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arXiv:2002.00048 · Physical Review Letters

Observation of Collisions between Two Ultracold Ground-State CaF Molecules

Lawrence W. Cheuk, Loïc Anderegg, Yicheng Bao, Sean Burchesky, Scarlett S. Yu, Wolfgang Ketterle, Kang-Kuen Ni, John M. Doyle

Published Jul 20, 2020• 103 citations
arXivHTMLPDF

Abstract

We measure inelastic collisions between ultracold CaF molecules by combining two optical tweezers, each containing a single molecule. We observe collisions between ^{2}Σ CaF molecules in the absolute ground state |X,v=0,N=0,F=0⟩, and in excited hyperfine and rotational states. In the absolute ground state, we find a two-body loss rate of 7(4)×10^{-11} cm^{3}/s, which is below, but close to, the predicted universal loss rate.

DOI
10.1103/physrevlett.125.043401

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We measure inelastic collisions between ultracold CaF molecules by combining two optical tweezers, each containing a single molecule. We observe collisions between ^{2}Σ CaF molecules in the absolute ground state |X,v=0,N=0,F=0⟩, and in excited hyperfine and rotational states. In the absolute ground state, we find a two-body loss rate of 7(4)×10^{-11} cm^{3}/s, which is below, but close to, the predicted universal loss rate.
We prepare mixtures of ultracold CaF molecules and Rb atoms in a magnetic trap and study their inelastic collisions. When the atoms are prepared in the spin-stretched state and the molecules in the spin-stretched component of the first rotationally excited state, they collide inelastically with a rate coefficient k_{2}=(6.6±1.5)×10^{-11} cm^{3}/s at temperatures near 100 μK. We attribute this to rotation-changing collisions. When the molecules are in the ground rotational state we see no inelastic loss and set an upper bound on the spin-relaxation rate coefficient of k_{2}<5.8×10^{-12} cm^{3}/s with 95% confidence. We compare these measurements to the results of a single-channel loss model based on quantum defect theory. The comparison suggests a short-range loss parameter close to unity for rotationally excited molecules, but below 0.04 for molecules in the rotational ground state.
Harnessing the potential wide-ranging quantum science applications of molecules will require control of their interactions. Here, we used microwave radiation to directly engineer and tune the interaction potentials between ultracold calcium monofluoride (CaF) molecules. By merging two optical tweezers, each containing a single molecule, we probed collisions in three dimensions. The correct combination of microwave frequency and power created an effective repulsive shield, which suppressed the inelastic loss rate by a factor of six, in agreement with theoretical calculations. The demonstrated microwave shielding shows a general route to the creation of long-lived, dense samples of ultracold polar molecules and evaporative cooling.
Ultracold collisions between Cs atoms and Cs2{\mathrm{Cs}}_{2}Cs2​ dimers in the electronic ground state are observed in an optically trapped gas of atoms and molecules. The Cs2{\mathrm{Cs}}_{2}Cs2​ molecules are formed in the triplet ground state by cw photoassociation through the outer well of the ${0}_{g}^{\ensuremath{-}}({P}_{3/2})$ excited electronic state. Inelastic atom-molecule collisions converting internal excitation into kinetic energy lead to a loss of Cs2{\mathrm{Cs}}_{2}Cs2​ molecules from the dipole trap. Rate coefficients are determined for collisions involving Cs atoms in either the F=3F=3F=3 or F=4F=4F=4 hyperfine ground state, and Cs2{\mathrm{Cs}}_{2}Cs2​ molecules in either highly vibrationally excited states (${v}^{\ensuremath{'}}=32--47$) or in low vibrational states (${v}^{\ensuremath{'}}=4--6$) of the $a\text{ }^{3}\ensuremath{\Sigma}_{u}^{+}$ triplet ground state. The rate coefficients $\ensuremath{\beta}\ensuremath{\sim}{10}^{\ensuremath{-}10}\text{ }\text{ }{\mathrm{cm}}^{3}/\mathrm{s}$ are found to be largely independent of the vibrational and rotational excitation indicating unitary limited cross sections.
Abstract We study inelastic collisions between CaF molecules and 87Rb atoms in a dual-species magneto-optical trap. The presence of atoms increases the loss rate of molecules from the trap. By measuring the loss rates and density distributions, we determine a collisional loss rate coefficient k 2 = (1.43 ± 0.29) × 10−10 cm3 s−1 at a temperature of 2.4 mK. We show that this is not substantially changed by light-induced collisions or by varying the populations of excited-state atoms and molecules. The observed loss rate is close to the universal rate expected in the presence of fast loss at short range, and can be explained by rotation-changing collisions in the ground electronic state.
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Abstract Understanding ultracold collisions involving molecules is of fundamental importance for current experiments, where inelastic collisions typically limit the lifetime of molecular ensembles in optical traps. Here we present a broad study of optically trapped ultracold RbCs molecules in collisions with one another, in reactive collisions with Rb atoms, and in nonreactive collisions with Cs atoms. For experiments with RbCs alone, we show that by modulating the intensity of the optical trap, such that the molecules spend 75% of each modulation cycle in the dark, we partially suppress collisional loss of the molecules. This is evidence for optical excitation of molecule pairs mediated via sticky collisions. We find that the suppression is less effective for molecules not prepared in the spin-stretched hyperfine ground state. This may be due either to longer lifetimes for complexes in the dark or to laser-free decay pathways. For atom–molecule mixtures, RbCs + Rb and RbCs + Cs, we demonstrate that the rate of collisional loss of molecules scales linearly with the density of atoms. This indicates that, in both cases, the loss of molecules is rate-limited by two-body atom–molecule processes. For both mixtures, we measure loss rates that are below the thermally averaged universal limit.