optical transition, it was already clear that the
clocks where heading towards 100 times better than the cesium fountains.
The line of the stability improved very quickly at that time, and even
if the slope changed there is still improvements occurring.

What was also already known was that a handful of optical transitions
was occurring, such as Aluminium, Strontium etc. At the same time, what
was apparent was that multiple physical package approaches was being
used. Both neutral atoms and ions was used. Laser cooling with various
methods was applied. Single atom, lattice, etc. State-squeezing also
applied. We see super-Ramsey interrigation being used to compensate
first degree doppler. There is a richness of methods and tricks that is
applied to further improve the performance. They all end up in a similar
range of performance.

Now, this is in contrast with the times when Cesium was selected.
According to the criterions at the time, Thallium 205 was actually
considered, but in the only physical realisation of atomic beam device,
Thallium 205 had one performance edge over all other, in the magnetic
shift sensitivity is 20 times better than the next competitor of Cesium
133. However, Thallium had two techical drawbacks at the time making
physical implementation difficult, the transition frequency of
21.3108339461 GHz was at the time considerably harder to achieve than
the Cesium 9.192631770 GHz. Secondly the ionization was harder to do in
the masspectrometer needed for detection, resulting in weaker signal,
worsening the signal to noise ratio and predicted lower stability
properties.

Notice how the performance of a single type of physical package shaped
the field. Today we have removed ionization as measure for state-flip in
the Ramsey interigation, we can do that using lasers. Optical readout
was done at NIST-7, and the continuation of that is in the form of
todays fountain clock realizations that also address some of the other
issues of the original beam clocks. Observation time has been
considerably improved in the folded beam of the fountains. Some effects
is first-degree compensated for but subtle micro-lensing effects occurs
and is compensated for. Also, the sensitivity to magnetic field has been
removed as effect. Through observation of the splitting of fields, the
magnetic field can be measured and even stabilized or compensated, to
create a stable and well known shift. This have even made it into the
commercial clocks such as FTS-4040, FTS-4065, HP5071A and others.

For some time, it was even thought that Rubidium was better suited for
fountain operation than Cesium, but learning to control the issues the
shift can be estimated and compensated for and they are more or less on
par after that.

So, what we can see is that the realization form now varies greatly and
that then in combination with the spieces (atom and its particular
isotope) it becomes a race to combine all the tricks and perfecting it.
The exact frequency in the optical range varies a little, and higher
should be better, but other factors also combine to a rather complex
mixture. It's not a very clear race of speices, it's a much more
multi-dimensional field.

So, history teaches us that our selection criterions may be completely
wrong down the line, and that development can take interesting turns
that we do not fully can forsee.

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.

Another aspect is that you can end up locking yourself to a definition
that actually turns out to be a particular bad choice, that you can have
trouble to develop further, and then you end up having to redefine later.

In metrology, there is a concept of primary reference and secondary
reference which is often misunderstood or even missused. The primary
reference is a definition you can realize with a repeatability that is
well understood and require no corrections. Secondary references may
perform well, but need calibration towards the primary to correct for
it. Rubidum have had a secondary reference relationship to cesium,
despite that for compareable realizations they perform similar enough,
not withstanding the normal incorrecty distinction of cesium vs.
rubidium when they really differen in beam device vs. gas cell device.
So, we now have multiple ways of realizing an optical transition
standard for multiple spieces, which all is potential primary references.

One way of approaching this is to take a pick of a spieces and define
that, but then keep maintaing the comparison measurments that is done
between spieces and define and refine relationships to these other
spieces and let them be very well known secondary standards. That could
work well. However, we now have the technological bet here. If we now
choose the wrong spieces, it could turn out that we have trouble to
develop it's performance at the same speed as the other. Then it wasn't
as good candiate for primary standard as we thought, and we bet on the
wrong horse.

However, look at the EAL/TAI/UTC realization of SI second and
coordinated time-scales. We've learend to ensamble dissimilar spieces
and clock types to provide a really good realisation and overall
performance. We already do a range of relative clock comparisions
between the optical clocks that currently beat any measurement back
towards cesium with two digits since 1E-16 and 1E-18 has that factor of
100 in it. Actually, the limitation in realization of cesium transition
prohibit us of knowing two digits for any of our potential definitions,
we have essentially random numbers there. As we do the redefinition we
will end up guessing those and it is likely that we will not be able to
ever measure and estabilsh the actual offset we create. So, now comes
the mental leap of consider the actual definition to be an ensemble of a
number of species, in which the relative comparison measurments is
continued and the definition then keeps changing with known
relationships. As our technology develop, we can harvest the benefit of
that and the ensemble update will slowly shift in smaller and smaller
fractions at the edge of our continued development of technology. If a
spieces turns out to be a limit, its contribution will become lower, if
another spieces turns out particularly useful it's contribution will
become stronger, all as consequence of the ensemble balance.

It should also be stated that the constant of nature approach that has
been used succsessfully in the redefinition of the other SI base units
does not work for frequency, because we currently lack the sufficient
clarity in theoretical models that come even close to match the
precision we can realize in our labs. Therefore that option was dropped.

So, when Fritz Riele of PTB presented the concept of redefinitions and
asked openly for any input on how to approach this, then I presented
this view in November 2015. He was friendly enough to also point out
that his article do provide a reference to my email to him.

Having an ensamble definition is however not in most physics people
comfort zone. 7 years later, it has however not been written of, because
it does have interesting merits going forward.

While my own contribution is naturally something I enjoy pointing out, I
think it is really a very interesting field and that we do need many
solutions that we can rule out in order to learn deeper what the many
aspects of concerns is. As I have been tracking the issue since, much of
the analysis still holds. I do not see any of the spieces having a
significant benefit in performance or realizations to others. As
development continues, the research groups learn of each other and keep
reiterate their realizations to squeeze more performance, easier to
maintain, better availability (PTB reported an optical clock that was
over a two-week time had 99.5% availability of observing the optical
transition, and they did not see any major obsticle maintaining that for
periods of years).

So, while this post turns out in the long form, I think it is a very
interesting little corner of problems, and it covers more than just pure
physics or skill of making physical devices, is a whole range of issues
that needs to be considered as we move forward. We need to learn from
history and look forward.

BTW, this year it is 50 years since the speed of light was last measured
at NIST. At the time, it was a major achievement. I have suggested that
the topic should be revisited, so we could with our advancement in
technology measure it again, and to see how much we got it wrong back
then. I reminded NIST and it was brought up with relevant BIPM and UFFC
parties, but I have not seen any outcome. With some difficultie we could
setup a Krypton-86 source and measure it with todays technology, and
with difficultie means also relating it to the conditions as stated at
the time.

So, no, I was not down there to vote. I have nothing to officially say.
Hopefully I did provide some inspiration, but I doubt anyone will trace
it back to me and give me a beer for it. It was a fun mental exercise
thought, and I want to share that aspect with you.

Cheers,
Magnus

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

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

Hi Attila,

On 2022-11-24 18:14, Attila Kinali via time-nuts wrote:

Indeed. It is assumed knowledge that we have that few primary
references, and we do not want to drive it to fewer labs, but to more.
Still may not be to many more, but consider the improved availability of
doubled set of labs.

Back in 2017, the optical clocks for sure had challenges in continuous
operation, and I pointed out that it needs to improve as a requirement
for being considered a new primary standard, and we have now seen
significant improvement and that show the maturity for sure is coming in
that aspect. The maturity is needed since we need availability to the
primary standard, and we need redundancy for both resilience of failure
of individual realizations as well as the improved performance of
multiple labs. The geographical dispersion is for sure also relevant.

Yes, and we should encourage the proliferation into more labs to ensure
both availability and the improved TAI/UTC performance as a result.

We are not yet there to have commercial optical clocks, but for sure see
multiple efforts nearing that. More of the tools that goes into one has
become commercialized. There is a range of challenges. The race for
related technologies such as quantum computers creates a larger market
for the components, such that stability of market and volume benefits is
starting to be seen.

You are not wrong. This is indeed how it works. None of the commercial
clocks available is a primary standard in the metrology sense, but many
NMIs use one or more of the commercial clocks for their realization and
through the calibration to UTC achieves the calibration link. They
typically steer their clock product towards UTC to keep the UTC(k)-UTC
difference within bounds. This is not strictly needed, as calibration is
enough to know the relationship and compensate for it, but it is
desireable to have an actual clock being close to UTC at hand.

To remind people, the clocks reported into BIPM contribute to the EAL
time-scale, which is synthesized paper clock to just be optimally
stable. A number of measurements between labs establish the
UTC(k)-UTC(l) differences and the lab report the difference of
individual clocks to the UTC(k). The phase, frequency and drift of each
individual clock is estimated, and so for each UTC(k) time-scale. The
stability of each time-scale and clock is calculated and a weight of
contribution is set for each clock depending on their noise such that
the sum of the weighted clocks achieves an optimally low noise level.
The produced EAL clock will however be somewhat of in frequency, so the
small set of primary references is used to frequency reference and the
EAL error is estimated and corrected for into the TAI time-scale. Once
TAI is used, the leap-second information is added into forming the UTC
time-scale. Once this is known the UTC(k)-UTC can be calculated but also
each clock to UTC. Since the noise estimation using ADEV is linear drift
sensitive, linear drift removal needs to be done in order to use
secondary clocks experiencing linear drift. This include hydrogen
masers. Once this was added into the EAL-processing, hydrogen masers
could be directly reported and now enjoys a much heavier weight than
cesiums. Historically some rubidium clocks have also contributed to EAL/TAI.

This however keep changing. We see how sub 1 ns is more reachable, and
100 ps is closer to availabile with commercial needs. Optical frequency
comparison is regularly done with 1E-19 level of performance, matching
the optical clocks being used. Mindboggeling as it is. Optical
comparisons is only frequency comparisons, which is easier but to reach
that level of performance require quite a bit of care to compensate the
microphonics of optical fiber, but it is done and operational. Optical
comparison using satellites is also looked at and techniques is
improving. The many tools needed for international comparisons of
optical frequency clocks is coming into place for sure.

It is. It really is. Some time-nuts is on par or even better than some NMIs.

Well, you can pay to get legal traceability. It's a monetary issue in
the end.

Cheers,
Magnus

Magnus Danielson via time-nuts writes:

If optical clocks actually shone light, you could point the light
from two of them at a screen, and use the resulting pattern of
interference fringes to align them to the same exact height :-)

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
FreeBSD committer      | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

Hi Poul-Henning,

On 2022-11-25 22:47, Poul-Henning Kamp wrote:

To within the precision of the realization, yes. It's done with counters
and not interference patterns, in fact it is done with disimilar
frequencies. The optical link is some suitable free 1550 nm range
frequency which is tied to the optical transition usinga stable transfer
laser and similarly in the other end. This is done using the optical
combs to jump between frequencies and make comparisons using regular
frequency counters.

It's just like the microwave mixing stuff but at higher frequencies.

Currently the leveling on dm level can be measured.

Cheers,
Magnus

Dear fans of precise time and frequency measurements,

On Friday, 25 November 2022 23:00:12 CET Magnus Danielson via time-nuts wrote:

            accuracy

You would also have to make sure that the air in the lab is really quiet
where you do the interference fringe experiment:

The gravitational red-shift is about 10^-18/dm. The optical frequencies
optical clocks run at lie around 450 THz~=4.5*10^-14 Hz. You would have to
keep the interference fringe drift controlled to better than one per hour to
get your height measurement right!

Another fun-fact for all time-nerds on the other side of the Atlantic trying
to do the intercontinental version of this experiment:
The Doppler-red-shift due to continental drift of ~1cm/year is of the same
order of magnitude: (1 cm / year)/c = 10^-18 as well.

Cheers,
Jürgen