Highly charged ions for atomic clocks and search for variation of the fine structure constant

TL;DR

This paper reviews highly charged ions with optical transitions suitable for atomic clocks and explores their sensitivity to variations in the fine structure constant, highlighting specific ions with the highest potential for fundamental physics tests.

Contribution

It identifies and analyzes specific highly charged ions that are promising for atomic clock development and for detecting variations in the fine structure constant.

Findings

Californium and einsteinium ions have the highest sensitivity to alpha variation.

Holmium ion is most suitable for experimental studies of alpha variation.

Spectral features of these ions are suitable for frequency standards.

Abstract

We review a number of highly charged ions which have optical transitions suitable for building extremely accurate atomic clocks. This includes ions from Hf$^{12+}$ to U$^{34+}$, which have the $4f^{12}$ configuration of valence electrons, the Ir$^{17+}$ ion, which has a hole in almost filled $4f$ subshell, the Ho$^{14+}$, Cf$^{15+}$, Es$^{17+}$ and Es$^{16+}$ ions. Clock transitions in most of these ions are sensitive to variation of the fine structure constant, $\alpha$ ($\alpha = e^2/\hbar c$). E.g., californium and einsteinium ions have largest known sensitivity to $\alpha$-variation while holmium ion looks as the most suitable ion for experimental study. We study the spectra of the ions and their features relevant to the use as frequency standards.