High-precision determination of $g$ factors and masses of ${}^{20}{\text{Ne}}^{9+}$ and ${}^{22}{\text{Ne}}^{9+}$

Abstract

We present the measurements of individual bound electron $g$ factors of ${}^{20}{\text{Ne}}^{9+}$ and ${}^{22}{\text{Ne}}^{9+}$ on the relative level of $0.1\text{parts}$ per billion. The comparison with theory represents the most stringent test of bound-state QED in strong electric fields. A dedicated mass measurement results in $m\left({}^{20}\text{Ne}\right)=19.99244016877(9)\text{u}$, which improves the current literature value by a factor of nineteen, disagrees by $4$ standard deviations and represents the most precisely measured mass value in atomic mass units. Together, these measurements yield an electron mass on the relative level of $0.1\text{ppb}$ with $m_{\text{e}}=5.48579909099(59)\times 10^{-4}\text{u}$ as well as a factor of eight improved $m\left({}^{22}\text{Ne}\right)=21.9913850982(26)\text{u}$.