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New Atomic Clocks May Someday Redefine the Length of a Second

Researchers used three atomic clocks to measure time accurately down to the quadrillionth of a percent

This is an atomic clock that uses the predictable frequency of ytterbium atoms absorbing and emitting light to tell time. A new experiment paired a ytterbium-based atomic clock with two others that used aluminum and strontium atoms, respectively, to create an even more accurate measure of time. (Burrus / NIST)

New experiments bring researchers one step closer to redefining the length of the second, reports Emily Conover for Science News.

For decades, atomic clocks have been the gold standard when it comes to measuring the passage of time. When atomic clocks first appeared in the 1960s, they defined the second based on the properties of cesium atoms, which absorb and emit light at a reliable frequency.

These cesium-based atomic clocks “tick” about nine billion times per second, and they’re used to keep our modern, connected world in sync, report Karen Zamora, Christopher Intagliata and Farah Eltohamy for NPR.

"Every time you want to find your location on the planet, you're asking what time it is from an atomic clock that sits in the satellite that is our GPS system," Colin Kennedy, a physicist at the Boulder Atomic Clock Optical Network (BACON) Collaboration, tells NPR.

But newer atomic clocks use different atoms that oscillate or tick even faster, which means they dice up each second into even smaller pieces, according to NPR.

“There have been a lot of improvements in atomic clocks,” David Hume, a physicist at the National Institute of Standards and Technology, tells Science News.

Using these new atomic clocks to redefine the length of a second could help physicists conduct new, more accurate experiments testing weighty concepts such as relativity and dark matter, reports Sarah Wells for Inverse.

But that requires a thorough study of the differences between these new-fangled clocks. A new paper, published last week in the journal Nature, pitted three different atomic clocks against each other, per Science News. Each of the three clocks used different atoms to measure time: one used strontium, one used ytterbium and the third used just one electrically charged aluminum atom.

The ytterbium and aluminum clocks were housed in one lab in Boulder, Colorado, and the strontium clock was housed in another lab just under a mile across town, according to NPR. Researchers used a laser beam and fiber optic cable to connect the three clocks and compare their measurements.

This trio of networked atomic clocks was able to tell time with uncertainties less than a quadrillionth of a percent, according to Science News.

"These comparisons are really defining the state of the art for both fiber-based and free-space measurements--they are all close to 10 times more accurate than any clock comparisons using different atoms performed so far," says Hume in a statement.

The experiment, which lasted months, also showed that the so-called free-space link created by the laser beam provided measurements that were just as accurate as the more cumbersome optical fiber connection. Per Inverse, this opens up new experimental possibilities outside the laboratory such as land surveying.

Scientists will need to conduct more tests on these and other atomic clocks to better understand their properties before the second is officially redefined, according to Inverse.

In the meantime, Jun Ye, a physicist at the University of Colorado, Boulder and one of the study’s collaborators, tells NPR that networks of these new atomic clocks could perhaps be used as sensors by researchers hoping to detect subtle perturbations in Earth’s gravity or passing waves of dark matter.


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