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arXiv:2509.01618 · Physical Review Research

Optical trapping of SrOH molecules for dark matter and T -violation searches

Hiromitsu Sawaoka, Abdullah Nasir, Annika Lunstad, Mingda Li, Jack Mango, Zack Lasner, John M. Doyle

Published Dec 17, 2025• 8 citations
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Abstract

We report an optical dipole trap of strontium monohydroxide (SrOH) with 1400(300) trapped molecules. Through optical pumping, we access vibrational states that are proposed for improved probes of the electron’s electric dipole moment (eEDM) and ultralight dark matter (UDM). For each of these states, the lifetime of trapped molecules is measured and found to be consistent with spontaneous radiative decay and blackbody excitation limits, making this platform viable for these eEDM and UDM searches.

DOI
10.1103/2h36-myzp

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We report an optical dipole trap of strontium monohydroxide (SrOH) with 1400(300) trapped molecules. Through optical pumping, we access vibrational states that are proposed for improved probes of the electron’s electric dipole moment (eEDM) and ultralight dark matter (UDM). For each of these states, the lifetime of trapped molecules is measured and found to be consistent with spontaneous radiative decay and blackbody excitation limits, making this platform viable for these eEDM and UDM searches.
Polyatomic molecules are promising platforms for conducting precision measurements to probe the Standard Model, owing to their unique degrees of freedom including vibrations and rotations. In particular, ultracold SrOH has sensitivity to the variation of the proton to electron mass ratio μ\muμ and subsequently to scalar ultralight dark matter (UDM) fields. We discuss the laser slowing, sub-Doppler cooling, and trapping of SrOH and present an order of magnitude improved trap number through the use of transverse cooling of the molecular beam resulting in ∼\sim∼10410^4104 optically trapped molecules. This would allow for a two order of magnitude improvement on the current best sensitivities to UDM over four decades of mass. Probing μ\muμ-variation is not only important for UDM studies, but has more general implications for a variety of fundamental physics searches. Additionally, this increase in trapped molecule number makes SrOH a viable candidate in the search for the electron's electric dipole moment (eeeEDM) for fundamental symmetry violations. These techniques are broadly applicable, and make cooling and trapping heavier and more complex molecules with greater sensitivity to fundamental physics more viable.
Polyatomic molecules are projected to be powerful tools in searches for physics beyond the Standard Model (BSM), including new CP-violating (CPV) interactions and ultralight dark matter (UDM) particles. Certain degrees of freedom present in polyatomic molecules enhance the sensitivity of these searches, as well as reject systematic errors, but necessitate extensive high-precision spectroscopy to identify pathways for optical cycling and quantum state readout. Here we show how a magneto-optical trap (MOT) can be used to locate weak optical transitions and identify rovibronic states for optical cycling and quantum control. We demonstrate this spectroscopic approach with strontium monohydroxide (SrOH), which is a candidate for both CPV and UDM searches. We identify two new repumping transitions in SrOH and implement them in a deeper optical cycle to achieve 32400(4700) trapped molecules, a 4.5-fold increase over the previous, shallower cycle. In addition, we determine the energy spacing between the X2Σ+(200)X^{2}Σ^{+}(200)X2Σ+(200) and X2Σ+(0310)X^{2}Σ^{+}(03^10)X2Σ+(0310) vibrational manifolds of SrOH, confirming the existence of numerous low-frequency rovibrational transitions that are sensitive to temporal variations of the proton-to-electron mass ratio, a predicted effect of the existence of UDM.
We report a magneto-optical trap of strontium monohydroxide (SrOH) containing 2000(600) molecules at a temperature of 1.2(3) mK. The lifetime is 91(9) ms, which is limited by decay to optically unaddressed vibrational states. This provides the foundation for future sub-Doppler cooling and optical trapping of SrOH, a polyatomic molecule suited for precision searches for physics beyond the standard model including new CP violating particles and ultralight dark matter. We also identify important features in this system that guide cooling and trapping of complex and heavy polyatomic molecules into the ultracold regime.
We demonstrate a two-dimensional Sisyphus laser cooling method that increases the number of strontium monohydroxide (SrOH) molecules loaded into a magneto-optical trap by a factor of 12. Subsequent loading into an optical dipole trap (ODT) achieves 2.2(3)×1042.2 (3)\times10^42.2(3)×104 ultracold SrOH molecules with a peak density of ∼2(1)×1010 cm−3\sim2(1)\times10^{10}~\mathrm{cm^{-3}}∼2(1)×1010 cm−3. The lifetime of molecules in the ODT is limited by two-body collisions characterized by a measured collision rate constant β∼4×10−10 cm3/sβ\sim 4\times10^{-10}~\mathrm{cm^3/s}β∼4×10−10 cm3/s. The cooling method developed here is generally applicable to all known cases of direct molecular laser cooling, including symmetric and asymmetric top molecules. Increases in trapped molecule number will directly improve the search for ultralight dark matter, position polyatomic molecules as a platform for probing CP-violating new particles with masses $\gg$10 TeV, and facilitate a broad range of further research in quantum science.
Ultracold polyatomic molecules containing heavy nuclei, such as SrOH and YbOH, are sensitive to T- symmetry violating and P-symmetry violating physics beyond the Standard Model through the electron’s electric dipole moment. Furthermore, SrOH is sensitive to ultralight Dark Matter through measuring time variations of fundamental constants. However, in order to make full use of these molecules, they need to be cooled to ultracold temperatures and held in optical traps. We have realized a magneto-optical trap (MOT) of SrOH and used this as the essential element to achieve optical trap- ping in the microKelvin temperature regime. We observed MOT lifetimes approaching 200 ms with temperatures ∼ 1 mK and observed damped oscillations in the MOT. We further cooled the SrOH molecules to ∼ 40 μ K using Λ-enhanced gray molasses, and compressed the cloud to about 100 μm size using the conveyor belt MOT technique. Using this ultracold dense cloud of SrOH, we loaded an optical dipole trap (ODT), with a molecular lifetime of 1.2(1) seconds. In separate experiments with YbOH, which is heavier than SrOH, we demonstrated a novel deceleration method (Zeeman- Sisyphus deceleration) that can slow YbOH to under 20 m/s with only a few photons scattered utilizing high magnetic fields up to 2.5 T. By driving spin-flip transitions at the maxima and minima of the magnetic fields, slowing was achieved, which is a critical first step toward loading of a MOT for this species. We addressed the challenges associated with driving spin-flip transitions in molecules that have molecular perturbations among the electronic excited states and developed an understanding of the physics of such systems that is applicable to a wide range of molecules.
Ultracold polyatomic molecules are promising candidates for experiments in quantum science and precision searches for physics beyond the Standard Model. A key requirement is the ability to achieve full quantum control over the internal structure of the molecules. In this work, we established coherent control of individual quantum states in calcium monohydroxide (CaOH) and demonstrated a method for searching for the electron electric dipole moment (eEDM). Optically trapped, ultracold CaOH molecules were prepared in a single quantum state, polarized in an electric field, and coherently transferred into an eEDM-sensitive state where an electron spin precession measurement was performed. To extend the coherence time, we used eEDM-sensitive states with tunable, near-zero magnetic field sensitivity. Our results establish a path for eEDM searches with trapped polyatomic molecules.