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

• Published in: Physical review letters Vol. 125; no. 17; pp. 1 - 173002
• Main Authors: Beloy, K., Dzuba, V. A., Brewer, S. M.
• Format: Journal Article
• Language: English
• Published: College Park American Physical Society 23-10-2020
• Subjects: Barium; Black body radiation; Clocks; Electron states; Fine structure; Many body interactions; Quadrupoles; Sensitivity
• ISSN: 0031-9007; 1079-7114
• DOI: 10.1103/PhysRevLett.125.173002

We identify Ba4+ (Te-like) as a promising candidate for a high-accuracy optical clock. The lowest-lying electronic states are part of a 3PJ fine structure manifold with anomalous energy ordering, being nonmonotonic in J. We propose a clock based on the 338.8 THz electric quadrupole transition between the ground (3P2) and first-excited (3P0) 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.