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arXiv:1812.00219 · Physical Review X

Dipolar Collisions of Ultracold Ground-State Bosonic Molecules

Mingyang Guo, Xin Ye, Junyu He, Maykel Leonardo González-Martínez, Romain Véxiau, Goulven Quéméner, Dajun Wang

Published Dec 10, 2018• 63 citations
arXivHTMLPDF

Abstract

The dipolar collision between ultracold polar molecules is an important topic both by its own right from the fundamental point of view and for the successful exploration of many-body physics with strong and longrange dipolar interactions.Here, we report the investigation of collisions between ultracold ground-state sodium-rubidium molecules in electric fields with induced electric dipole moments as large as 0.7 D. We observe a stepwise enhancement of losses due to the coupling between different partial waves induced by the increasingly stronger anisotropic dipolar interactions.Varying the temperature of our sample, we find good agreement with theoretical loss rates assuming complex formation as the main loss process.Our results shed new light on the understanding of complex molecular collisions in the presence of strong dipolar interactions and also demonstrate the versatility of modifying molecular interactions with electric fields.

DOI
10.1103/physrevx.8.041044

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The dipolar collision between ultracold polar molecules is an important topic both by its own right from the fundamental point of view and for the successful exploration of many-body physics with strong and longrange dipolar interactions.Here, we report the investigation of collisions between ultracold ground-state sodium-rubidium molecules in electric fields with induced electric dipole moments as large as 0.7 D. We observe a stepwise enhancement of losses due to the coupling between different partial waves induced by the increasingly stronger anisotropic dipolar interactions.Varying the temperature of our sample, we find good agreement with theoretical loss rates assuming complex formation as the main loss process.Our results shed new light on the understanding of complex molecular collisions in the presence of strong dipolar interactions and also demonstrate the versatility of modifying molecular interactions with electric fields.
Creation of a strongly dipolar gas of ultracold ground-state 23Na87Rb^{23}\rm{Na}^{87}\rm{Rb}23Na87Rb molecules
We report the successful production of an ultracold sample of absolute ground-state 23Na87Rb^{23}\rm{Na}^{87}\rm{Rb}23Na87Rb molecules with a large induced electric dipole moment. Starting from weakly-bound Feshbach molecules formed via magneto-association, the lowest rovibrational and hyperfine level of the electronic ground state is populated following a high efficiency and high resolution two-photon Raman process. The high purity absolute ground-state samples have up to 8000 molecules and densities of over 101110^{11}1011 cm−3^{-3}−3. With an external electric field, we have induced an effective dipole moment over one Debye, enough to provide truly strong and long-range dipolar interactions. Bimolecular reaction between ground-state 23Na87Rb^{23}\rm{Na}^{87}\rm{Rb}23Na87Rb molecules is endothermic, but we still observed a rather fast decay of the molecular sample. Our results pave the way toward investigation of ultracold molecular collisions in a fully controlled manner, and possibly to quantum gases of ultracold bosonic molecules with strong dipolar interactions.
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We report the successful production of an ultracold sample of absolute ground-state ^{23}Na^{87}Rb molecules. Starting from weakly bound Feshbach molecules formed via magnetoassociation, the lowest rovibrational and hyperfine level of the electronic ground state is populated following a high-efficiency and high-resolution two-photon Raman process. The high-purity absolute ground-state samples have up to 8000 molecules and densities of over 10^{11} cm^{-3}. By measuring the Stark shifts induced by external electric fields, we determined the permanent electric dipole moment of the absolute ground-state ^{23}Na^{87}Rb and demonstrated the capability of inducing an effective dipole moment over 1 D. Bimolecular reaction between ground-state ^{23}Na^{87}Rb molecules is endothermic, but we still observed a rather fast decay of the molecular sample. Our results pave the way toward investigation of ultracold molecular collisions in a fully controlled manner and possibly to quantum gases of ultracold bosonic molecules with strong dipolar interactions.
We theoretically study collisions between ultracold polar $^{1}\mathrm{\ensuremath{\Sigma}}$ molecules that are polarized by microwave or static electric fields. We systematically study the dependence on field strength, microwave polarization, and detuning from rotational transitions. We calculate the loss in two-body collisions that is observable experimentally and compare with the results expected for purely first-order dipolar interactions. For ground-state molecules polarized by a static electric field, the dynamics are accurately described by first-order dipolar interactions. For microwave dressing, instead, resonant dipolar collisions dominate the collision process, in which molecules reorient along the intermolecular axis and interact with the full strength of the transition dipole. For red detuning, reorientation can only be suppressed at extreme Rabi frequencies. For blue detuned microwaves, resonant dipolar interactions dominate even for the highest Rabi frequencies, leading to microwave shielding for circular polarization and structured losses due to resonances for linear polarization. The results are presented numerically for fermionic 23Na40K^{23}\mathrm{Na}^{40}\mathrm{K}23Na40K and bosonic 23Na39K^{23}\mathrm{Na}^{39}\mathrm{K}23Na39K molecules.
Recent experiments on ultracold nonreactive dipolar molecules have observed high two-body losses, even though these molecules can undergo neither inelastic, nor reactive (as they are in their absolute ground state), nor light-assisted collisions (if they are measured in the dark). In the presence of an electric field these losses seem to be near universal (the probability of loss at short range is near unity) while in the absence of it the losses seem nonuniversal. To explain these observations we propose a simple model based on the mixing effect of an electric field on the states of the two diatomic molecules at long range and on the density of states of the tetramer complex formed at short range, believed to be responsible for the losses. We apply our model to collisions of ground-state molecules of endothermic systems, of current experimental interest.
Ultracold polar molecular quantum gases promise to open new research directions ranging from the study of ultra-cold chemistry, precision measurements to novel quantum phase transitions. Based on the preparation of high-phase space density gases of polar KRb molecules [1–3], I will discuss the control of dipolar collisions and chemical reactions of polar molecules in a regime where quantum statistics, single scattering partial waves, and quantum threshold laws play a dominant role [4]. In particular, I will point out the crucial role of electric dipole-dipole interactions [5] and external confinement [6] in determining the chemical reaction rate. Finally, I will discuss prospects of reaching quantum degeneracy in bialkali samples of polar molecules and prospects for these systems as novel dipolar quantum many-body systems.