Toward a transportable, magnetically insensitive inertial sensor: Proposal and demonstration of a Mach–Zehnder atom interferometer using barium

Atom interferometers provide precise measurements of acceleration and rotation and are promising candidates for next-generation inertial sensors. However, conventional interferometers based on the hyperfine structure of alkali atoms suffer from residual magnetic sensitivity caused by the second-order Zeeman effect. To overcome this limitation, we propose an atom-interferometric scheme using the fine-structure levels of the metastable states of the alkaline-earth atom barium (Ba). In our experiment, a thermal atomic beam of ¹³⁸Ba emitted from an effusive oven is used as the source. Ba atoms are prepared in the metastable ³D₂ state through a two-photon Raman process from the ¹S₀ ground state via the ¹P₁ state using 554 nm and 1131 nm light. A Mach–Zehnder–type interferometer is then constructed by driving Raman transitions between the ³D₁ (m=0) and ³D₂ (m=0) states, which enables magnetic-field insensitivity by several orders of magnitude. The interferometer output is read out via fluorescence detection on the ³D₁-³P₀ transition at 602 nm. We have successfully observed interference fringes from the thermal beam, demonstrating the feasibility of this magnetically insensitive Ba atom interferometer. Ongoing work aims to quantify its residual magnetic sensitivity and evaluate its potential for precision inertial sensing.