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

Alexander Frenett1,*, Dorothy Gan1, Nicholas Emtage1, Monika Fouad1, Sebastian Miki-Silva1, and Xing Wu1,2

  • *Contact author: frenetta@msu.edu, frenett@frib.msu.edu
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Phys. Rev. Research 8, 033236 – Published 27 August, 2026

DOI: https://doi.org/10.1103/wzf3-7h5g

Abstract

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 alkalilike 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†) [Andreev et al., Nature (London) 562, 355 (2018)], does not fall into this category. In this work, we demonstrate the first optical cycling on this nonconventional class over a range of experimental parameter space, including laser intensity, polarization switching rate, and interaction time. We show that both the J=1 and J=2 rotational levels of ThO molecule are capable of cycling an average of 11(2) photons with a single laser, at scattering rates of 1.9(6)×106 and 2.3(7)×106 s−1, 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 noncycling 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 ∼100 photons, which would enable a wider range of quantum control and sensing using ThO molecules.

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Phys. Rev. Research 8, 033236– Published 27 August, 2026
  • Received 9 June 2026
  • Accepted 31 July 2026
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