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arXiv:1510.06601 · Physical Review A

Shielding Σ 2 ultracold dipolar molecular collisions with electric fields

Goulven Quéméner, John L. Bohn

Published Jan 11, 2016• 65 citations
arXivHTMLPDF

Abstract

The prospects for shielding ultracold, paramagnetic, dipolar molecules from inelastic and chemical collisions are investigated. Molecules placed in their first rotationally excited states are found to exhibit effective long-range repulsion for applied electric fields above a certain critical value, as previously shown for nonparamagnetic molecules. This repulsion can safely allow the molecules to scatter while reducing the risk of inelastic or chemically reactive collisions. Several molecular species of $^{2}\mathrm{\ensuremath{\Sigma}}$ molecules of experimental interest---RbSr, SrF, BaF, and YO---are considered, and all are shown to exhibit orders of magnitude suppression in quenching rates in a sufficiently strong laboratory electric field. It is further shown that, for these molecules described by Hund's coupling case (b), electronic and nuclear spins play the role of spectator with respect to the shielding.

DOI
10.1103/physreva.93.012704

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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)•Jun 21, 2021•1 citation
The prospects for shielding ultracold, paramagnetic, dipolar molecules from inelastic and chemical collisions are investigated. Molecules placed in their first rotationally excited states are found to exhibit effective long-range repulsion for applied electric fields above a certain critical value, as previously shown for nonparamagnetic molecules. This repulsion can safely allow the molecules to scatter while reducing the risk of inelastic or chemically reactive collisions. Several molecular species of 2\ensuremathΣ^{2}\mathrm{\ensuremath{\Sigma}}2\ensuremathΣ molecules of experimental interest---RbSr, SrF, BaF, and YO---are considered, and all are shown to exhibit orders of magnitude suppression in quenching rates in a sufficiently strong laboratory electric field. It is further shown that, for these molecules described by Hund's coupling case (b), electronic and nuclear spins play the role of spectator with respect to the shielding.
Ultracold Chemistry Because reactive collisions limit the lifetime of ultracold molecular ensembles, controlling chemical reactivity at ultralow temperatures has been a long-standing goal. Using large electric fields that trigger resonant dipolar interactions between potassium-rubidium molecules trapped in a quasi–two-dimensional geometry, Matsuda et al. report suppression of the reactive loss rate in the vicinity of the dipolar-mediated resonances by up to an order of magnitude below the background value. The proposed shielding mechanism is general and is expected to be effective in three-dimensional geometry. It could also be used for creating long-lived quantum molecular gases of other polar molecules under strong electric fields. Science , this issue p. [1324][1] [1]: /lookup/doi/10.1126/science.abe7370
Full control of molecular interactions, including reactive losses, would open new frontiers in quantum science. We demonstrate extreme tunability of ultracold chemical reaction rates by inducing resonant dipolar interactions by means of an external electric field. We prepared fermionic potassium-rubidium molecules in their first excited rotational state and observed a modulation of the chemical reaction rate by three orders of magnitude as we tuned the electric field strength by a few percent across resonance. In a quasi-two-dimensional geometry, we accurately determined the contributions from the three dominant angular momentum projections of the collisions. Using the resonant features, we shielded the molecules from loss and suppressed the reaction rate by an order of magnitude below the background value, thereby realizing a long-lived sample of polar molecules in large electric fields.
We investigate the electric field shielding of ultracold collisions of dipolar rotors, initially in their first rotational excited state, using an adimensional approach. We establish a map of good and bad candidates for efficient evaporative cooling based on this shielding mechanism, by presenting the ratio of elastic over quenching processes as a function of a rescaled rotational constant $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{B}=B/{s}_{{E}_{3}}$ and a rescaled electric field $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{F}=dF/B$. B,d,F,andsE3B,d,F,\phantom{\rule{4.pt}{0ex}}\text{and}\phantom{\rule{4.pt}{0ex}}{s}_{{E}_{3}}B,d,F,andsE3​​ are respectively the rotational constant, the full electric dipole moment of the molecules, the applied electric field, and a characteristic dipole-dipole energy. We identify two groups of bi-alkali-metal dipolar molecules. The first group, including RbCs, NaK, KCs, LiK, NaRb, LiRb, NaCs, and LiCs, is favorable with a ratio over 1000 at collision energies equal to (or even higher than) their characteristic dipolar energy. The second group, including LiNa and KRb, is not favorable. More generally, for molecules well described by Hund's case b, our adimensional study provides the conditions of efficient evaporative cooling. The range of appropriate rescaled rotational constant and rescaled field is approximately $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{B}\ensuremath{\ge}{10}^{8}$ and $3.25\ensuremath{\le}\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{F}\ensuremath{\le}3.8$, with a maximum ratio reached for $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{F}\ensuremath{\simeq}3.4$ for a given $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{B}$. We also discuss the importance of the electronic van der Waals interaction on the adimensional character of our study.
We propose a method to suppress collisional loss in strongly dipolar, rotationally excited ultracold molecules using a combination of static (dc) and microwave (ac) electric fields. By tuning two excited pair molecular rotational states into a Förster resonance with a dc field, simultaneously driving excited rotational transitions with an ac field removes all long-range bound states, allowing near complete suppression of all two- and three-body collisional loss channels. While permitting tunable dipolar and anti-dipolar interactions, this bound-state-free ac/dc scheme is not subject to photon-changing collisions that are the primary source of two-body loss in shielding with two microwave fields, used to achieve the first molecular Bose-Einstein condensate [Bigagli et al., Nature 631, 289 (2024)]. Using NaCs as a representative example for strongly dipolar molecules, close-coupling calculations are performed to show that bound-state-free shielding can achieve ratios of elastic-to-loss rates ≳106\gtrsim 10^{6}≳106 at 100 nK, with currently accessible ac and dc field generation technologies. This work opens new opportunities for realizing large, long-lived samples of strongly interacting degenerate molecular gases with tunable long-range interactions.
Collisions of polar $^{1}\ensuremath{\Sigma}$ state molecules at ultralow energies are considered, within a model that accounts for long-range dipole-dipole interactions, plus rotation of the molecules. We predict a substantial suppression of dipole-driven inelastic collisions at high values of the applied electric field, namely, field values of several times ${B}_{0}∕\ensuremath{\mu}$. Here B0{B}_{0}B0​ is the rotational constant, and $\ensuremath{\mu}$ is the electric dipole moment of molecules. The sudden large drop in the inelastic cross section is attributed to the onset of degeneracy between molecular rotational levels, which dramatically alters the scattering Hamiltonian. This capability could, in principle, be used to stabilize ultracold gases against collisional losses.
Polar quantum gases represent promising platforms for studying many-body physics and strongly correlated systems with possible applications e.g. in quantum simulation or quantum computation. Due to their large permanent electric dipole moment polar molecules in electric field exhibit strong long-range anisotropic dipole-dipole interactions (DDIs). The creation and trapping of ultracold dipolar diatomic molecules of various species are feasible in many experimental groups nowadays. However long time trapping is still a challenge even in the case of the so called nonreactive molecules which are supposed to be immune against inelastic collisions in their absolute ground state [1] . Various hypothesis have been invoked to explain the possible mechanisms behind the trapping loss, however the check of their pertinence is not trivial since the final products can not be easily detected [2] . To overcome the loss processes we propose the suppression of inelastic collisions between molecules using optical shielding (OS). OS relies on the modification of long-range interactions between ground-state and excited molecules by laser light. The first experimental proof of OS process was demonstrated in case of the collision between identical ultracold alkali-metal atoms [3] .