The title of the world’s most accurate clock now belongs to researchers at the National Institute of Standards and Technology (NIST). Their newly developed optical atomic clock, built around a single trapped aluminum ion, boasts a fractional frequency uncertainty of 5.5 × 10−19. This remarkable precision means that it would take longer than the current age of the universe for the clock to lose or gain a single second.
This advanced clock also features a fractional frequency stability of 3.5 × 10−16 / √τ seconds, making it more than two and a half times more stable than any existing ion clock. Researchers assess optical clocks based on two primary factors: accuracy, which measures how closely they align with “true” time, and stability, which evaluates how consistently they can maintain that accuracy.
Two Decades of Innovation
The clock’s record-breaking performance is the culmination of 20 years of continuous improvements to the aluminum ion clock’s laser, ion trap, and vacuum chamber. “It’s exciting to work on the most accurate clock ever,” said Mason Marshall, a researcher at NIST and the first author of the study detailing this achievement.
The clock operates using quantum logic spectroscopy of a single 27Al+ ion, with a 25Mg+ ion trapped alongside it to assist with sympathetic cooling and to help read out the aluminum ion’s state. Aluminum is particularly well-suited for timekeeping due to its extremely steady “ticks,” which are less influenced by temperature fluctuations or magnetic fields. However, controlling aluminum with lasers has posed challenges. Thus, magnesium serves as a partner, cooling the aluminum ion and facilitating indirect measurements.
One significant upgrade involved extending the Rabi probe duration to 1 second, achieved by transferring laser stability from a remote cryogenic silicon cavity in Jun Ye’s laboratory at JILA through a 3.6 km fiber link. This advancement reduced instability by a factor of three compared to prior aluminum ion clocks.
Innovative Design Enhancements
The team also redesigned the ion trap to minimize excess micromotion—tiny unwanted movements that can disrupt timing accuracy. By utilizing a thicker diamond wafer and adjusting gold coatings on the electrodes, they were able to correct electrical imbalances in the system.
Additionally, the vacuum chamber has been reconstructed from titanium, which decreases background hydrogen gas by a factor of 150, thereby lowering collisional shifts. This redesign allows the clock to operate for extended periods without needing to reload ions. Researchers enhanced their measurement techniques by assessing the ac magnetic field from the radio-frequency trap in a direction-sensitive manner, eliminating uncertainties caused by field orientation.
These advancements mean the clock can now achieve 19-decimal-place precision in approximately 36 hours instead of the previous three weeks. “With this platform, we’re poised to explore new clock architectures—like scaling up the number of clock ions and even entangling them—further improving our measurement capabilities,” noted graduate student Willa Arthur-Dworschack.
The implications of this achievement extend beyond merely keeping time. The enhanced precision could redefine the second and open new avenues in Earth science and fundamental physics, including the ability to test whether the constants of nature remain consistent over time. The clock represents not only a significant technical milestone but also a potential tool for exploring the very fabric of reality itself.