# NIST Ytterbium Atomic Clocks Set Record For Stability

#### timothy posted 1 year,23 days | from the old-faithful-is-an-ambling-slacker dept.

85
New submitter bryanandaimee writes *"An optical lattice clock like the one discussed earlier on Slashdot has broken the stability record. Comparing two OLC's using trapped atoms of Ytterbium, the stability of the clocks was measured to 2 parts per quintillion (10^18). While the previously reported OLC used strontium, these clocks, built by another group, use Ytterbium. Interestingly, while the stability of the clocks is now the best in the world, the accuracy has yet to be measured."*

## Weird choice of measurements (3, Interesting)

## elfprince13 (1521333) | 1 year,23 days | (#44662305)

## Re:Weird choice of measurements (5, Funny)

## vomitology (2780489) | 1 year,23 days | (#44662317)

## Re:Weird choice of measurements (-1)

## Anonymous Coward | 1 year,23 days | (#44662335)

Fuck off.

## Re:Weird choice of measurements (0)

## Anonymous Coward | 1 year,23 days | (#44662829)

Its like a series of wobbly tubes.

## Re:Weird choice of measurements (-1)

## Anonymous Coward | 1 year,23 days | (#44662395)

but is it dividable of the orbits of Jupiter and Saturn?

## What? (-1, Offtopic)

## leuk_he (194174) | 1 year,23 days | (#44662803)

what?

## Re:What? (-1)

## Anonymous Coward | 1 year,22 days | (#44663721)

## Re:Weird choice of measurements (1)

## Freshly Exhumed (105597) | 1 year,22 days | (#44664073)

I have read your reply, am reading your reply, and will always read your reply. Hello and good bye.

## Re:Weird choice of measurements (1)

## Kizul Emeraldfire (2719487) | 1 year,21 days | (#44670847)

Time. It's a big ball of wibbly-wobbly timey-wimey stuff.

It kind of got away from you, 'eh?

## Re:Weird choice of measurements (4, Informative)

## Anonymous Coward | 1 year,23 days | (#44662389)

I guess accuracy here refers to the cycle-to-cycle jitter. That is, how much the clock edge position varies when looking at single cycles.

I guess stability here refers to wandering. That is, how much the single cycle error accumulates when looking at e.g. 1 million cycles.

## Re:Weird choice of measurements (0)

## Anonymous Coward | 1 year,23 days | (#44662617)

+5, poster actually knows what they're talking about. Oh wait, Anonymous Coward? ZERO FOR YOU!

## Re:Weird choice of measurements (4, Informative)

## AmiMoJo (196126) | 1 year,23 days | (#44663177)

I work with atomic clocks for a living. You are on the right track.

Stability is the amount of variation over time. Clock frequency changes as parts age, temperature varies and so forth. Sometimes the variation is predictable and can be compensated for, sometimes not.

Accuracy is how close the frequency is to the specified one. A cheap ebay atomic clock can do 1 second per century.

There is also jitter which is the cycle to cycle variation of the output, which could be a square or sine wave. For some applications it matters.

## Re:Weird choice of measurements (1)

## oldhack (1037484) | 1 year,22 days | (#44664933)

## Re:Weird choice of measurements (4, Informative)

## AmiMoJo (196126) | 1 year,22 days | (#44665051)

What is the reference clock against which other clocks are measured?Good question. For things like drift you can compare a new and an old clock, or compare one in a cold environment to one in a cool environment. Actually, most atomic clocks have a heater built in that maintains a stable temperature internally.

Even just comparing two clocks of the same age under the same conditions is useful, because they will never be at

exactlythe same frequency so will eventually drift apart.Beyond that it's a question of determining what the uncertainties are using physics. In other words it's a mathematical proof, rather than the result of an actual measurement. Obviously measurements are made to make sure the clock is close to other known accurate clocks, but the ultimate determination of frequency accuracy is a paper exercise.

## Re:Weird choice of measurements (1)

## oldhack (1037484) | 1 year,22 days | (#44665199)

Thanks for the reply.

In an overly simplified nutshell, we are averaging out whole lot of stuff with circular dependencies, because, given the nature of the concept that is time, that has been the best we can do, and it seems to hold up reasonably well?

## Re:Weird choice of measurements (2)

## AmiMoJo (196126) | 1 year,22 days | (#44665665)

The laws of physics that we think are probably correct state that certain things take a certain amount of time. We based our units of time off these things. Therefore we know that under perfect conditions a given clock should perform in a certain way. What we then have to do is determine how close to perfect the conditions are.

Atomic clocks are basically an engineering problem. You have to build a machine that measures something difficult to measure and then turns that measurement into some kind of usable output. All of the error comes from this side of things, and there are some very complex tests that can be done on individual parts and the output to determine how they perform.

## Re:Weird choice of measurements (1)

## oldhack (1037484) | 1 year,22 days | (#44666071)

And those laws of physics are, in turn, premised on the concept of time, which is non too simple or clearly defined, measured with clocks that are constructed based on the physics, which, in turn..

No, I'm not knocking it. In the end, we accept it (time, clock accuracy, the whole physics) as long as it works when thrown up against the wall called nature.

The subject of clock accuracy brings up the nature of time, and more I think about it, more a rabbit hole it becomes in my mind.

Thanks for the replies - they were informative.

## Re:Weird choice of measurements (1)

## AmiMoJo (196126) | 1 year,22 days | (#44666135)

Your welcome. I'm not sure what you mean about the concept of time not being clearly defined. It is, conceptually by general relativity and physically by the things that atomic clocks try to measure.

## Re:Weird choice of measurements (1)

## Man On Pink Corner (1089867) | 1 year,22 days | (#44666055)

The ultimate reference is the SI second standard that's based on the difference in energy levels associated with a particular ground-state hyperfine transition in the caesium-133 atom, approximately 9.192 631 770 GHz.

A clock is 'accurate' to the extent that its long-term stability can be traced to the SI second. It is 'stable' if its stability over a given time interval (which must be specified for the term 'stability' to have any meaning) is consistent between intervals.

## Re:Weird choice of measurements (1)

## oldhack (1037484) | 1 year,22 days | (#44666081)

Which leads me to a question:

Does time define cessium electron's frequency, or does cessium electron defines what time is?

## Re:Weird choice of measurements (1)

## Man On Pink Corner (1089867) | 1 year,22 days | (#44666369)

I'm not qualified to explain the details, but it comes down to the fact that we had to pick an easily-reproducible definition of the second, and at the time, the ground-state Cs-133 hyperfine transition was the one that made sense.

Today there are more stable clock designs based on other quantum transitions but the caesium standard serves well enough for almost all purposes that it's not worth the trouble to change it.

There is no such thing as "perfect time," or perfect knowledge of just what time it is, but we have a very good wiorking approximation of it, one that has actually outpaced Moore's Law in terms of increasing stability over the past few decades. Time is still the ultimate basis for virtually all other physical measurements, so it pays to define and understand it as precisely as we can.

## Re:Weird choice of measurements (0)

## Anonymous Coward | 1 year,23 days | (#44663297)

You guess wrong.

You guess wrong again.

## Re:Weird choice of measurements (1)

## elfprince13 (1521333) | 1 year,22 days | (#44663415)

## Re:Weird choice of measurements (1)

## msauve (701917) | 1 year,22 days | (#44663525)

## Re:Weird choice of measurements (1)

## elfprince13 (1521333) | 1 year,22 days | (#44664237)

## Re:Weird choice of measurements (1)

## ceoyoyo (59147) | 1 year,22 days | (#44664649)

Except it's highly unlikely they collect a time series. The summary suggests they have two clocks and they're comparing the two of them. Relative drift between the two indicates instability, but it doesn't say anything about how accurate they are.

## Re:Weird choice of measurements (1)

## elfprince13 (1521333) | 1 year,22 days | (#44664885)

of some sort. If I draw two lines on the bank of a river, and say "this line is data showing that a stick floated from this point to the far end of the line, and the other is data showing how far another stick floated", there really are no meaningful comparisons you can make without knowingsomethingabout the respective times over which those processes occurred.## Re: Weird choice of measurements (1)

## ceoyoyo (59147) | 1 year,22 days | (#44664977)

No, you don't. The easiest way is just to sum peaks, or rising edges, or whatever. Come back tomorrow and compare the totals for a bunch of clocks. There's your stability measurement with one number per clock. No time series.

Collecting a time series is hard, and probably impossible in this case. You have to be able to sample fast enough, and your sampling must be more consistent and more accurate than what you're measuring.

## Re: Weird choice of measurements (1)

## elfprince13 (1521333) | 1 year,22 days | (#44665019)

## Re: Weird choice of measurements (1)

## ceoyoyo (59147) | 1 year,22 days | (#44667325)

I'm not sure what you mean by amplitude. I doubt very much I'm talking about it.

Think about how you're going to measure the time between peaks without a timer that is more accurate than the one you're testing. You can't. One solution is to measure the relative difference between the time indicated by one clock, and the time indicated by another. You could do this simply by counting peaks for a period of time, or by doing something fancier such as looking at the relative phase of the two outputs. Both methods let you measure the stability (which is NOT defined as your quote suggests, according to another poster who actually works on atomic clocks), but neither method requires a more accurate time source, and neither method lets you determine the accuracy of your clock.

## Re: Weird choice of measurements (1)

## elfprince13 (1521333) | 1 year,22 days | (#44667453)

## Re: Weird choice of measurements (1)

## ceoyoyo (59147) | 1 year,22 days | (#44667607)

Counting is a special case of summing when every entry is a one. The mathematical operation is summation. To count a bunch of digital clock pulses you'd use an adder circuit. To do the equivalent in the analog world you use an integrator.

## Re:Weird choice of measurements (0)

## Anonymous Coward | 1 year,22 days | (#44663561)

No. To measure stability, you only have to measure a bunch of clock pulses and see how similar they all look

to each othernotto an ideal clock pulse.To measure accuracy, you count the number of clock pulses in X seconds, and see if that number is right. If you measure over a very long time, you may find that the clock is, on average, pretty accurate, even if its stability is not great. That is, you could have a bunch of block pulses that are sometimes longer, sometimes shorter, but on the whole average out.

Those are both different measurements. Obviously, you'd like an atomic clock to have both good stability and good accuracy, but I imaging doing either measurement well is difficult and time-consuming. The article states that they have measured stability, and so next they will move on to measuring accuracy.

## Re:Weird choice of measurements (1)

## elfprince13 (1521333) | 1 year,22 days | (#44664169)

## Re:Weird choice of measurements (1)

## ChrisMaple (607946) | 1 year,22 days | (#44664439)

## Re:Weird choice of measurements (5, Informative)

## AdamHaun (43173) | 1 year,23 days | (#44662415)

Accuracy measures how close the frequency is to the target, on average. Stability measures how the frequency drifts over time (and temperature, etc.). Accuracy is more of an absolute measurement while stability is more of a relative measurement. From the article:

and

So it sounds like accuracy is defined in terms of how well the clock reproduces the ideal frequency of the physical process it's based on. Hopefully there's a physicist or two around who can give us the exciting details.

## Re:Weird choice of measurements (1)

## elfprince13 (1521333) | 1 year,22 days | (#44663407)

## Re:Weird choice of measurements (2)

## msauve (701917) | 1 year,22 days | (#44663607)

If changes in the environment have less effect on a Y clock, or the physical processes upon which it's based have less random variation, then in might be more stable. Stability can be determined relatively quickly, using the 3-cornered hat method [wriley.com] . But with the scales at issue, it may take a considerable length of time comparing the Y clock to the official realization of time in order to determine its accuracy (or perhaps more properly, calibrate it to be accurate).

## Re:Weird choice of measurements (1)

## elfprince13 (1521333) | 1 year,22 days | (#44664151)

## Re:Weird choice of measurements (1)

## msauve (701917) | 1 year,22 days | (#44664227)

## Re:Weird choice of measurements (1)

## elfprince13 (1521333) | 1 year,22 days | (#44664305)

## Re:Weird choice of measurements (1)

## sFurbo (1361249) | 1 year,21 days | (#44674903)

To discuss what is *really* being measured gets absurd pretty quickly with modern instrumentation.

## Re:Weird choice of measurements (5, Informative)

## maxwell demon (590494) | 1 year,23 days | (#44662523)

Imagine a mechanical clock that has such a heavy minute hand that it goes much faster down than up. But it returns after exactly one hour, and even after ten years, it shows the full hour accurate to the second. This is a clock with very low stability, but quite high accuracy (for a mechanical clock, for an atomic clock that would of course still be terrible accuracy).

On the other hand, imagine a mechanic clock which doesn't have this stability problem, but the pendulum is not compensated for temperature changes. That is, when it gets warmer, the pendulum gets slightly longer and the clock goes slightly slower. Now temperature doesn't change very fast, so you'll not notice the effect in a short time span. However over time, the clock will drift away from the correct time, unless you manually correct it. This is clock with good stability, but not so good accuracy.

## Re:Weird choice of measurements (0)

## Anonymous Coward | 1 year,23 days | (#44662945)

very nice analogies

## Re:Weird choice of measurements (1)

## elfprince13 (1521333) | 1 year,22 days | (#44663391)

## Re:Weird choice of measurements (1)

## maxwell demon (590494) | 1 year,22 days | (#44663865)

Well, in one case you let it run a relatively short time and see how well it matches another clock, in the other case you let it run a very long time. compensate for short-time errors, and again check how well it matches another clock.

Clearly the accuracy measurement needs longer than the stability measurement (and moreover you need to know the stability in order to compensate for your accuracy measurement).

## Re: Weird choice of measurements (2)

## David Lee (3029417) | 1 year,22 days | (#44665601)

## Re: Weird choice of measurements (1)

## elfprince13 (1521333) | 1 year,22 days | (#44667239)

## Re: Weird choice of measurements (1)

## David Lee (3029417) | 1 year,22 days | (#44667459)

## Re: Weird choice of measurements (1)

## elfprince13 (1521333) | 1 year,22 days | (#44667541)

## Re:Weird choice of measurements (1)

## Razgorov Prikazka (1699498) | 1 year,23 days | (#44662915)

## Re:Weird choice of measurements (1)

## unixisc (2429386) | 1 year,23 days | (#44663013)

lessaccurate, not more?## Re:Weird choice of measurements (2)

## c0lo (1497653) | 1 year,23 days | (#44663039)

They measured the stability but not the accuracy? Aren't both essentially frequency measurements? Can someone explain what data you would collect that would allow you to determine one but not the other?

Preliminaries [wikipedia.org] : stability (reproducibility) determines precision - in this case, how repeatable is the result obtained by measuring a defined duration. Accuracy would be how close is the measured value to the real one.

As long as the the stability is high, one can organise different experiments/measurements to determine the accuracy.

Assume you measure the difference between the time required for a laser beam to bounce from a mirror on the Moon. If your timer is not accurate, you can't be sure on how close the measured distance is to the reality.

But here comes the benefit of being 10^18 precise: assume you want to measure the

variation of the measured distancebetween two consecutive bounces. Now, the Moon-Earth distance is roughly 3.844e+8 meters - take it twice into consideration. Assuming perfect sensors, a precision of 10^18 allows you to detect a variation of7.688e+8 / 1e+18 = 7.688 e-10 m = 0.76 nm; to put it in perspective: you could measure the displacement, caused by a cosmic radiation impact, of the atoms in a CPU gate placed on the Moon.Where's the applicability you ask? Theoretically, being able to measure that precise (even if not accurate), one should be able to organize experiments on gravitational waves (causing space metric variations) without asking a supernova explosion as the trigger for the experiment.

## Re:Weird choice of measurements (1)

## Chemisor (97276) | 1 year,22 days | (#44665881)

A stopped clock is perfectly stable, but is only accurate twice a day.

## Re:Weird choice of measurements (1)

## sFurbo (1361249) | 1 year,21 days | (#44674887)

## Re:Weird choice of measurements (1)

## MW_NIST_Scientist (3036755) | 1 year,15 days | (#44727137)

## ACCORDING TO WHOM ?? (0)

## Anonymous Coward | 1 year,23 days | (#44662321)

Them !!

TWO !!

## Does anybody really know what time it is? (-1)

## Anonymous Coward | 1 year,23 days | (#44662353)

Does anybody really know what time it is?

Does anybody really care? [youtube.com]

## Even a stopped clock... (2, Insightful)

## Anonymous Coward | 1 year,23 days | (#44662387)

has great stability.

## Re:Even a stopped clock... (1)

## maxwell demon (590494) | 1 year,23 days | (#44662557)

But absolutely terrible accuracy.

## Re:Even a stopped clock... (0)

## Anonymous Coward | 1 year,23 days | (#44662725)

Its still perfectly accurate and stable twice a day.

## Re:Even a stopped clock... (1)

## msauve (701917) | 1 year,22 days | (#44663703)

## Nice to know (0)

## Anonymous Coward | 1 year,23 days | (#44662431)

There ain't no jitter in the ytter!

## Kathryn17812706 (-1, Offtopic)

## Kathryn (3028993) | 1 year,23 days | (#44662439)

## Re:Kathryn17812706 (0)

## Anonymous Coward | 1 year,23 days | (#44662529)

My dog owes your mom and sister 2 silver dimes for the three way last night.

## Re:Kathryn17812706 (1)

## maxwell demon (590494) | 1 year,23 days | (#44662535)

She makes lots of question marks? Sounds like a boring thing to do.

## Re:Kathryn17812706 (1)

## Ganty (1223066) | 1 year,23 days | (#44662943)

Well, let's see, if your buddy's step-sister made ?21143 last month and her rate is ?61 per hour then she worked 346.6 hours. That's about an 87 hour week, no thank you!!

Ganty

## Re:Kathryn17812706 (0)

## Anonymous Coward | 1 year,22 days | (#44664797)

You're a bit fucking pathetic - can't even include the link in your crappy spam post (which is the *only fucking point of it*). You're not even worth the 0.0001c you might have got paid for this.

## a newer more stable clock? (2)

## Gravis Zero (934156) | 1 year,23 days | (#44662495)

so... it has come to this.

## Re:a newer more stable clock? (0)

## Anonymous Coward | 1 year,23 days | (#44662929)

+1 xkcd to you, good sir.

## haha ok (0)

## Anonymous Coward | 1 year,23 days | (#44662543)

NIST? They're the least accurate "scientific" community in existence.

## Re:haha ok (0)

## Anonymous Coward | 1 year,22 days | (#44663527)

I think you misspelled "IPCC."

## hello!` (0)

## Anonymous Coward | 1 year,23 days | (#44662549)

so slashdot apparently automatically deletes any comment i make, fun stuff.

## Re:hello!` (1)

## maxwell demon (590494) | 1 year,23 days | (#44662559)

so slashdot apparently automatically deletes any comment i make, fun stuff.

If it automatically deletes your comments, how come that I can read this one?

Maybe you've not set your threshold appropriately.

## Re:hello!` (0)

## Anonymous Coward | 1 year,22 days | (#44664999)

so slashdot apparently automatically deletes any comment i make, fun stuff.No, you're just a retard who either doesn't understand how to click the submit button or doesn't understand how to look at your own 0 or -1 posts. Try rebooting yourself with a new OS install (use a baseball bat or similar).

## Stability and Precision (5, Interesting)

## fahrbot-bot (874524) | 1 year,23 days | (#44662571)

From the following newspaper article: http://www.latimes.com/science/sciencenow/la-sci-sn-atomic-clock-stability-nist-20130822,0,6785801.story [latimes.com]

The ytterbium optical lattice clocks at the [ NIST ], achieved a so-called stability of one part in 10^18. In plain English, that means that "

if a clock had been running since the Big Bang, by now it would only be off by one second,” said Vladan Vuletic, a physicist at MIT who was not involved in the work.[these clocks] could help industry build GPS systems that can rapidly pinpoint locations with sub-centimeter-scale precision.

In addition, I heard a report on NPR that said researchers studying Einstein's theory of general relativity could make use of this clock to more precisely measure how time is different depending on the surrounding gravitational force -

over a change in altitude of 1 inch.## Re:Stability and Precision (1)

## Anonymous Coward | 1 year,23 days | (#44662741)

In addition, I heard a report on NPR that said researchers studying Einstein's theory of general relativity could make use of this clock to more precisely measure how time is different depending on the surrounding gravitational force -

over a change in altitude of 1 inchSuch advanced technology.....such archaic measurement units....

## Re:Stability and Precision (2)

## maxwell demon (590494) | 1 year,23 days | (#44662755)

Yeah, they should have specified the height as 72 DTP points instead. :-)

## Re:Stability and Precision (1)

## ebno-10db (1459097) | 1 year,22 days | (#44663777)

Archaic, nothing - the metric system is for people who can't do arithmetic.

## Re:Stability and Precision (0)

## Anonymous Coward | 1 year,22 days | (#44665047)

s/ the metric system is for people who can't do arithmetic./ the metric system is for people who don't want to do pointless arithmetic./

## Re:Stability and Precision (1)

## yusing (216625) | 1 year,22 days | (#44666615)

[these clocks] could help industry build GPS systems that can rapidly pinpoint locations with sub-centimeter-scale precisionNow, if only I could get transceivers to strap to fruit flies, I might find out where they're all coming from.

## Re:Stability and Precision (1)

## davewoods (2450314) | 1 year,16 days | (#44716029)

Thanks for the article link!

## They weren't so sure, at first (1)

## Provocateur (133110) | 1 year,23 days | (#44663287)

Ytterbium, better than the prototype at least.

Thanks, I'll be here all week. Sorry.

## Right use of the material? (1)

## Erbo (384) | 1 year,22 days | (#44666163)

## Re:Right use of the material? (1)

## davewoods (2450314) | 1 year,16 days | (#44717457)

Thank you.