Ion clock and search for the variation of the fine structure constant using optical transitions in Nd$^{13+}$ and Sm$^{15+}$

TL;DR

This paper investigates ultranarrow optical transitions in Nd$^{13+}$ and Sm$^{15+}$ ions, proposing their use in highly accurate atomic clocks and as probes for detecting potential variations in the fine structure constant over time.

Contribution

It identifies specific optical transitions in highly charged ions that are both insensitive to external perturbations and highly sensitive to variations in the fine structure constant, enabling precise tests of fundamental physics.

Findings

Transitions lie in the optical region and are ultranarrow.

Transitions are insensitive to external perturbations.

Potential to detect variations in $eta$ at levels of $10^{-20}$ per year.

Abstract

We study ultranarrow $5s_{1/2}$ - $4f_{5/2}$ transitions in Nd$^{13+}$ and Sm$^{15+}$ and demonstrate that they lie in the optical region. The transitions are insensitive to external perturbations. At the same time they are sensitive to the variation of the fine structure constant $\alpha$. The fractional accuracy of the frequency of the transitions can be smaller than $10^{-19}$, which may provide a basis for atomic clocks of superb accuracy. Sensitivity to the variation of $\alpha$ approaches $10^{-20}$ per year.