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arXiv:2106.00622 · Physical Review Letters

Polarization Enhanced Deep Optical Dipole Trapping of Λ -Cooled Polar Molecules

Thomas Langin, Varun Jorapur, Yuqi Zhu, Qian Wang, David P. DeMille

Published Oct 12, 2021• 45 citations
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Abstract

We demonstrate loading of SrF molecules into an optical dipole trap (ODT) via in-trap $\mathrm{\ensuremath{\Lambda}}$-enhanced gray molasses cooling. We find that this cooling can be optimized by a proper choice of relative ODT and cooling beam polarizations. In this optimized configuration, we observe molecules with temperatures as low as $14(1)\text{ }\text{ }\ensuremath{\mu}\mathrm{K}$ in traps with depths up to $570\text{ }\text{ }\ensuremath{\mu}\mathrm{K}$. With optimized parameters, we transfer $\ensuremath{\sim}5%$ of molecules from our radio-frequency magneto-optical trap into the ODT, at a density of $\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{9}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$, a phase space density of $\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$, and with a trap lifetime of $\ensuremath{\sim}1\text{ }\text{ }\mathrm{s}$.

DOI
10.1103/physrevlett.127.163201

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We demonstrate loading of SrF molecules into an optical dipole trap (ODT) via in-trap \ensuremathΛ\mathrm{\ensuremath{\Lambda}}\ensuremathΛ-enhanced gray molasses cooling. We find that this cooling can be optimized by a proper choice of relative ODT and cooling beam polarizations. In this optimized configuration, we observe molecules with temperatures as low as 14(1)  \ensuremathμK14(1)\text{ }\text{ }\ensuremath{\mu}\mathrm{K}14(1)  \ensuremathμK in traps with depths up to 570  \ensuremathμK570\text{ }\text{ }\ensuremath{\mu}\mathrm{K}570  \ensuremathμK. With optimized parameters, we transfer \ensuremath∼5\ensuremath{\sim}5%\ensuremath∼5 of molecules from our radio-frequency magneto-optical trap into the ODT, at a density of \ensuremath∼2\ifmmode×\else\texttimes\fi109  cm\ensuremath−3\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{9}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}\ensuremath∼2\ifmmode×\else\texttimes\fi109  cm\ensuremath−3, a phase space density of \ensuremath∼2\ifmmode×\else\texttimes\fi10\ensuremath−7\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}\ensuremath∼2\ifmmode×\else\texttimes\fi10\ensuremath−7, and with a trap lifetime of \ensuremath∼1  s\ensuremath{\sim}1\text{ }\text{ }\mathrm{s}\ensuremath∼1  s.
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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 demonstrate the efficient transfer of molecules from a magneto-optical trap into a conservative magnetic quadrupole trap. Our scheme begins with a blue-detuned optical molasses to cool SrF molecules to ≈50 μK. Next, we optically pump the molecules into a strongly trapped sublevel. This two-step process reliably transfers ≈40% of the molecules initially trapped in the magneto-optical trap into a single quantum state in the magnetic trap. Once loaded, the molecule cloud is compressed by increasing the magnetic field gradient. We observe a magnetic trap lifetime of over 1 s. This opens a promising new path to study ultracold molecular collisions, and potentially to produce quantum-degenerate molecular gases via sympathetic cooling with co-trapped atoms.
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 report optical trapping of laser-cooled molecules at sufficient density to observe molecule-molecule collisions for the first time in a bulk gas. SrF molecules from a red-detuned magneto-optical trap (MOT) are compressed and cooled in a blue-detuned MOT. Roughly 30% of these molecules are loaded into an optical dipole trap with peak number density n_{0}≈3×10^{10} cm^{-3} and temperature T≈40 μK. We observe two-body loss with rate coefficient β=2.7_{-0.8}^{+1.2}×10^{-10} cm^{3} s^{-1}. Achieving this density and temperature opens a path to evaporative cooling towards quantum degeneracy of laser-cooled molecules.