NIST atomic clocks now keep time well enough to improve models of Earth

Ytterbium lattice clock with state-of-the-art performance.

Simplified experimental scheme.

A newly developed experimental atomic clock can precisely tick to improve timekeeping and navigation. It additionally can detect signals from gravity, the early universe and perhaps even dark matter.

Developed at the NIST, the clock trap a thousand ytterbium molecules in optical cross sections, matrices made of laser beams. The toms tick by vibrating or exchanging between two energy levels.

The clock has achieved three new performance records:

  • Systematic uncertainty: How well the clock represents the natural vibrations, or frequency, of the atoms. NIST researchers found that each clock ticked at a rate matching the natural frequency to within a possible error of just 1.4 parts in 1018—about one billionth of a billionth.
  • Stability: How much the clock’s frequency changes over a specified time interval, measured to a level of 3.2 parts in 1019 (or 0.00000000000000000032) over a day.
  • Reproducibility: How closely the two clocks tick at the same frequency, shown by 10 comparisons of the clock pair, yielding a frequency difference below the 10-18 level (again, less than one billionth of a billionth).
NIST physicist Andrew Ludlow and colleagues achieved new atomic clock performance records in a comparison of two ytterbium optical lattice clocks. Laser systems used in both clocks are visible in the foreground, and the main apparatus for one of the clocks is located behind Ludlow.  Credit: Burrus/NIST
NIST physicist Andrew Ludlow and colleagues achieved new atomic clock performance records in a comparison of two ytterbium optical lattice clocks. Laser systems used in both clocks are visible in the foreground, and the main apparatus for one of the clocks is located behind Ludlow.
Credit: Burrus/NIST

Project leader Andrew Ludlow said, “Systematic uncertainty, stability, and reproducibility can be considered the ‘royal flush’ of performance for these clocks. The agreement of the two clocks at this unprecedented level, which we call reproducibility, is perhaps the single most important result because it essentially requires and substantiates the other two results.”

“This is especially true because the demonstrated reproducibility shows that the clocks’ total error drops below our general ability to account for gravity’s effect on time here on Earth. Hence, as we envision clocks like these being used around the country or world, their relative performance would be, for the first time, limited by Earth’s gravitational effects.”

Einstein’s theory of relativity predicts that an atomic clock’s ticking, that is, the frequency of the atoms’ vibrations, is reduced—shifted toward the red end of the electromagnetic spectrum—when observed in stronger gravity. That is, time passes more slowly at lower elevations.

While these so-called redshifts degrade a clock’s timekeeping, this same sensitivity can be turned on its head to exquisitely measure gravity. Super-sensitive clocks can map the gravitational distortion of space-time more precisely than ever. Applications include relativistic geodesy, which measures the Earth’s gravitational shape and detecting signals from the early universe such as gravitational waves and perhaps even as-yet-unexplained dark matter.

NIST’s ytterbium clocks now exceed the conventional capability to measure the geoid or the shape of the Earth based on tidal gauge surveys of sea level. Comparisons of such clocks located far apart such as on different continents could resolve geodetic measurements to within 1 centimeter, better than the current state of the art of several centimeters.

Scientists noted, “In the past decade of new clock performance records announced by NIST and other labs around the world, this latest paper showcases reproducibility at a high level. Furthermore, the comparison of two clocks is the traditional method of evaluating performance.”

“Among the improvements in NIST’s latest ytterbium clocks was the inclusion of thermal and electric shielding, which surround the atoms to protect them from stray electric fields and enable researchers to better characterize and correct for frequency shifts caused by heat radiation.”

Scientists have reported about the clock in the journal Nature.

YOU'LL ALSO LIKE