New Optical-Atomic Hybrid Clock Could Redefine the Length of a Second

May 26, 2016 | Joanne Kennell

Graphic design of a spiralling clock
Photo credit: daybeezho/flickr (CC by SA 2.0)

It could reduce the number of “lost” seconds since the start of the universe.

Time. It is fundamental to our lives, and although we can’t physically touch it, it is perceivable through motion — day to night, changing seasons, and everyone’s favorite, the aging process. We use time for GPS-based navigation, communication systems, electrical power grids, and even financial networks, where precise time is kept by a nexus of 500 atomic clocks located around the world.

In The Optical Society’s journal for high impact research, Optica, researchers recently presented a way to use optical clocks, rather than atomic ones, for more accurate time keeping. Researchers also measured the optical clock’s frequency, or its “ticking,” with unparalleled precision.

Scientists have known that optical clocks are more accurate than atomic clocks for some time, but optical clocks experience a lot of downtime (where they stop working), making them impractical to use for worldwide timekeeping. But that may be about to change.

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"We showed that even with the downtimes of today's optical clocks, they still can improve timekeeping," said Christian Grebing of The National Metrology Institute of Germany and member of the research team, in a press release.

Clocks work by counting a recurrent event with a known frequency, such as the swinging of a pendulum. With traditional atomic clocks, this recurrent event is the natural oscillation of the cesium atom, which has a frequency in the microwave region of the electromagnetic spectrum — a term used for all possible frequencies of electromagnetic radiation.

But even the best microwave (atomic) clocks can accumulate an error of about one nanosecond over a month. That is still extremely accurate, but when it comes to science, we always strive to do better.

This is where optical clocks come in. They work similar to microwave clocks, but use atoms or ions that oscillate about 100,000 times faster than microwave frequencies in the visible part of the electromagnetic spectrum.

To deal with optical clock’s downtime dilemma, the research team combined a maser with a strontium optical lattice clock. The maser, which acts like a laser except that it operates in the microwave spectral range, was used as a pendulum to bridge the downtime of the optical clock.

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

Researchers estimate they “lose” around 100 seconds or so over the entire 13.8 billion-year age of the universe due to the uncertainty of exactly how long a second is, but using this new technique, it may be possible to cut this number to just a handful of “lost seconds.” what this means is that the error involved in estimating the length of a second will be improved, reducing the current error of 0.00000000000000025.

But that doesn’t mean scientists are going to redefine the second just yet. Since 1967, the second has been defined by the International System of Units (SI) as the time that elapses during 9,192,631,770 cycles of the microwave signal, and although optical clocks do keep time about 100 times better than atomic ones, a true redefinition of the second likely won’t happen for 10 years.

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