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SYMPATHETIC COOLING OF LITHIUM BY LASER-COOLED CESIUM

Stephan Kraft, Marcel Carto Constantin Mudrich, Kilian Singer, Rudolf Grimm, Allard P. Mosk, Matthias Weidemüller

Published Jan 1, 2001• 0 citations
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Abstract

We present first indications of sympathetic cooling between two neutral trapped atomic species. Lithium and cesium atoms are simultaneously stored in an optical dipole trap formed by the focus of a CO2 laser, and allowed to interact for a given period of time. The temperature of the lithium gas is found to decrease when in thermal contact with cold cesium. The timescale of thermalization yields an estimate for the Li-Cs cross-section. Mixtures of atomic gases enrich the field of physics with ultracold gases by making many interesting phenomena available for experimental study. A prime example is the exchange of thermal energy between different components in the mixture. This cross-thermalization can be used for determining inter-species collision cross-sections, and, more importantly, for sympathetic cooling. The process of sympathetic cooling, where the gas of interest is cooled through thermalization with a coolant gas, is especially advantageous if the cooled gas (Li in our case) is hard to cool by other methods, while the coolant gas (Cs) allows for efficient laser cooling. The Li can only be

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SYMPATHETIC COOLING OF LITHIUM BY LASER-COOLED CESIUM

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We present first indications of sympathetic cooling between two neutral, optically trapped atomic species. Lithium and cesium atoms are simultaneously stored in an optical dipole trap formed by the focus of a CO$_2$ laser, and allowed to interact for a given period of time. The temperature of the lithium gas is found to decrease when in thermal contact with cold cesium. The timescale of thermalization yields an estimate for the Li-Cs cross-section.

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We simultaneously trap ultracold lithium and cesium atoms in an optical dipole trap formed by the focus of a ${\mathrm{CO}}_{2}$ laser and study the exchange of thermal energy between the gases. The optically cooled cesium gas efficiently decreases the temperature of the lithium gas through sympathetic cooling. Equilibrium temperatures down to $25\ensuremath{\mu}\mathrm{K}$ have been reached. The measured cross section for thermalizing ${}^{133}\mathrm{Cs}{\ensuremath{-}}^{7}\mathrm{Li}$ collisions is $8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}{\mathrm{cm}}^{2}$, for both species unpolarized in their lowest hyperfine ground state. Besides thermalization, we observe evaporation of lithium purely through elastic cesium-lithium collisions (sympathetic evaporation).

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A cold atomic gas formed by two different species represents an intriguing system for a deeper understanding of atom-atom interactions at ultralow temperatures. We present experiments on a mixture of atomic lithium and cesium which are of particular interest regarding the formation of heteronuclear molecules on the one hand, and the prospects for sympathetic cooling of atomic gases through mutual thermalization on the other hand. A first series of experiments on interaction in presence of a near-resonant light field is performed in a two-species magneto-optical trap. The collisional properties of the lithium-cesium mixture are investigated through detailed analysis of trap-loss processes induced by the trap light. Photoassociation in an additional near-resonant laser field yields high-resolution spectra of the excited Cs_2 dimers, but shows no unambiguous indication of LiCs molecule formation. A second series of experiments on pure ground-state collisional properties utilizes an optical dipole trap formed by light that is detuned extremely far below atomic resonance (quasi-electrostatic trap). Storage times of many minutes are achieved in a particularly simple and versatile setup for both atomic species. Cooling of cesium through evaporation and thermalization by elastic collisions is observed. The evolution of temperature and particle number is compared with model simulations of evaporative cooling. Direct laser cooling of trapped cesium in the absolute energetic ground state is demonstrated. Homonuclear spin-changing collisions of ground-state cesium and lithium atoms are analyzed, and first evidence for pure ground-state collisions between atoms of different species is found. Based on the current achievements, prospects for future experiments are discussed. (orig.)

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Scattering and Sympathetic Cooling within an Ion-Neutral Hybrid Trap

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Evidence of sympathetic cooling of Na + ions by a Na magneto-optical trap in a hybrid trap

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A hybrid ion-neutral trap provides an ideal system to study collisional dynamics between ions and neutral atoms. This system provides a general cooling method that can be applied to species that do not have optically accessible transitions and can also potentially cool internal degrees of freedom. The long-range polarization potentials ($V\ensuremath{\propto}\ensuremath{-}\ensuremath{\alpha}/{r}^{4}$) between ions and neutrals result in large scattering cross sections at cold temperatures, making the hybrid trap a favorable system for efficient sympathetic cooling of ions by collisions with neutral atoms. We present experimental evidence of sympathetic cooling of trapped Na${}^{+}$ ions, which are closed shell and therefore do not have a laser-induced atomic transition from the ground state, by equal-mass cold Na atoms in a magneto-optical trap.

arXiv:1209.2106•Physical Review A•Dec 26, 2012•59 citations
We present first indications of sympathetic cooling between two neutral trapped atomic species. Lithium and cesium atoms are simultaneously stored in an optical dipole trap formed by the focus of a CO2 laser, and allowed to interact for a given period of time. The temperature of the lithium gas is found to decrease when in thermal contact with cold cesium. The timescale of thermalization yields an estimate for the Li-Cs cross-section. Mixtures of atomic gases enrich the field of physics with ultracold gases by making many interesting phenomena available for experimental study. A prime example is the exchange of thermal energy between different components in the mixture. This cross-thermalization can be used for determining inter-species collision cross-sections, and, more importantly, for sympathetic cooling. The process of sympathetic cooling, where the gas of interest is cooled through thermalization with a coolant gas, is especially advantageous if the cooled gas (Li in our case) is hard to cool by other methods, while the coolant gas (Cs) allows for efficient laser cooling. The Li can only be
We present first indications of sympathetic cooling between two neutral, optically trapped atomic species. Lithium and cesium atoms are simultaneously stored in an optical dipole trap formed by the focus of a CO2_22​ laser, and allowed to interact for a given period of time. The temperature of the lithium gas is found to decrease when in thermal contact with cold cesium. The timescale of thermalization yields an estimate for the Li-Cs cross-section.
We simultaneously trap ultracold lithium and cesium atoms in an optical dipole trap formed by the focus of a CO2{\mathrm{CO}}_{2}CO2​ laser and study the exchange of thermal energy between the gases. The optically cooled cesium gas efficiently decreases the temperature of the lithium gas through sympathetic cooling. Equilibrium temperatures down to $25\ensuremath{\mu}\mathrm{K}$ have been reached. The measured cross section for thermalizing ${}^{133}\mathrm{Cs}{\ensuremath{-}}^{7}\mathrm{Li}$ collisions is $8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}{\mathrm{cm}}^{2}$, for both species unpolarized in their lowest hyperfine ground state. Besides thermalization, we observe evaporation of lithium purely through elastic cesium-lithium collisions (sympathetic evaporation).
A cold atomic gas formed by two different species represents an intriguing system for a deeper understanding of atom-atom interactions at ultralow temperatures. We present experiments on a mixture of atomic lithium and cesium which are of particular interest regarding the formation of heteronuclear molecules on the one hand, and the prospects for sympathetic cooling of atomic gases through mutual thermalization on the other hand. A first series of experiments on interaction in presence of a near-resonant light field is performed in a two-species magneto-optical trap. The collisional properties of the lithium-cesium mixture are investigated through detailed analysis of trap-loss processes induced by the trap light. Photoassociation in an additional near-resonant laser field yields high-resolution spectra of the excited Cs_2 dimers, but shows no unambiguous indication of LiCs molecule formation. A second series of experiments on pure ground-state collisional properties utilizes an optical dipole trap formed by light that is detuned extremely far below atomic resonance (quasi-electrostatic trap). Storage times of many minutes are achieved in a particularly simple and versatile setup for both atomic species. Cooling of cesium through evaporation and thermalization by elastic collisions is observed. The evolution of temperature and particle number is compared with model simulations of evaporative cooling. Direct laser cooling of trapped cesium in the absolute energetic ground state is demonstrated. Homonuclear spin-changing collisions of ground-state cesium and lithium atoms are analyzed, and first evidence for pure ground-state collisions between atoms of different species is found. Based on the current achievements, prospects for future experiments are discussed. (orig.)
Summary form only given. A quasi-electrostatic trap (QUEST) realized with a CO/sub 2/ laser relies on the electrostatic polarizability of the trapped particles and therefore represents a universal storage device for ultracold particles. All intriguing application of the QUEST is the simultaneous storage of different atomic species. Elastic and inelastic ground-state interactions can be investigated, one species may act as an agent to sympathetically cool the other species, and the formation and storage of cold ground-state molecules becomes possible in the same trapping volume. Our experiments start with a two-species magneto-optical trap for lithium and cesium.
Dilute ion-neutral gas mixtures are an interesting system to study due to the rich variety and enormous strength of their scattering interactions, especially in the cold and ultracold regimes. Ion-neutral collisions are dominated by long-range polarization potentials that lead to cross sections several orders of magnitude larger than typical neutral-neutral van der Waals interactions. These large ion-neutral interactions can play an important role in atomic and molecular optical physics, astrophysics, and quantum chemistry. Thanks to advances in theoretical modeling of ion-neutral scattering and the development of new ion-neutral hybrid trapping and cooling technology, interest in this previously unexplored area of cold atomic and molecular physics has surged over the past decade.\nThe ion-neutral hybrid trap, composed of a magneto-optical trap concentric within a linear Paul trap, offers the ideal playground for studying and manipulating ion-neutral interactions over a wide energy range (~ 1 μK - 104 K). This dissertation discusses the development of UConn's hybrid trap, compares experimental measurements of the Na+ - Na system and Ca+ - Na system's low-energy collision rates with previously reported theory, and reports on experimental and simulation results of the sympathetic cooling of Na+ ions by laser cooled Na atoms.
A hybrid ion-neutral trap provides an ideal system to study collisional dynamics between ions and neutral atoms. This system provides a general cooling method that can be applied to species that do not have optically accessible transitions and can also potentially cool internal degrees of freedom. The long-range polarization potentials ($V\ensuremath{\propto}\ensuremath{-}\ensuremath{\alpha}/{r}^{4}$) between ions and neutrals result in large scattering cross sections at cold temperatures, making the hybrid trap a favorable system for efficient sympathetic cooling of ions by collisions with neutral atoms. We present experimental evidence of sympathetic cooling of trapped Na+{}^{+}+ ions, which are closed shell and therefore do not have a laser-induced atomic transition from the ground state, by equal-mass cold Na atoms in a magneto-optical trap.