Optical Cycling of ThO for Improved Tests of Beyond Standard Model Physics

Optical cycling, or the repeated electronic excitation of an atom or molecule using only a few lasers, is necessary for most modern techniques of quantum readout and control in neutral species. Despite its ubiquity and importance, optical cycling in molecules has hitherto been mainly limited to species composed of an alkaline- or alkaline-earth metal and a halogenic or hydridic ligand. ThO, a molecule of interest to both electronic and nuclear symmetry violation experiments[1,2], does not fall into this category. However, existing spectroscopy[3] suggests that there may be a transition in ThO that allows a degree of cycling sufficient to enable improved readout, and perhaps even laser cooling (either of which could improve existing or future experiments significantly). In this talk, we confirm the closure of this transition and demonstrate the first optical cycling of ThO. To systematically confirm a cycle of ~10 photons with a single laser, we compare fluorescence measurements with Optical Bloch simulations on several rotational transitions, as well as measure population loss to vibronic leakage channels with known branching ratios. Towards future laser cooling, we also discuss spectroscopy on the L and U states in ThO to find optimal repumping schemes. We conclude with a roadmap to translating this work into a design for a next-generation Schiff moment measurement using ²²⁷ThO.

[1] Chen et al., J. Phys. Chem. A 128 (2024)

[2] ACME Collaboration. Nature 562, (2018)

[3] Kokkin et al., Phys. Rev. A 90, (2014)