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arXiv:2006.01769 · Physical Review A

Establishing a nearly closed cycling transition in a polyatomic molecule

Louis Baum, Nathaniel B. Vilas, Christian Hallas, Benjamin L. Augenbraun, Shivam Raval, Debayan Mitra, John M. Doyle

Published Apr 15, 2021• 40 citations
arXivHTMLPDF

Abstract

We study optical cycling in the polar free radical calcium monohydroxide (CaOH) and establish an experimental path towards scattering $\ensuremath{\sim}{10}^{4}$ photons. We report rovibronic branching ratio measurements with precision at the $\ensuremath{\sim}{10}^{\ensuremath{-}4}$ level and observe weak symmetry-forbidden decays to bending modes with nonzero vibrational angular momentum. Calculations are in excellent agreement with these measurements and predict additional decay pathways. Additionally, we perform high-resolution spectroscopy of the $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{X}{\phantom{\rule{1.0pt}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+}({12}^{0}0)$ and $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{X}{\phantom{\rule{1.0pt}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+}({12}^{2}0)$ hybrid vibrational states of CaOH. These advances establish a path towards radiative slowing, three-dimensional magneto-optical trapping, and sub-Doppler cooling of CaOH.

DOI
10.1103/physreva.103.043111

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We study optical cycling in the polar free radical calcium monohydroxide (CaOH) and establish an experimental path towards scattering \ensuremath∼104\ensuremath{\sim}{10}^{4}\ensuremath∼104 photons. We report rovibronic branching ratio measurements with precision at the \ensuremath∼10\ensuremath−4\ensuremath{\sim}{10}^{\ensuremath{-}4}\ensuremath∼10\ensuremath−4 level and observe weak symmetry-forbidden decays to bending modes with nonzero vibrational angular momentum. Calculations are in excellent agreement with these measurements and predict additional decay pathways. Additionally, we perform high-resolution spectroscopy of the X\ifmmode~\else˜\fi2\ensuremathΣ+(1200)\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{X}{\phantom{\rule{1.0pt}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+}({12}^{0}0)X\ifmmode~\else˜\fi​2\ensuremathΣ+(1200) and X\ifmmode~\else˜\fi2\ensuremathΣ+(1220)\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{X}{\phantom{\rule{1.0pt}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+}({12}^{2}0)X\ifmmode~\else˜\fi​2\ensuremathΣ+(1220) hybrid vibrational states of CaOH. These advances establish a path towards radiative slowing, three-dimensional magneto-optical trapping, and sub-Doppler cooling of CaOH.
We report optical trapping of a polyatomic molecule, calcium monohydroxide (CaOH). CaOH molecules from a magneto-optical trap are sub-Doppler laser cooled to $20(3)\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{K}$ in free space and loaded into an optical dipole trap. We attain an in-trap molecule number density of $3(1)\ifmmode\times\else\texttimes\fi{}{10}^{9}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$ at a temperature of $57(8)\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{K}$. Trapped CaOH molecules are optically pumped into an excited vibrational bending mode, whose $\ensuremath{\ell}$-type parity doublet structure is a potential resource for a wide range of proposed quantum science applications with polyatomic molecules. We measure the spontaneous, radiative lifetime of this bending mode state to be $\ensuremath{\sim}0.7\text{ }\text{ }\mathrm{s}$.
The rich internal structure of polyatomic molecules has inspired a number of theoretical proposals exploring a myriad of unique scientific and engineering frontiers, including quantum simulation and next-generation searches for physics beyond the standard model. Establishing full control over these complex quantum objects is likely necessary to harness their full potential; however, the same rich internal structure that gives rise to the immense scientific potential of polyatomic molecules also makes manipulating them a daunting task. In the past decade there has been considerable progress in the production, cooling, and control of diatomic species using direct laser cooling and magneto-optical trapping. This dissertation focuses on extending direct laser cooling and trapping to polyatomic molecules. Direct laser cooling of triatomic radical calcium monohydroxide (CaOH) is demonstrated. This molecules is a prototypical example of a broad class of polyatomic molecules that includes symmetric and asymmetric tops. Transverse magneto-optical trapping is demonstrated, establishing the ability to scatter an unprecedented number of photons in a polyatomic species. An understanding of previously unappreciated intricacies of molecular structure is developed, that explains observations of novel vibrational decay pathways. High resolution spectroscopy of hybrid vibrational modes is performed. A laser cooling scheme is constructed that will support future efforts to use the full range of laser cooling and high-fidelity detection techniques required to implement an optical tweezer array of polyatomic molecules. Our experimental efforts demonstrate the feasibility of direct laser cooling applied to CaOH, up ending long-standing assumptions of the inextricable complexity of polyatomic molecules.
We report magneto-optical trapping (MOT) of a polyatomic molecule, calcium monohydroxide (CaOH). The MOT contains 2.0(5)×1042.0(5)\times 10^42.0(5)×104 CaOH molecules at a peak density of 3.0(8)×1063.0(8)\times10^{6}3.0(8)×106 cm−3^{-3}−3. CaOH molecules are further sub-Doppler laser cooled in an optical molasses, to a temperature of 110(4) μμμK. The temperatures and densities achieved here make CaOH a viable candidate for a wide variety of quantum science applications, including the creation of optical tweezer arrays of CaOH molecules. This work also suggests that laser cooling and magneto-optical trapping of many other polyatomic species will be both feasible and practical.
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.
Polyatomic molecules contain diverse structures – including rotational and vibrational degrees of freedom, nuclear and electronic spins, and electric dipole moments – that make them promising for a range of quantum science applications. These include quantum information science, quantum simulation, studies of ultracold collisions and ultracold chemistry, and precision searches for physics beyond the Standard Model. However, maximizing the potential of polyatomic molecules for these applications requires them to be cooled to ultracold temperatures ( mK), trapped in three dimensions, and controlled at the single quantum state level. Significant progress has been made with diatomic molecules over the last fifteen years, but the increased complexity of polyatomic species (defined as molecules containing more than two atoms) makes them more challenging to control at the same level. In this thesis, we describe our work bringing a linear triatomic molecule, calcium monohydroxide (CaOH), into the quantum regime. We demonstrate laser cooling of CaOH molecules to microkelvin temperatures, confinement in a magneto-optical trap (MOT), and loading of conservative optical dipole traps and optical tweezer arrays. We characterize the lifetime of low-lying vibrational states of CaOH in optical traps, including the vibrational bending mode, whose parity-doublet states are a key resource for many applications. Next, we develop techniques for preparing CaOH molecules in a single internal quantum state and for coherently manipulating the internal state with microwave and radio-frequency fields. We also show that we can nondestructively and state-selectively detect trapped CaOH molecules, including single molecules in optical tweezers. We demonstrate the applicability of these tools to applications including precision measurements, quantum information science, and ultracold collisions. We establish a method for future CP-violating physics searches with optically trapped polyatomic molecules, by identifying states that are sensitive to the electron electric dipole moment (eEDM) and showing that these states have long coherence times in CaOH. We identify potential qubit states in the CaOH bending mode and show that they can be coherently controlled at the single-molecule level in an optical tweezer array. Finally, we observe and characterize single quantum-state-controlled collisions between ultracold CaOH molecules, and identify states in the bending mode that could potentially be used for evaporative cooling to quantum degeneracy. Compared to linear triatomic molecules, nonlinear molecules contain even more structures that can be harnessed for quantum science applications, at the cost of making them even more challenging to cool and control. Towards this goal, in this thesis we describe work demonstrating 1D laser cooling of a beam of CaOCH3, a symmetric top molecule. This contributes to a growing body of evidence that complex polyatomic molecules could soon be cooled, trapped, and controlled at ultracold temperatures.