The world’s most accurate clock now belongs to researchers at the National Institute of Standards and Technology (NIST). Its new optical atomic clock, built around a single trapped aluminum ion, can keep time so precisely that its fractional frequency uncertainty is 5.5 × 10⁻¹⁹. That means it would take longer than the age of the universe for it to lose or gain a second. It also has a fractional frequency stability of 3.5 × 10⁻¹⁶ / √τ seconds, making it 2.6 times more stable than any other ion clock.
Optical clocks are judged on accuracy, which is how close they get to “true” time, and stability, which is how consistently they measure it. This record is the result of 20 years of steady 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, NIST researcher and first author of the study.
The clock works using quantum logic spectroscopy of a single ²⁷Al⁺ ion. A ²⁵Mg⁺ ion is trapped alongside it to help with sympathetic cooling and to read out the aluminum ion’s state. Aluminum is excellent for timekeeping because its “ticks” are extremely steady and less affected by temperature or magnetic fields, but it is hard to control with lasers. Magnesium is easier to handle, so it acts as a partner, cooling the aluminum ion and letting researchers measure it indirectly.
One big upgrade was extending the Rabi probe duration to 1 second, made possible by transferring laser stability from a remote cryogenic silicon cavity in Jun Ye’s lab at JILA via a 3.6 km fiber link. This cut instability by a factor of three compared to earlier aluminum ion clocks.
The team also redesigned the ion trap to reduce excess micromotion, tiny unwanted movements that can throw off timing. They used a thicker diamond wafer and adjusted gold coatings on the electrodes to fix electrical imbalances. The vacuum chamber was rebuilt from titanium, which reduced background hydrogen gas by 150 times and lowered collisional shifts, allowing the clock to run for days without reloading ions.
They also measured the ac magnetic field from the radio-frequency trap in a direction-sensitive way, removing uncertainty caused by field orientation.
These changes mean the clock can now reach 19-decimal-place precision in about 36 hours instead of 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,” said graduate student Willa Arthur-Dworschack.
The achievement could help redefine the second with far greater precision and open new possibilities in Earth science and fundamental physics, including testing whether the constants of nature are truly constant.
This article was generated with some help from AI and reviewed by an editor. Under Section 107 of the Copyright Act 1976, this material is used for the purpose of news reporting. Fair use is a use permitted by copyright statute that might otherwise be infringing.