• NIST researchers have made 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.
• This new result contributes to the international effort to define the second with a much greater level of accuracy than before, enabling new scientific and technological advances.

(From left to right) Mason Marshall, David Hume, Willa Arthur-Dworschack and Daniel Rodriguez Castillo stand in front of the aluminum ion clock at NIST. With its recent improvements, the clock can pave the way for the campaign to redefine the second as we

There’s a new record holder for the most accurate clock in the world. Researchers at the National Institute of Standards and Technology (NIST) have improved their atomic clock based on a trapped aluminum ion. Part of the latest wave of optical atomic clocks, it can perform timekeeping with 19 decimal places of accuracy.   

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). This 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 published its results in Physical Review Letters

“It’s exciting to work on the most accurate clock ever,” said Mason Marshall, NIST researcher and first author on the paper. “At NIST we get to carry out these long-term plans in precision measurement that can push the field of physics and our understanding of the world around us.”

NIST physicist David Hume holds the newly modified ion trap for the aluminum ion clock. By modifying the trap, the aluminum ion and its magnesium ion partner were able to “tick” unperturbed.

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. And the aluminum ion isn’t as sensitive to some environmental conditions, like temperature and magnetic fields.

But the aluminum ion is kind of shy, 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. Magnesium doesn’t have the beautiful ticking properties of aluminum, but 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. Even with this coordination, there was still an array of physical effects to characterize, said Daniel Rodriguez Castillo, also a graduate student on the project.

“It’s a big, complex challenge, because every part of the clock’s design affects the clock,” Rodriguez Castillo said.

One challenge was the design of the trap where the ions are held, which was causing tiny movements of the ions, called excess micromotion, that were lowering the clock’s accuracy. That excess micromotion throws off the ions’ tick rate. Electrical imbalances at opposite sides of the trap were creating extra fields that disturbed the ions. The 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 newly modified ion trap for NIST’s aluminum ion clock, with an inset showing a CCD image of the aluminum-magnesium ion pair. The circle shows the position of the aluminum ion, which is dark to the camera as it can only be read out using quantum logic

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. That limited how long the experiment could run before the ions needed to be reloaded. 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.