Optical Clocks—Already Ridiculously Accurate—Get Synchronized

A grandfather clock?s swinging pendulum marks time in seconds. A quartz watch?s oscillating crystal can segment time better: into thousandths of a second. Then there are super-powerful optical clocks, which can slice time into quadrillionths (one million times smaller than a billionth) of a second.

But there?s a problem with the precision of these new optical clocks: They are actually more precise than the technology that exists to take advantage of them. But a new advancement, detailed recently in the journal Science, presents a solution and opens up wide new possibilities for what ultra-accurate clocks can do.

The Problem

The clocks familiar to most people use oscillations like those from a pendulum or crystal as a reference point for telling time. Atomic clocks monitor atoms jumping between energy states, and for the last decade or so, scientists have been working on developing atomic clocks that use frequencies in the optical range?the kind of radiation we see as visible light?to make extremely stable clocks. "These are the best clocks we know how to make," says Bruce Warrington of the National Measurement Institute in Australia.

The problem is that these clocks are so accurate that you can?t compare one to another. If you try to send a signal from one optical clock to another optical clock in a different lab, you encounter noise from the radio or satellite transmission you?re using to send the signal. This noise distorts the signal. A regular clock isn?t precise enough in the first place to be impaired as a result of this, but an optical clock is. "We hit a limit," says Warrington. "We can?t compare the clocks at the level of their own performance."

The Solution

A team from Germany connected two optical clocks in different cities using an optical fiber?a hair-thin glass wire that transmits signals with light. Optical fibers allow you to send signals long distances with little distortion. The German team was able to send a clear signal from an optical clock at one lab in Garching, Germany, to another in Braunschweig, Germany, across a distance of 572 miles.

"This is really spectacular work," says Scott Diddams, a scientist at the National Institute of Standards and Technology (NIST) in Boulder, Colo. There are still obstacles to overcome?how to transmit these signals overseas, for example, which could require advances in satellite technology or running an optical cable under the ocean, a huge undertaking. But the achievement opens the door for future technologies to take advantage of optical clocks? extraordinary precision.

"An optical clock is so sensitive, it?s like having a super-powerful microscope," Diddams says. "You just have to find some physical aspect that the frequency of the clock depends on." To put it another way, you need to find a physical phenomenon for which the optical clocks? incredible accuracy provides a useful measurement. Once you find that, you need to be able to compare two clocks in order to measure the phenomenon: one to take readings, and a reference clock to compare those measurements with.

What to Do With Optical Clocks

Optical clocks are now one instance of a technology going ahead of any particular use for it. That is, now that scientists can synch up these extremely accurate clocks, they need to figure out what they can do with them.

One possible use is in measuring Earth?s gravitational field, which is tricky to do in full detail because it requires adjusting for the theory of relativity, since gravity can affect time. "If you take a clock and shift it from the bottom of a mountain to the top, it ticks a little faster at the top, because time is passing differently," Warrington says. Scientists already do this with GPS satellites, which adjust for relativity to match corresponding clocks on the ground. Super-accurate synced clocks could allow researchers to create more precise maps of the planet?s gravitational field, which could clue us in to the locations of underground features such as mineral deposits and the water table.

Other possible applications could emerge in radio astronomy and financial markets. In radio astronomy, readings from many earthbound radio telescopes separated by vast distances could be combined using synchronized optical clocks to make more accurate observations of the universe. "It will allow us to look much more sensitively and deeper into space than anything so far," Warrington says.

In the current financial market, computers do a lot of the trading. So, as the speed of computing continues to increase, so does the speed of the trading. "What that does is set tighter and tighter requirements on the clocks that are time-stamping the transactions," Warrington says. "It might start to matter what happens from microsecond to microsecond, or even finer." Optical clocks that can be accurately synced will be able to handle these rapid-fire transactions.

Scientists say the most exciting uses for optical clocks probably are things we haven?t come up with yet. When less sensitive atomic clocks were first built in the 1950s, there were not yet any real-world applications for them. "Then, GPS was created," Warrington says. "The history of clocks is one where the applications always follow the invention."

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