For a Second-Hand Clock, It's First in Reliability

A new atomic clock may be used to redefine the second.

A team of physicists at the Paris Observatory has been testing an advanced form of atomic clock that is so accurate it can measure time to within a single second in 300 million years. The clock could one day be used to redefine the second itself.

The new timekeeper, called an optical lattice clock, uses lasers and the oscillations of strontium atoms to parcel out time. It is so unerring in its accuracy that if such a clock had been ticking since the instant of the Big Bang, some 13.8 billion years ago, it would have lost only about 46 seconds in all that time.

Impressive as that is, it's the new clock's stability and consistency in parsing out these precise units of time that sets it apart from other cutting-edge atomic clocks. Because of its precision, it could ultimately be used to establish a new and more accurate standard for the length of a second.

"For example, you might have a wristwatch that gains a second one day and loses it the next," says the Paris Observatory's Jerome Lodewyck, who led the research. "And while that might average out so that your watch seems to keep perfect time during the course of a million years, that is still not a stable watch." (See Brain Games: "It's About Time.")

The new optical clock, however, has turned out to be very stable indeed. Lodewyck and his team demonstrated that two separate optical clocks could remain as perfectly in sync as could be measured. The step up in accuracy and stability could have implications for a host of applications, ranging from astronomy to theoretical physics, to telecommunications, to GPS mapping (GPS satellites rely on precise time measurements for triangulation).

Since the 1960s, atomic clocks have been used to set the standards in timekeeping. Just like an old-fashioned grandfather clock, which uses a pendulum to measure time, atomic clocks use the very regular and measurable oscillations of caesium atoms—9,192,631,770 of them make up the standard official second. (Related: "Leap Year: How the World Makes Up for Lost Time.")

The new optical clocks use laser beams and strontium atoms, which oscillate much faster, breaking down time into much smaller intervals so that the duration of a second can be measured more precisely. (Related: "An Extra Day for Everyone—Lobbying for Leap Year Status.")

"These sorts of colorful descriptions—accurate to within one second in 300 million years—are useful for illustrating to a layman the degree of precision we can achieve, but they don't really represent our motivation for doing this sort of work," says Lodewyck. "It isn't the super-long timescales that interests us, but rather the very short ones.

"Even an accuracy of a second in 300 million years still means a lag of about 0.01 of a nanosecond over the course of a day," he says. "And that is not really so little when you think about fiberoptic communications and realize that a single telecommunications slot is 0.1 of a nanosecond.

"While I am not saying that optical clocks are needed yet in optical telecommunications, that may well happen in the future. I am confident that practical applications will arise for optical clocks, much like what happened with caesium atomic clocks," Lodewyck adds.

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