Optical cycling on thorium monoxide (ThO) for an improved test of fundamental symmetries

Optical cycling refers to repeated excitation and spontaneous emission on an electronic transition in an atom or molecule. Optical cycling in molecules can enable a wide range of quantum control and readout techniques, but unfortunately it has only been demonstrated on a small class of alkali-like or alkaline-earth-like molecules. Thorium monoxide (ThO), a molecule used in one of the most precise permanent electron electric dipole moment (eEDM) searches (ACME [1]), does not fall into this category. In this work, we demonstrate the first optical cycling on this non-conventional class over a range of experimental parameter space, including laser intensity, polarization switching rate, and interaction time. We show that both the $J = 1, 2$ rotational levels of ThO molecule are capable of cycling $11(2)$ photons on average with a single laser, at $1.9(6) \times 10^{6}~\mathrm{s}^{-1}$ and $2.3(7)\times 10^6~\mathrm{s}^{-1}$ scattering rate, respectively, before population is lost to other vibronic levels. We outline a scheme to apply this demonstrated optical cycling in an ACME-style eEDM measurement, improving the detection efficiency by over fourfold compared to non-cycling fluorescence detection. This would lead to over a twofold enhancement in the statistical sensitivity of the eEDM search. This optical cycling scheme can be further extended to scatter about 100 photons, which would enable a wider range of quantum control and sensing using ThO molecules. [1] V. Andreev, D. G. Ang, D. DeMille, J. M. Doyle, G. Gabrielse, J. Haefner, N. R. Hutzler, Z. Lasner, C. Meisenhelder, B. R. O`Leary, C. D. Panda, A. D. West, E. P. West, and X. Wu, Nature 562, 355 (2018).