The National Institute of Standards and Technology has constructed experimental atomic clocks – extremely accurate clocks that work based on the vibrations of atoms – that have achieved three new performance records. They are so precise that they are susceptible to Earth’s gravity and could be used to study some of the frontiers of physics. They could also improve timekeeping and navigation.
As reported in Nature, the two clocks are made of thousands of ytterbium atoms trapped in a laser grid, a so-called optical lattice. The clocks ticked at a rate that matched the natural frequency of the atoms within 1.4 parts in a billionth of a billionth (1.4x1018). That’s the systematic uncertainty of the devices and it is truly minuscule.
The clocks are also very stable. The clocks’ frequency changed by just 3.2 parts in 1019 over a day. The measurements were also reproducible. The two clocks were tested against each other and only differed at the level of less than one billionth of a billionth.
"Systematic uncertainty, stability, and reproducibility can be considered the 'royal flush' of performance for these clocks," project leader Andrew Ludlow said in a statement. "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."
Gravity on the surface of the Earth is not uniform. There are subtle variations due to the heterogeneous nature of the materials beneath our feet. And gravity affects space-time. While not as dramatically as a black hole or even a star, the gravity of our planet slows down clocks compared to an ideal clock outside a gravitational field, and these ultraprecise clocks can detect these subtle changes.
If placed in different locations, these clocks would be able to precisely measure the shape of the gravitational field of Earth to within 1 centimeter (0.4 inches). Current tech has it down to several centimeters.
The ytterbium clocks are also being tipped to eventually produce a better estimation of the second, changing the unit. The second is currently estimated using cesium atom vibration.