Keeping time with highly charged ions

Optical clocks offer the most precise frequency measurements to date, reaching fractional uncertainties of 10⁻¹⁸, roughly 1 second in 30 billion years. (See the article by James Bergquist, Steven Jefferts, and David Wineland, Physics Today, March 2001, page 37 .) Until now all of them have used singly charged or neutral atoms , but highly charged ions (HCIs), which are stripped of multiple or even most of their electrons, have been proposed as a promising alternative. In an HCI, the outermost electron is more tightly bound than in its neutral or singly charged counterparts. The electron is thus less sensitive to external electromagnetic influences. It’s also more influenced by the structure of the ion’s nucleus.

But HCI-based clocks have faced hurdles. For example, HCIs are tricky to cool. The megakelvin environments needed to produce such high ionization mean the ions have a lot of energy to lose to reach the millikelvin temperatures needed for a clock. What’s more, HCIs are hard to laser cool directly because their allowed transitions fall outside optical frequencies. Researchers recently found a work-around: a trick called sympathetic cooling, in which easy-to-cool ions slow down nearby HCIs through the Coulomb interaction.

Building on that and other advances, Piet Schmidt of the National Metrology Institute of Germany and his colleagues have now demonstrated the first HCI optical clock. Their proof-of-principle clock, which used an argon ion with just five electrons, Ar¹³⁺, reached a fractional uncertainty of 2.2 × 10⁻¹⁷.

The Ar¹³⁺ frequency was measured relative to a clock based on a singly charged ytterbium ion, ¹⁷¹Yb⁺. Schmidt and his colleagues found that the new clock’s primary source of uncertainty was time dilation from the Ar ion’s tiny motions in its ion trap. In the future, an improved trap design should be able to reduce the issue and its resulting uncertainty.

The researchers used the system to measure the quantum electrodynamic component of the nuclear recoil for the first time in a many-electron system. They determined the total recoil from the difference in frequency between two isotopes, ⁴⁰Ar¹³⁺ and ³⁶Ar¹³⁺. The result was nearly nine orders of magnitude more precise than previous measurements of the isotope shift and was precise enough to meaningfully compare with the predictions of atomic-structure calculations. Schmidt and his colleagues found that the theoretical predictions matched the experimental results only when the effects of quantum electrodynamics were included, even though in the literature, those effects are often ignored.

The strategies the researchers employed should translate to other HCIs. In addition to quantum electrodynamics, HCI clock transitions are highly sensitive to possible changes in the fine-structure constant and certain dark-matter candidates. (S. A. King et al., Nature 611, 43, 2022 .)