# Precision measurements on the $^{138}$Ba$^+$ $6s^2S_{1/2}-5d^2D_{5/2}$ clock transition

## TL;DR

This paper reports highly precise measurements of the $^{138}$Ba$^+$ $6s^2S_{1/2}-5d^2D_{5/2}$ clock transition frequency and Landé $g_J$ factor, improving the accuracy of the $g_J$ measurement by a factor of 30.

## Contribution

The study provides the most accurate measurement to date of the $g_J$-factor for the $D_{5/2}$ level and precise clock transition frequency for $^{138}$Ba$^+$.

## Key findings

- Clock transition frequency measured with sub-Hz accuracy.
- Landé $g_J$-factor for $D_{5/2}$ level improved 30-fold.
- Systematic effects from ac-magnetic fields are significant and corrected.

## Abstract

Measurement of the $^{138}$Ba$^+$ ${}^2S_{1/2} - {}^2D_{5/2}$ clock transition frequency and $D_{5/2}$ Land\'e $g_J$ factor are reported. The clock transition frequency $\nu_{\mathrm{Ba}^+}=170\,126\,432\,449\,333.31\pm(0.39)_\mathrm{stat}\pm(0.29)_\mathrm{sys}\,$Hz, is obtained with accuracy limited by the frequency calibration of the maser used as a reference oscillator. The Land\'{e} $g_J$-factor for the ${}^2D_{5/2}$ level is determined to be $g_{D}=1.200\,367\,39(24)$, which is a 30-fold improvement on previous measurements. The $g$-factor measurements are corrected for an ac-magnetic field from trap-drive-induced currents in the electrodes, and data taken over a range of magnetic fields underscores the importance of accounting for this systematic.