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arXiv:2607.16913

Order of Magnitude Improved Optical Trapping of Molecules Through Transverse Cooling

Abdullah Nasir, Annika Lunstad, Mingda Li, Saif Salim, Liu S, Christian Hallas, Zack Lasner, John M. Doyle

Published Jul 18, 2026• 0 citations
arXivHTMLPDF

Abstract

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 $2.2 (3)\times10^4$ ultracold SrOH molecules with a peak density of $\sim2(1)\times10^{10}~\mathrm{cm^{-3}}$. The lifetime of molecules in the ODT is limited by two-body collisions characterized by a measured collision rate constant $β\sim 4\times10^{-10}~\mathrm{cm^3/s}$. 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.

DOI
10.48550/arxiv.2607.16913

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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.
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.
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.
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.
We perform magnetically assisted Sisyphus laser cooling of the triatomic free radical strontium monohydroxide (SrOH). This is achieved with principal optical cycling in the rotationally closed P(N^{''}=1) branch of either the X[over ˜]^{2}Σ^{+}(000)↔A[over ˜]^{2}Π_{1/2}(000) or the X[over ˜]^{2}Σ^{+}(000)↔B[over ˜]^{2}Σ^{+}(000) vibronic transitions. Molecules lost into the excited vibrational states during the cooling process are repumped back through the B[over ˜](000) state for both the (100) level of the Sr-O stretching mode and the (02^{0}0) level of the bending mode. The transverse temperature of a SrOH molecular beam is reduced in one dimension by 2 orders of magnitude to ∼750 μK. This approach opens a path towards creating a variety of ultracold polyatomic molecules by means of direct laser cooling.
We perform magnetically assisted Sisyphus laser cooling of the triatomic free radical strontium monohydroxide (SrOH).This is achieved with principal optical cycling in the rotationally closed PðN 00 ¼ 1Þ branch of either the X2 Σ þ ð000Þ ↔ Ã2 Π 1=2 ð000Þ or the X2 Σ þ ð000Þ ↔ B2 Σ þ ð000Þ vibronic transitions.Molecules lost into the excited vibrational states during the cooling process are repumped back through the Bð000Þ state for both the (100) level of the Sr-O stretching mode and the ð02 0 0Þ level of the bending mode.The transverse temperature of a SrOH molecular beam is reduced in one dimension by 2 orders of magnitude to ∼750 μK.This approach opens a path towards creating a variety of ultracold polyatomic molecules by means of direct laser cooling.
Ultracold molecules are a promising platform for a wide range of applications in quantum science, including searches for new particles (beyond the Standard Model), quantum computing and simulation, and studies of ultracold collisions and chemistry. Over the last decade, tremendous progress has been made in developing techniques to trap diatomic molecules (molecules containing only two atoms) and cool them to ultracold temperatures using laser cooling. Polyatomic molecules (molecules containing more than two atoms) provide new, unique scientific opportunities not afforded by their diatomic counterparts and represent the next frontier in laser cooling and quantum science with molecules. In this thesis, we present our work on extending laser cooling and quantum control to polyatomic molecules. We describe our demonstration of laser cooling and the creation of a magneto-optical trap (MOT) of a polyatomic molecule, calcium monohydroxide (CaOH). We further demonstrate loading of the trapped molecules into an optical dipole trap (ODT) and loading of individual CaOH molecules into the conservative potential an optical tweezer array. We also describe our realization of a new type of MOT developed for molecules, a "conveyor belt MOT", which provides increased compression compared to traditional molecular MOTs and was discovered using numerical simulations of the CaOH MOT that were developed as part of this thesis. The increased MOT density and ODT loading afforded by the conveyor belt MOT provides the density required to perform studies of collisions between ultracold CaOH molecules. We also describe our work on the laser cooling the symmetric top molecule CaOCH₃, opening the possibility for 3-D laser cooling of even larger, more complex molecules into the ultracold regime. With the ability to laser cool and trap CaOH molecules, our most recent work has focused on developing tools for quantum control and on initial explorations of some of the new features afforded by polyatomic molecules. We describe a novel protocol established to perform a measurement of the electron's electric dipole moment ("eEDM"). Looking towards quantum information processing with polyatomic molecules, we create a new type of qubit using parity-doublet states in the bending mode of CaOH and characterize its coherence properties. Finally, we provide a brief outlook towards the future of laser cooling of polyatomic molecules for quantum science and provide a few suggestions for new directions opened up by the work presented in this thesis.