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arXiv:2512.24167 · Communications Physics

Optical pumping and laser slowing of a heavy molecule

Shuhua Deng, Shoukang Yang, Zixuan Zeng, Bo Yan

Published Dec 2, 2025• 0 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1038/s42005-025-02397-3

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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.
As a heavy molecule, barium monofluoride (BaF) presents itself as a promising candidate for measuring permanent electric dipole moment. Here we report the realization of three-dimensional magneto-optical trapping (MOT) of BaF molecules. Through the repumping of all the vibrational states up to v=3, and rotational states up to N=3, we effectively close the transition to a leakage level lower than 10^{-5}. This approach enables molecules to scatter a sufficient number of photons required for laser cooling and trapping. By employing a technique that involves chirping the slowing laser frequency, BaF molecules are decelerated to near-zero velocity, resulting in the capture of approximately 3×10^{3} molecules in a MOT. Our findings represent a significant step towards the realization of ultracold BaF molecules and the conduct of precision measurements with cold molecules.
Laser cooling of molecules is a powerful technique for producing cold, slow beams for precision measurements and quantum control, yet its implementation remains challenging due to molecular complexity. Here, we combine a cryogenic buffer gas beam, an electrostatic hexapole lens, and 2D transverse Doppler laser cooling to produce a bright beam of barium monofluoride (138Ba19F) molecules. We study both numerically and experimentally the laser cooling effect as a function of laser detuning, laser power, laser alignment, and interaction time. We find a scattering rate of 6.1(1.4) × 105 s−1 on the laser cooling transition (14% of the expected maximum) and identify suboptimal dark Zeeman state remixing, suboptimal laser sideband powers and detunings, and a lack of vibrational repump laser intensity as possible causes of such a low rate. Using 3 tuneable lasers with appropriate sidebands and detuning, each molecule scatters approximately 400 photons during 2D laser cooling, limited by the interaction time and scattering rate. Leaks to dark states are less than 10%. Finally, we use the experimental results to benchmark the trajectory simulations to predict the achievable flux 3.5 m downstream for a planned eEDM experiment. Laser cooling is a powerful technique that enables precision measurements and quantum control, yet its implementation in molecules remains challenging due to their complex structures. Here, the authors apply 2D transverse laser cooling to a focused beam of cold barium monofluoride (138Ba19F) molecules to significantly increase beam brightness.
Diatomic molecules are considered good candidates for the study of strongly correlated systems and precision measurement searches due to their combination of complex internal states and strong long-range interactions. Cooling molecules down to ultracold temperatures is often a necessary step for fully utilizing the power of the molecule. This requires a trap for molecules and the ability to cool molecules to the mK regime and below. A magneto-optical trap (MOT) is a good tool for achieving mK temperatures. However, extra care is needed for molecules to form the necessary quasi-closed cycling transitions due to molecule's complicated energy structure. In our work with CaF, we use two repump lasers to block vibrational leakage and selection rules for the rotational degree of freedom to achieve about 10^{5} photon cycles.\n\n\n\nThe two-stage buffer gas beam source is a general method to generate a cold and slow beam of molecules with a forward velocity of about 50 m/s. The compatibility of the buffer-gas source with a MOT is studied and we confirm that such beams can be nicely compatible with MOTs using various atomic species. \n\n\n\nIn order to load molecules into a MOT from even such a slow beam, additional slowing is required due to the low capture velocity of a molecular MOT (< 10 m/s). We apply a frequency-broadened “white-light” slowing on CaF from a two-stage source, demonstrating slowing of CaF below 10 m/s. An AC MOT, which provides active remixing of dark substates, is also developed and Li atoms are slowed and trapped. These are crucial ingredients for co-trapping CaF molecules and Li atoms and study their collisional properties, which would lead to sympathetic cooling of molecules down to ultracold temperatures. \n\n\n\nThe achievement of slowing and development of this system allowed for the detailed study of the CaF laser cooling system, as well as physical processes involved with AC MOTs and the proposed MOT for CaF. Crucial knowledge of this archetypal system provides significant progress toward manipulation and control of molecules similar to what has been achieved with atoms and what is necessary for searches for new physics with ultracold molecules.
Abstract Measurements of the electron’s electric dipole moment (eEDM) are demanding tests of physics beyond the standard model. We describe how ultracold YbF molecules could be used to improve the precision of eEDM measurements by two to three orders of magnitude. Using numerical simulations, we show how the combination of magnetic focussing, two-dimensional transverse laser cooling, and frequency-chirped laser slowing, can produce an intense, slow, highly-collimated molecular beam. We show how to make a magneto-optical trap of YbF molecules and how the molecules could be loaded into an optical lattice. eEDM measurements could be made using the slow molecular beam or using molecules trapped in the lattice. We estimate the statistical sensitivity that could be reached in each case and consider how sources of noise can be reduced so that the shot-noise limit of sensitivity can be reached. We also consider systematic effects due to magnetic fields and vector light shifts and how they could be controlled.
Permanent electric dipole moments (EDMs) are signatures of time-reversal and parity violation and act as sensitive probes for physics beyond the Standard Model (BSM). The work in this thesis has been done as part of the NL-eEDM collaboration, which plans to measure the electron EDM using a beam of BaF molecules. The sensitivity of such a table-top precision experiment benefits from utilizing an intense, slow beam of molecules. We plan to achieve that through a combination of Stark deceleration and transverse laser cooling of a cryogenic beam of BaF. The work presented in this thesis has primarily been done to produce intense, slow molecular beams for an electron EDM measurement. We constructed a supersonic beam source of SrF and BaF molecules. The source produces high intensity beams with low translational and rotational temperatures. Time-resolved detection of laser-induced fluorescence from pulsed excitation of electronic states in BaF molecules was performed to determine the lifetimes of the A^{2}Pi_{1/2} and A^{2}Pi_{3/2} states. The lifetimes of these states were determined to be 57.1(3) ns and 47.9(7) ns, respectively. The transition between the A^{2}Pi_{1/2} and X^{2}Sigma^{+} states is the main laser cooling transition in BaF. We decelerated a beam of SrF molecules from a cryogenic source to a standstill inside a traveling-wave Stark decelerator, with the number of decelerated molecules on the order of 1000. BaF molecules will be decelerated with the same technique to perform an electron EDM measurement.
We demonstrate buffer-gas cooling, high-resolution spectroscopy, and cycling fluorescence of cold barium monofluoride (BaF) molecules. Our source produces an intense and internally cold molecular beam containing the different BaF isotopologues with a mean forward velocity of 190m/s190\phantom{\rule{0.16em}{0ex}}\mathrm{m}/\mathrm{s}190m/s. For a well-collimated beam of 138Ba19F^{138}\mathrm{Ba}\phantom{\rule{0.16em}{0ex}}^{19}\mathrm{F}138Ba19F we observe a flux of >1010>{10}^{10}>1010 molecules ${\mathrm{sr}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{pulse}}^{\ensuremath{-}1}$ in the $X{\phantom{\rule{0.16em}{0ex}}}^{2}\mathrm{\ensuremath{\Sigma}}$, N=1N=1N=1 state in our downstream detection region. Studying the absorption line strength of the intermediate ${A}^{\ensuremath{'}}\mathrm{\ensuremath{\Delta}}$ state we infer a lifetime of ${\ensuremath{\tau}}_{\mathrm{\ensuremath{\Delta}}}=790\ifmmode\pm\else\textpm\fi{}346\phantom{\rule{0.16em}{0ex}}\mathrm{ns}$, significantly longer than previously estimated. Finally, highly diagonal Franck-Condon factors and magnetic remixing of dark states allow us to realize a quasicycling transition in 138Ba19F^{138}\mathrm{Ba}\phantom{\rule{0.16em}{0ex}}^{19}\mathrm{F}138Ba19F that is suitable for future laser cooling of this heavy diatomic molecule.