Could optical clocks redefine the length of a second?

May 25, 2016
The Cs clock transition frequency is comparedagainst the maser flywheel frequency. The acquired offset Cs yH is used to correct the classical timescale TS(Cs) generated from the maser utilizing a phase stepper (Df). An equivalent scheme is applicable when referencing the timescale TS(Sr)to an optical frequency standard. For that purpose, the clock laser light is down-converted to the microwave regime using a femtosecond frequency comb (FC) before comparing against the flywheel. Moreover, both maser offsets can be analyzed toyield the Sr clock frequency in SI units. Credit: Christian Grebing

GPS-based navigation, communication systems, electrical power grids and financial networks all rely on the precise time kept by a network of around 500 atomic clocks located around the world.

In The Optical Society's journal for high impact research, Optica, researchers present a way to use optical clocks for more accurate timekeeping than is possible with today's system of traditional . The researchers also measured an optical clock's frequency—analogous to it's "ticking"—with unprecedented precision.

A more accurate global time keeping system would allow financial networks to use more precise time stamps and thus handle even more transactions in shorter amounts of time. It would also allow GPS and other satellite-based navigation systems to provide even more precise location information.

Although optical clocks have been more accurate than clocks for some time, their complexity and resulting long downtimes have made it unpractical to use them for worldwide timekeeping.

"We showed that even with the downtimes of today's optical clocks, they still can improve timekeeping," said Christian Grebing, Physikalisch-Technische Bundesanstalt (PTB), The National Metrology Institute of Germany, who is a member of the research team. "We achieved a better performance compared to the very best microwave fountain clocks which have generally been considered less reliable and thus less suitable for the actual implementation of a practical timescale."

How long is a second?

Clocks work by counting a recurrent event with a known frequency, such as the swinging of a pendulum. For traditional atomic clocks, the recurrent event is the natural oscillation of the cesium atom, which has a frequency in the microwave region of the electromagnetic spectrum. Since 1967, the International System of Units (SI) has defined the second as the time that elapses during 9,192,631,770 cycles of the microwave signal produced by these oscillations.

Atomic clocks are extremely accurate because they are based on natural and universal atom vibrations. However, even the best atomic microwave clocks can still accumulate an error of about 1 nanosecond over a month.

Optical clocks work in a manner somewhat similar to microwave clocks but use atoms or ions that oscillate about 100,000 times higher than microwave frequencies, in the optical, or visible, part of the electromagnetic spectrum. These higher frequencies mean that optical clocks "tick" faster than microwave atomic clocks, and this contributes to their higher accuracy and stability over time. However, optical clocks do experience significant downtimes because of their higher technical complexity.

Making optical clocks practical

To deal with the downtimes that plague today's optical clocks, the researchers combined a commercially available maser with a strontium optical lattice clock at PTB, Germany's national metrology institute. The maser, which is like a laser except that it operates in the microwave spectral range, can be used as a type of reliable pendulum with limited accuracy to bridge the downtime of the optical clock. The researchers spanned the large spectral gap between the optical clock's optical frequency and the maser's microwave frequency with an optical frequency comb, which effectively divides the slower microwave-based "ticks" to match the faster "ticks" of the optical clock.

"We compared the continuously running maser with our optical clock and corrected the maser frequency as long as we had data available from the optical clock," said Grebing. "During the optical clock's downtimes, the maser runs on its own stably."

The researchers operated the maser and optical clock for 25 days, during which the optical clock ran about 50 percent of the time. Even with optical clock downtimes ranging from minutes to two days, the researchers calculated a time error of less than 0.20 nanoseconds over the 25 days.

Redefining the second

To redefine a second based on optical clocks not only requires making sure that optical clocks are practical, but it also requires comparing their frequency, or "ticking," to the old definition of the SI second. To do this, the researchers compared their strontium optical clock with two microwave clocks at PTB. Incorporating the maser strongly improved the statistical uncertainty of these measurements, allowing the researchers to measure the absolute frequency of the optical clock's strontium oscillations with the lowest uncertainty ever achieved. The obtained relative uncertainty of about 2.5×10-16 corresponds to losing only 100 seconds over the age of the universe—about 14 billion years.

"Our study is a milestone in terms of practical implementation of optical clocks," said Grebing. "The message is that we could today implement these optical clocks into the time-keeping infrastructure that we have now, and we would gain."

Although optical clocks keep time about one hundred times better than atomic clocks, Grebing said that he thinks that a true redefinition of a second might still be a decade away. It makes sense to hold off on redefining the SI second until it is clear which of the several available types of optical clock is the best for global timekeeping. Also, with the very fast pace at which optical clock technology is improving, the accuracy limit of these clocks is not yet fully known.

"We want to improve the timekeeping infrastructure all over the world by building better and better clocks and integrating them into the time-keeping infrastructure," said Grebing. "What we demonstrated is a first step towards a global improvement of timekeeping."

Explore further: Optical strontium atomic clock sets new stability record

More information: C. Grebing, A. Al-Masoudi, S. Dörscher, S. Häfner, V. Gerginov, S. Weyers, B. Lipphardt, F. Riehle, U. Sterr, C. Lisdat, "Realization of a timescale with an accurate optical lattice clock," Optica, 3, 6, 563(2016) DOI: 10.1364/optica.3.000563

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not rated yet May 25, 2016
It would seem to suggest an alternative solution: use 2 optical clocks in parallel so when one needs maintenance, the other still keeps the microwave clock accurate. When the optical clock gets back in service, it is set to sync with the other optical which can then be taken out of service to get its needed maintenance. No interruption of clocks at all with that method.
not rated yet May 25, 2016
it's not a matter of needing multiple clocks, or maintenance cycling; they already do that. It's more about getting finer precision in the measurements. More ticks per second = higher precision measure of time.
5 / 5 (1) May 25, 2016
"A more accurate global time keeping system would allow financial networks to use more precise time stamps and thus handle even more transactions in shorter amounts of time."

hahaha, just what we need, high frequency stock trading with prices fluctuating in nanaseconds
4 / 5 (1) May 25, 2016
It would seem to suggest an alternative solution: use 2 optical clocks in parallel so when one needs maintenance, the other still keeps the microwave clock accurate. When the optical clock gets back in service, it is set to sync with the other optical which can then be taken out of service to get its needed maintenance. No interruption of clocks at all with that method.

What's the mechanism for the changeover, and how do you ensure glitches in switching or slight phase differences between the clocks don't put the timing off?
not rated yet May 25, 2016
I was one of those guys involved with Apollo (a few months ago:) and my job was Apollo Tracking and timing. The timing part was atomic clocks, 2 of them, a cesium beam as primary, Rubidium as secondary (about 1/10th the accuracy of the Cesium clock, and a tertiary temperature compensated quartz crystal. Each tracking station had this arrangement and of course things were a bit looser time wise back then but when a tracking station went out of view, lost sync, we had 100 nanoseconds to sync up to the next tracking station to ensure we didn't lose data.

We managed to sync all those clocks around the world to maintain sync 24/7 in 1970 and I suspect modern clocks a thousand times more stable would be able to be sync'd just as we did back in the day but of course also a thousand times faster. If two clocks are in phase, even if it is optical, either one can, through proper sync circuits both be up at the same time and if one has to drop out, there won't be loss of sync.
5 / 5 (1) May 25, 2016

Congrats on the great job you guys did back then. After all that work, it's too bad NASA recorded over the data on the tapes.

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