Exciting development G. Edward Marti/JILA

It’s about time. The most precise atomic clock ever made has been created by arranging strontium atoms in a grid-like pattern and then stacking those grids like pancakes.

Most atomic clocks use atoms of the isotope caesium-133. The ticking of time is measured through microwaves emitted by the electrons around those atoms jumping from a lower to higher orbit as they absorb and then lose energy from a laser.

But these clocks are constrained in how precisely they can divide time because when caesium electrons jump from a certain state to another they emit radiation with a frequency of only 9 giga-hertz, or 9 billion cycles per second. The electrons in strontium atoms emit radiation at 429,500 giga-hertz.


“In 2014, the world’s most accurate optical clock wouldn’t lose or gain one second in the entire age of the universe,” says Jun Ye at the University of Colorado at Boulder. Previous caesium clocks kept time accurately to within a second over the course of 300 million years.

Now, Ye’s group has built a strontium clock that is so precise, out of every 10 quintillion ticks only 3.5 would be out of sync – the first atomic clock ever to reach that level of precision.

Controlling the atoms

To build a more precise clock, Ye and his team designed a 3D structure that let them measure signals from more atoms at once within the width of the laser beam. But if you pack in too many, the signal can get blurred by collisions between atoms.

The latticework enabled the researchers to survey atoms that were much more densely packed together – 10 trillion atoms per cubic centimetre compared with previous clocks with 10 billion atoms per cubic centimetre – and have better control of those atoms’ interactions, minimising how often they crash into each other.

They cooled the atoms to -273˚C and trapped each one in its own spot to control the interactions between them. “Imagine a scenario where you have single-person housing in a city block. One person lives in each house and neighbours are never allowed in,” says Ye. “Each atom fits in one particular site.”

Ye says the ultracold temperatures turn the atoms into what is known as a quantum gas. “When atoms in the gaseous phase are very hot, they’re moving apart and colliding with each other,” he says. “This changes when you lower the temperature of the gas so much that these particles start to move like waves – they start to avoid each other.”

Time will tell

More precise clocks will allow us to test theories like Einstein’s general relativity – which says clocks run differently should they experience different gravitational forces – and search for gravitational waves that alter the passage of time in deep space.

Although it is the most precise atomic clock ever made, it hasn’t run long enough to confirm how accurate it will remain over time, Ye says.

Christian Lisdat at the Physikalisch-Technische Bundesanstalt in Germany is interested in how the 3D-lattice arrangement will influence the clock’s accuracy, but he is convinced the results are sound. “It is an impressive demonstration of what one can do,” Lisdat says.

Journal reference: Science, DOI: 10.1126/science.aam5538

Read more: About time: The world’s most accurate clock

We corrected the mechanism of atomic clock frequency emissions.