Optical pumping and laser slowing of a heavy molecule

Precision measurements of the electron’s electric dipole moment (eEDM) are critical for testing fundamental symmetries in particle physics, and heavy polar molecules—such as barium monofluoride (BaF)—have emerged as promising candidates for advancing the sensitivity. However, the achievement of a 3D magneto-optical trap (MOT) required slowing BaF molecules to near-zero velocity by scattering over 104 photons per molecule, demanding a quasi-cycling transition with minimal leakage. We present a detailed study of the leakage channels, including higher vibrational and rotational states. By combining microwave remixing with optical pumping of rotational and vibrational dark states, we reduced the total leakage fraction to 10−5. Using frequency-chirped laser slowing, we slowed a subset of buffer-gas-cooled BaF molecules from approximately 80 ms−1 to near-zero velocity, which is critical for efficient MOT loading. This work establishes the technical foundation for precision eEDM measurements using laser-cooled heavy molecules. Heavy polar molecules like BaF are key to boosting measurement sensitivity of the electron’s electric dipole moment (eEDM), but slowing heavy molecules to near-zero velocity for experiments is challenging. This study achieved reducing BaF’s total leakage fraction to 10−5 using microwave remixing and optical pumping and slowing buffer-gas-cooled BaF molecules from ~ 80 m/s to near-zero velocity.