Quadruply Ionized Barium as a Candidate for a High-Accuracy Optical Clock

TL;DR

This paper proposes using quadruply ionized barium (Ba$^{4+}$) as a high-precision optical clock candidate, highlighting its favorable electronic structure, long-lived excited state, and low sensitivity to environmental perturbations.

Contribution

The study introduces Ba$^{4+}$ as a novel optical clock candidate, providing detailed relativistic calculations of its properties and demonstrating its potential advantages over existing clocks.

Findings

Excited state lifetime of several seconds.

Small, negative differential scalar polarizability.

Enhanced sensitivity to fine structure constant variation.

Abstract

We identify Ba$^{4+}$ (Te-like) as a promising candidate for a high-accuracy optical clock. The lowest-lying electronic states are part of a $^3P_J$ fine structure manifold with anomalous energy ordering, being non-monotonic in $J$. We propose a clock based on the 338.8 THz electric quadrupole transition between the ground ($^3P_2$) and first-excited ($^3P_0$) electronic states. We perform relativistic many-body calculations to determine relevant properties of this ion. The lifetime of the excited clock state is found to be several seconds, accommodating low statistical uncertainty with a single ion for practical averaging times. The differential static scalar polarizability is found to be small and negative, providing suppressed sensitivity to blackbody radiation while simultaneously allowing cancellation of Stark and excess micromotion shifts. With the exception of Hg$^+$ and Yb$^+$, sensitivity to variation of the fine structure constant is greater than other optical clocks thus far demonstrated.