Atomic clock achieves record stability

[neufer]

Atomic clocks at the National Institute of Standards and Technology in Colorado have set a record for stability, scientists reported Thursday. Next, scientists will begin to measure the clocks' accuracy in keeping time. (Burrus / NIST)

[b][color=#0000FF][size=150]NIST's ytterbium atomic clock[/size][/color][/b]

---

Atomic clock achieves record stability, holds promise for tech
By Eryn Brown, LA Times, August 22, 2013

<<Atomic clocks built at the official U.S. timekeeping laboratory tick with record-breaking regularity, scientists said — marking an advance that may someday allow researchers to perform new tests of the laws of physics and engineers to perfect technologies such as GPS systems.

The ytterbium optical lattice clocks at the National Institute of Standards and Technology in Boulder, Co., 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.

Atomic clocks already serve a number of practical purposes: standards bodies like NIST depend on cesium-based clocks to set international definitions of the second and the hertz (a measure of frequency), and global positioning systems use cesium or rubidium clocks for calibration purposes.

But before the record-setting ytterbium clocks can be put to wider use, researchers will have to show that they are accurate as well as stable, said Andrew Ludlow, a physicist at NIST and lead author of a study describing the achievement that was published Thursday in the online edition of the journal Science. That will involve evaluating uncertainty in the clocks: understanding exactly how various influences change the ticking rate of the atoms — such as gravity, magnetic field, and temperature. “At that point, we’ll have both ingredients for measuring time,” Ludlow said.

Since the first accurate atomic clocks were invented in the mid-1950s, scientists have worked on improving the technology. For many years, most atomic clocks were composed of a single charged atom, or ion, that scientists excited using a microwave signal. That signal would stimulate the ion, bringing about a switch between energy levels and creating the ticking of the clock.

So-called optical clocks, like the timepieces Ludlow and his colleagues have been working with at NIST, use lasers instead of a microwave signal. The lasers have a higher frequency than the microwave signals in the older atomic clocks, which means that the optical clocks tick more rapidly. The increased frequency of the ticks makes the clocks more useful, Ludlow said — much as a yardstick that measures down to sixteenths of an inch offers more precision than one that only indicates feet.

To set the new stability record, Ludlow’s team measured the exquisite regularity of their clocks — which shoot lasers at a lattice of 10,000 neutral atoms, obtaining a sort of average ticking rate across the group — by comparing two clocks’ ticking rates to each other. MIT’s Vuletic said the group’s achievement was notable because it happened so quickly. Optical lattice clocks have only been in use for about a decade, and as recently as five to seven years ago their performance lagged being that of microwave-based clocks, said Tom O’Brian, chief of the Time and Frequency division at NIST.

O’Brian, who was not a coauthor on the Science paper, said the huge improvements in the clocks’ stability was a result of improvements to the lasers in the systems — as well as a better understanding of atomic physics among scientists, which allows clock designers to tune the interactions between atoms in the lattices and obtain better performance.

Assuming that the team can also establish high accuracy in the clocks — and Ludlow said he thought it would — scientists will put the timepieces to work. Someday, optical lattice clocks could become the new international standard, or could help industry build GPS systems that can pinpoint locations with centimeter-scale precision.

By examining the subtle effects of gravity, magnetic fields, temperature and other influences on the clocks, users might be able to “turn the clocks around” — look at changes in their tick rates to describe gravitational fields in small areas, for instance. Such tools might be useful to a geologist mapping out an oil field or to a climate scientist tracking a large sheet of ice. “As the clocks get better and better, the number of things you can do with them increases,” said Ludlow.

The next generation of atomic clocks may also be useful to scientists working to test the laws of physics, he added. Atomic clocks have already been used at NIST to test Einstein’s theory of general relativity (it held up). In the future, Vuletic said, physicists might even use the devices to test, via lab experiments, whether fundamental constants in physics are indeed unchanging. (Some researchers have expressed doubts.) “It would be revolutionary if you could show that the speed of light isn’t a constant,” he said.>>

[stephen63]

Am I correct in stating that all clocks are unaffected by time dilation as long as our reference to them don't change? However, we can see the affects gravity waves on the clock?

[neufer]

Am I correct in stating that all clocks are unaffected by time dilation as long as our reference to them don't change?

What does "our reference to them" mean?
(Our references to them in this blog probably has no affect on them.)

Atomic clocks moving with respect to us generally run slower.

However, atomic clocks in free fall (e.g., aboard the ISS or in deep space) run faster.

However, we can see the affects gravity waves on the clock?

Gravitational waves have no first order contribution to the time metric and thus cannot affect clocks.

Ocean gravity waves used to affect pendulum clocks, however:

<<Hourglasses were very popular on board ships, as they were the most dependable measurement of time while at sea. Unlike the clepsydra, or water clock, the motion of the ship while sailing did not affect the hourglass. The fact that the hourglass also used granular materials instead of liquids gave it more accurate measurements, as the clepsydra was prone to get condensation inside it during temperature changes. Seamen found that the hourglass was able to help them determine longitude, distance east or west from a certain point, with reasonable accuracy. Not until the 18th century did the Harrison brothers, John and James, come up with a marine chronometer that significantly improved on the stability of the hourglass at sea. Taking elements from the design logic behind the hourglass, they were able to invent a marine chronometer that was able to accurately measure the journey from England to Jamaica, with only a miscalculation of five seconds, in 1761.>>

[stephen63]

What does "our reference to them" mean?
(Our references to them in this blog probably has no affect on them.)

Ha Ha. I meant our relative position to it.

[stephen63]

I read this article and thought that the experiment proved that gravity waves would affect the clock.
http://physicsworld.com/cws/article/new ... -confirmed

[neufer]

I read this article and thought that the experiment proved that gravity waves would affect the clock.
http://physicsworld.com/cws/article/new ... -confirmed

Müller hopes to further improve the precision of the redshift measurements by increasing the distance between the two superposition states of the caesium atoms. The distance achieved in the current research was a mere 0.1 mm, but, he says, by increasing this to 1 m it should be possible to detect gravitational waves, miniscule ripples in the fabric of space–time predicted by general relativity but never before observed.

Interesting article, Stephen.

My read of it is that high frequency gravity waves would affect their clock but only by modulating the distance between the caesium atoms within a strong gravitational field. It is essentially a 2nd order gravitational effect dependent upon the product of the Earth's field and the field of the gravitational wave. But I could be wrong (or Müller could be wrong).