Abstract: Robust and direct precision isotope shift measurements in Ba+ ions
Precision spectroscopy on atomic systems has become a powerful tool for fundamental physics tests and searches for physics beyond the standard model. Specifically, the King plot analysis method on precision mass and isotope shift data has been proposed to probe a hypothetical boson-mediated fifth force between hadrons and leptons. While King plot analyses of ytterbium and calcium have recently revealed significant anomalies, yielding important insights into higher-order nuclear structure effects, these effects have simultaneously obscured the sensitivity to possible new physics contributions. We aim to explore singly ionized barium (Ba+) as a platform for new physics searches via a King plot analysis. Owing to its large nuclear mass, it is as sensitive to new physics effects as Yb+, while it is expected to not suffer as much from nuclear deformation effects. Moreover, its two narrow quadrupole transitions, with natural linewidths of on the order of 10mHz, enable an order of magnitude higher spectroscopic resolution than Ca+. With a pair of entangled ions in a decoherence-free subspace (DFS), we aim to reach lifetime-limited interrogation times and read out isotope shifts directly via a parity measurement, circumventing the challenge of measuring optical frequencies. Further, uncertainties due to common mode systematic effects are strongly reduced in this manner. By realizing these techniques with Ba+ ions, we expect to reach a frequency resolution at the 10mHz level, potentially allowing a fifth force search at couplings 100 times weaker than in any previously reported King plot analysis. Here we present the recent progress for the realization of such a trapped Ba+ quantum sensor.