23 Jul 2025
In the latest wave of optical atomic clocks, it is accurate to 19 decimal places.
Researchers at the U.S. National Institute of Standards & Technology (NIST) say they have created the most accurate atomic clock to date — one that can measure time down to the 19th decimal place. This “quantum logic clock,” under continuous development for 20 years, relies on quantum computing techniques that pair an electrically charged aluminum atom (ion) with a magnesium ion.
NIST says that this new result contributes to the international effort to define the second with a much greater level of accuracy than before, and will “enable new scientific and technological advances.”
Optical clocks are typically evaluated on two levels — accuracy (how close a clock comes to measuring the ideal “true” time, also known as systematic uncertainty) and stability (how efficiently a clock can measure time, related to statistical uncertainty).
The NIST team says that the new record in accuracy comes out of 20 years of continuous improvement of the aluminum ion clock: “Beyond its world-best accuracy, 41% greater than the previous record, this new clock is also 2.6 times more stable than any other ion clock. Reaching these levels has meant carefully improving every aspect of the clock, from the laser to the trap and the vacuum chamber.”
The team has published its results in Physical Review Letters.
‘An exceptional clock’
The aluminum ion makes an exceptionally good clock, with an extremely steady, high-frequency “ticking” rate. Its ticks are more stable than those of cesium, which provides the current scientific definition of the second, said David Hume, the NIST physicist leading the aluminum ion clock project.
However the aluminum ion can be difficult to work with, Marshall explained. Aluminum is difficult to probe and cool with lasers, both necessary techniques for atomic clocks. The research group therefore paired the aluminum ion with magnesium. Although magnesium lacks the precise ticking properties of aluminum, it can be easily controlled with lasers.
“This ‘buddy system’ for ions is called quantum logic spectroscopy,” said Willa Arthur-Dworschack, a graduate student on the project. The magnesium ion cools the aluminum ion, slowing it down. It also moves in tandem with its aluminum partner, and the state of the clock can be read out via the magnesium ion’s motion, making this a “quantum logic” clock.
Redesigned ion trap
The NIST team redesigned the trap, putting it on a thicker diamond wafer and modifying the gold coatings on the electrodes to fix the imbalance of the electric field. They also made the gold coatings thicker to reduce resistance. Refining the trap this way slowed the ions’ motion and let them “tick” unperturbed.
The vacuum system in which the trap must operate was also causing problems. Hydrogen diffuses out of the steel body of a typical vacuum chamber, Marshall said. Traces of hydrogen gas collided with the ions, interrupting the clock’s operation.
The team redesigned the vacuum chamber and had it rebuilt out of titanium, which lowered the background hydrogen gas by 150 times. That meant they could go days without reloading the trap, rather than reloading every 30 minutes.
There was still one more ingredient they needed: a more stable laser to probe the ions and count their ticks. The 2019 version of the clock had to be run for weeks to average out quantum fluctuations — temporary random changes in the ions’ energy state — caused by its laser. To reduce that time, the team turned to NIST’s own Jun Ye, whose lab at JILA hosts one of the most stable lasers in the world. Ye’s strontium lattice clock, Strontium 1, held the previous record for accuracy.
Using fiber links under the street, Ye’s group at JILA sent the ultrastable laser beam 3.6 kilometers (a little more than 2 miles) to the frequency comb in the lab of Tara Fortier at NIST. The frequency comb allowed the aluminum ion clock group to compare its laser with Ye’s ultrastable one. The researchers could now probe the ions for a full second compared to their previous record of 150 ms. This improves the clock’s stability, reducing the time required to measure down to the 19th decimal place from three weeks to a day and a half.
The aluminum ion clock will also contribute to the international effort to redefine the second to much greater levels of accuracy than before. This clock can be a tool to make new measurements of Earth’s geodesy and explore physics beyond the Standard Model, such as the possibility that the fundamental constants of nature are not fixed values but actually changing.