Radium single-ion optical clock

TL;DR

This paper investigates radium ion transitions as potential optical clock standards, identifying promising candidates and analyzing their shift sensitivities to achieve high precision timekeeping at fractional uncertainties of 10^-17 to 10^-18.

Contribution

It introduces specific radium ion transitions suitable for optical clocks and evaluates their potential for high accuracy and robustness.

Findings

Identified radium ion transitions with low external field sensitivities.

Proposed a clock transition at 828 nm with minimal Zeeman and quadrupole shifts.

Projected fractional frequency uncertainties down to 10^-18 with further development.

Abstract

We explore the potential of the electric quadrupole transitions $7s\,^2S_{1/2}$ - $6d\,^2D_{3/2}$, $6d\,^2D_{5/2}$ in radium isotopes as single-ion optical frequency standards. The frequency shifts of the clock transitions due to external fields and the corresponding uncertainties are calculated. Several competitive $^A$Ra$^+$ candidates with $A=$ 223 - 229 are identified. In particular, we show that the transition $7s\,^2S_{1/2}\,(F=2,m_F=0)$ - $6d\,^2D_{3/2}\,(F=0,m_F=0)$ at 828 nm in $^{223}$Ra$^+$, with no linear Zeeman and electric quadrupole shifts, stands out as a relatively simple case, which could be exploited as a compact, robust, and low-cost atomic clock operating at a fractional frequency uncertainty of $10^{-17}$. With more experimental effort, the $^{223,225,226}$Ra$^+$ clocks could be pushed to a projected performance reaching the $10^{-18}$ level.