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Optical trapping and Feshbach spectroscopy of an ultracold Rb-Cs mixture

K. Pilch

Published Jan 1, 2009• 0 citations
OpenAlex

Abstract

We investigate quantum-mechanical interactions between ultracold rubidium and cesium in an optical trap at temperatures of a few microkelvin. Our results provide, on the one hand, an experimental key to understand the collisional properties and, on the other hand, a tool to control the interspecies interactions. By performing loss measurements we locate several Feshbach resonances, which provide insight into the energy structure of weakly bound RbCs molecules near the dissociation threshold and allow for the production of such heteronuclear Feshbach molecules. In the future we will transfer these loosely-bound molecules into the absolute internal ground state. The availability of ultracold heteronuclear ground state molecules will open the door to investigate phenomena associated with ultracold polar quantum gases. In our new experimental set-up we are able to trap and cool rubidium and cesium atoms in their lowest internal states. First we load both species into a two-color magneto-optical trap, having full control over the single-species atom number. We extend the technique of degenerate Raman-sideband cooling to a two-color version, which is able to simultaneously cool and polarize both rubidium and cesium. Thereafter we load the atoms into a levitated crossed optical dipole trap. Because of the presence of the gradient magnetic field the trap is highly state selective and consequently provides perfect spin-polarization of the sample. Furthermore, a coincidence of the magnetic-moment-to-mass ratios of the two species allows for simultaneous levitation of both, which assures an almost perfect spatial overlap between the species. We perform Feshbach spectroscopy in two different spin channels of the mixture within a magnetic field ranging from 20 to 300G. In the lowest spin combination of the species we locate 23 interspecies Feshbach resonances, while in a higher spin mixture we find 2 resonances. The high number of resonances found within this range of magnetic field is unusual for alkali mixtures. The presence of many resonances points to scattering properties of the mixture, which include higher-order coupling mechanisms. The obtained data on the Feshbach spectroscopy provide, on one hand, fundamental experimental input to characterize the Rb-Cs scattering properties and, on the other hand, identification of possible starting points for the association of ultracold heteronuclear RbCs molecules. In addition we show preliminary results on spectroscopy of the binding energy of RbCs dimers, based on a modulation of the magnetic field. The recently obtained Bose-Einstein condensate of Cs atoms, which represents a benchmark for the performance of the present apparatus, will be discussed as well as the potential pathways towards a double-degenerate mixture. We follow two main goals, first the production of a double Bose-Einstein condensate and, second, the transfer of shallow bound Feshbach molecules to the absolute internal ground state. The production of heteronuclear ground state molecules allows to enter the world of ultracold polar quantum systems.

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We report on the observation of interspecies Feshbach resonances in an ultracold, optically trapped mixture of Rb and Cs atoms. In a magnetic-field range up to 300 G we find 23 interspecies Feshbach resonances in the lowest spin channel and two resonances in a higher channel of the mixture. The extraordinarily rich Feshbach spectrum suggests the importance of different partial waves in both the open and closed channels of the scattering problem along with higher-order coupling mechanisms. Our results, on one hand, provide fundamental experimental input to characterize the Rb-Cs scattering properties and, on the other hand, identify possible starting points for the association of ultracold heteronuclear RbCs molecules.

arXiv:0812.3287•Physical Review A•Apr 30, 2009•112 citations

Creation of ultracold RbCs molecules

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This thesis reports the investigation of the scattering properties of a mixture of Rb and Cs and the formation of ultracold Feshbach molecules. The production of Feshbach molecules is a crucial step towards the production of ultracold polar molecules, which is of significant interest for a wide range of potential applications. We have investigated the scattering properties of a mixture of $^{85}$Rb and $^{133}$Cs in their lowest spin channel in a magnetic field range from 0 to 700\,G. Furthermore, we explored the Feshbach spectrum of $^{85}$Rb alone in both, the $(f=2,m_f=-2)$ and $(2,+2)$ states up to a magnetic field of 1000\,G. Additionally a Feshbach resonance in a $(2,+2)+(3,+3)$ spin mixture was experimentally confirmed. We associated Cs$_2$ Feshbach molecules using a Feshbach resonance at 19.9\,G. \mbox{$2.1(1) \times 10^4$} molecules with a temperature of $\sim60$\,nK were produced from a sample of $3.28(2) \times 10^5$ Cs atoms with a PSD of 0.20(1). Due to a magnetic field gradient, the molecules `bounce' at an avoided crossing between two states at 13.5\,G. This gradient field was also used to produce multiple molecular clouds from one atomic sample. A combination of both techniques led to a `collision' between two Cs$_2$ clouds. Furthermore, we associated up to $\sim5000$ heteronuclear $^{87}$RbCs Feshbach molecules using an interspecies resonance at 197.1\,G. Confined in the dipole trap the molecules have a lifetime of 0.21(1)\,s. We have measured the magnetic moment of the molecules in different internal states in a magnetic field range from 181 to 185\,G. Molecular loss spectroscopy on electronically excited states was performed to identify candidates for the intermediate state of a STIRAP transfer of the molecules into their rovibrational ground state. Subsequently, the binding energy of the rovibrational ground state was measured to be 3811.574(1)\,cm$^{-1}$, using two-photon spectroscopy.

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Towards the production of ultracold ground-state RbCs molecules: Feshbach resonances, weakly bound states, and the coupled-channel model

T. Takekoshi, Markus Debatin, Raffael Rameshan +8 more

We have studied interspecies scattering in an ultracold mixture of ${}^{87}$Rb and ${}^{133}$Cs atoms, both in their lowest-energy spin states. The three-body loss signatures of 30 incoming $s$- and $p$-wave magnetic Feshbach resonances over the range 0 to 667 G have been cataloged. Magnetic field modulation spectroscopy was used to observe molecular states bound by up to 2.5 $\text{MHz}\ifmmode\times\else\texttimes\fi{}h$. We have created RbCs Feshbach molecules using two of the resonances. Magnetic moment spectroscopy along the magnetoassociation pathway from 197 to 182 G gives results consistent with the observed and calculated dependence of the binding energy on magnetic field strength. We have set up a coupled-channel model of the interaction and have used direct least-squares fitting to refine its parameters to fit the experimental results from the Feshbach molecules, in addition to the Feshbach resonance positions and the spectroscopic results for deeply bound levels. The final model gives a good description of all the experimental results and predicts a large resonance near 790 G, which may be useful for tuning the interspecies scattering properties. Quantum numbers and vibrational wave functions from the model can also be used to choose optimal initial states of Feshbach molecules for their transfer to the rovibronic ground state using stimulated Raman adiabatic passage.

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Feshbach spectroscopy of an ultracold Rb-Cs mixture

Daniel L. Jenkin

This thesis reports the observation of interspecies Feshbach resonances in an ultracold mixture of Rb and Cs atoms. A versatile combined magnetic and optical potential has been designed and constructed which is capable of bringing both $^{87}\rm{Rb}$ and $^{133}\rm{Cs}$ to degeneracy, and reaching high phase-space density in $^{85}\rm{Rb}$. High phase-space density mixtures are the first step required in the production of ultracold polar molecules, the topic of much current research. The apparatus capitalises on the efficient capture of atoms by a magnetic trap from a magneto-optical trap, and the efficient sympathetic cooling of Cs by Rb therein. Upon transfer to the crossed optical dipole trap condensates in excess of $1\times10^{6}$ $^{87}\rm{Rb}$ atoms and approximately $1\times10^{5}$ $^{133}\rm{Cs}$ atoms are produced after direct evaporation and gravito-magnetic tilting of the potential. The observation of six interspecies $^{87}\rm{Rb}$-$^{133}\rm{Cs}$ Feshbach resonances are reported, three of which had only been predicted theoretically, allowing testing and development of the theoretical model. Furthermore, the extrapolation of this model has predicted numerous Feshbach resonances between $^{85}\rm{Rb}$ and $^{133}\rm{Cs}$, none of which have been experimentally observed prior to this work. The versatile nature of this apparatus is discussed, including the application of the current system to cooling of $^{85}\rm{Rb}$. Initial experiments observed seven interspecies resonances, including a broad s-wave resonance at a magnetic field of $(644\pm2)$ G which is in excellent agreement with the theoretical prediction. Further work has revealed that fourteen Feshbach resonances exist in the 0-700 G magnetic field range between $^{85}\rm{Rb}$ and $^{133}\rm{Cs}$ atoms in the $\left|2,+2\right\rangle$ and $\left|3,+3\right\rangle$ states, respectively. Several of these resonances would be ideal for magneto-association of RbCs molecules, prior to transfer to the rovibrational ground-state.

•Jan 1, 2012•0 citations

Interspecies Feshbach Resonances in an Ultracold, Optically Trapped Bose-Fermi Mixture of Cesium and Lithium

Marc Repp

This thesis reports on the tunability of interactions in ultracold Bose-Fermi mixtures of Cesium and Lithium. The \nfirst realization of an optically trapped \n6Li -133Cs mixture enabled to perform trap loss spectroscopy measurements to identify \nmagnetic Feshbach resonances. A total of 19 interspecies Feshbach resonances, all in the magnetic field range between 650 G and 950 G, were observed for the two energetically lowest spin states of each species. Two 5 G broad and especially two 60 G broad s-wave resonances give perspectives to produce a dipolar quantum gas of LiCs ground state molecules as well as to study the Efimov effect in highly mass imbalanced systems. In addition, a unique relative tunability of intra- and interspecies \nscattering lengths was found which makes the 6Li -133Cs system also well suited for the investigation of polarons. Evaporative cooling was performed on optically trapped samples which contained only one of the species. In this way, Bose-Einstein condensates of 6Li molecules as well as 133Cs samples at a phase-space density of p = 4 · 10^2 were prepared. All experiments were performed in a new apparatus, which has been designed and set up during this thesis.

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Mixtures of ultracold atoms and the quest for ultracold molecules

Matthias Weidemüller

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.)

•Jan 1, 2000•1 citation
We investigate quantum-mechanical interactions between ultracold rubidium and cesium in an optical trap at temperatures of a few microkelvin. Our results provide, on the one hand, an experimental key to understand the collisional properties and, on the other hand, a tool to control the interspecies interactions. By performing loss measurements we locate several Feshbach resonances, which provide insight into the energy structure of weakly bound RbCs molecules near the dissociation threshold and allow for the production of such heteronuclear Feshbach molecules. In the future we will transfer these loosely-bound molecules into the absolute internal ground state. The availability of ultracold heteronuclear ground state molecules will open the door to investigate phenomena associated with ultracold polar quantum gases. In our new experimental set-up we are able to trap and cool rubidium and cesium atoms in their lowest internal states. First we load both species into a two-color magneto-optical trap, having full control over the single-species atom number. We extend the technique of degenerate Raman-sideband cooling to a two-color version, which is able to simultaneously cool and polarize both rubidium and cesium. Thereafter we load the atoms into a levitated crossed optical dipole trap. Because of the presence of the gradient magnetic field the trap is highly state selective and consequently provides perfect spin-polarization of the sample. Furthermore, a coincidence of the magnetic-moment-to-mass ratios of the two species allows for simultaneous levitation of both, which assures an almost perfect spatial overlap between the species. We perform Feshbach spectroscopy in two different spin channels of the mixture within a magnetic field ranging from 20 to 300G. In the lowest spin combination of the species we locate 23 interspecies Feshbach resonances, while in a higher spin mixture we find 2 resonances. The high number of resonances found within this range of magnetic field is unusual for alkali mixtures. The presence of many resonances points to scattering properties of the mixture, which include higher-order coupling mechanisms. The obtained data on the Feshbach spectroscopy provide, on one hand, fundamental experimental input to characterize the Rb-Cs scattering properties and, on the other hand, identification of possible starting points for the association of ultracold heteronuclear RbCs molecules. In addition we show preliminary results on spectroscopy of the binding energy of RbCs dimers, based on a modulation of the magnetic field. The recently obtained Bose-Einstein condensate of Cs atoms, which represents a benchmark for the performance of the present apparatus, will be discussed as well as the potential pathways towards a double-degenerate mixture. We follow two main goals, first the production of a double Bose-Einstein condensate and, second, the transfer of shallow bound Feshbach molecules to the absolute internal ground state. The production of heteronuclear ground state molecules allows to enter the world of ultracold polar quantum systems.
We report on the observation of interspecies Feshbach resonances in an ultracold, optically trapped mixture of Rb and Cs atoms. In a magnetic-field range up to 300 G we find 23 interspecies Feshbach resonances in the lowest spin channel and two resonances in a higher channel of the mixture. The extraordinarily rich Feshbach spectrum suggests the importance of different partial waves in both the open and closed channels of the scattering problem along with higher-order coupling mechanisms. Our results, on one hand, provide fundamental experimental input to characterize the Rb-Cs scattering properties and, on the other hand, identify possible starting points for the association of ultracold heteronuclear RbCs molecules.
This thesis reports the investigation of the scattering properties of a mixture of Rb and Cs and the formation of ultracold Feshbach molecules. The production of Feshbach molecules is a crucial step towards the production of ultracold polar molecules, which is of significant interest for a wide range of potential applications. We have investigated the scattering properties of a mixture of 85^{85}85Rb and 133^{133}133Cs in their lowest spin channel in a magnetic field range from 0 to 700\,G. Furthermore, we explored the Feshbach spectrum of 85^{85}85Rb alone in both, the (f=2,mf=−2)(f=2,m_f=-2)(f=2,mf​=−2) and (2,+2)(2,+2)(2,+2) states up to a magnetic field of 1000\,G. Additionally a Feshbach resonance in a (2,+2)+(3,+3)(2,+2)+(3,+3)(2,+2)+(3,+3) spin mixture was experimentally confirmed. We associated Cs2_22​ Feshbach molecules using a Feshbach resonance at 19.9\,G. \mbox{2.1(1)×1042.1(1) \times 10^42.1(1)×104} molecules with a temperature of ∼60\sim60∼60\,nK were produced from a sample of 3.28(2)×1053.28(2) \times 10^53.28(2)×105 Cs atoms with a PSD of 0.20(1). Due to a magnetic field gradient, the molecules `bounce' at an avoided crossing between two states at 13.5\,G. This gradient field was also used to produce multiple molecular clouds from one atomic sample. A combination of both techniques led to a `collision' between two Cs2_22​ clouds. Furthermore, we associated up to ∼5000\sim5000∼5000 heteronuclear 87^{87}87RbCs Feshbach molecules using an interspecies resonance at 197.1\,G. Confined in the dipole trap the molecules have a lifetime of 0.21(1)\,s. We have measured the magnetic moment of the molecules in different internal states in a magnetic field range from 181 to 185\,G. Molecular loss spectroscopy on electronically excited states was performed to identify candidates for the intermediate state of a STIRAP transfer of the molecules into their rovibrational ground state. Subsequently, the binding energy of the rovibrational ground state was measured to be 3811.574(1)\,cm−1^{-1}−1, using two-photon spectroscopy.
We have studied interspecies scattering in an ultracold mixture of 87{}^{87}87Rb and 133{}^{133}133Cs atoms, both in their lowest-energy spin states. The three-body loss signatures of 30 incoming sss- and ppp-wave magnetic Feshbach resonances over the range 0 to 667 G have been cataloged. Magnetic field modulation spectroscopy was used to observe molecular states bound by up to 2.5 $\text{MHz}\ifmmode\times\else\texttimes\fi{}h$. We have created RbCs Feshbach molecules using two of the resonances. Magnetic moment spectroscopy along the magnetoassociation pathway from 197 to 182 G gives results consistent with the observed and calculated dependence of the binding energy on magnetic field strength. We have set up a coupled-channel model of the interaction and have used direct least-squares fitting to refine its parameters to fit the experimental results from the Feshbach molecules, in addition to the Feshbach resonance positions and the spectroscopic results for deeply bound levels. The final model gives a good description of all the experimental results and predicts a large resonance near 790 G, which may be useful for tuning the interspecies scattering properties. Quantum numbers and vibrational wave functions from the model can also be used to choose optimal initial states of Feshbach molecules for their transfer to the rovibronic ground state using stimulated Raman adiabatic passage.
This thesis reports the observation of interspecies Feshbach resonances in an ultracold mixture of Rb and Cs atoms. A versatile combined magnetic and optical potential has been designed and constructed which is capable of bringing both 87Rb^{87}\rm{Rb}87Rb and 133Cs^{133}\rm{Cs}133Cs to degeneracy, and reaching high phase-space density in 85Rb^{85}\rm{Rb}85Rb. High phase-space density mixtures are the first step required in the production of ultracold polar molecules, the topic of much current research. The apparatus capitalises on the efficient capture of atoms by a magnetic trap from a magneto-optical trap, and the efficient sympathetic cooling of Cs by Rb therein. Upon transfer to the crossed optical dipole trap condensates in excess of 1×1061\times10^{6}1×106 87Rb^{87}\rm{Rb}87Rb atoms and approximately 1×1051\times10^{5}1×105 133Cs^{133}\rm{Cs}133Cs atoms are produced after direct evaporation and gravito-magnetic tilting of the potential. The observation of six interspecies 87Rb^{87}\rm{Rb}87Rb-133Cs^{133}\rm{Cs}133Cs Feshbach resonances are reported, three of which had only been predicted theoretically, allowing testing and development of the theoretical model. Furthermore, the extrapolation of this model has predicted numerous Feshbach resonances between 85Rb^{85}\rm{Rb}85Rb and 133Cs^{133}\rm{Cs}133Cs, none of which have been experimentally observed prior to this work. The versatile nature of this apparatus is discussed, including the application of the current system to cooling of 85Rb^{85}\rm{Rb}85Rb. Initial experiments observed seven interspecies resonances, including a broad s-wave resonance at a magnetic field of (644±2)(644\pm2)(644±2) G which is in excellent agreement with the theoretical prediction. Further work has revealed that fourteen Feshbach resonances exist in the 0-700 G magnetic field range between 85Rb^{85}\rm{Rb}85Rb and 133Cs^{133}\rm{Cs}133Cs atoms in the ∣2,+2⟩\left|2,+2\right\rangle∣2,+2⟩ and ∣3,+3⟩\left|3,+3\right\rangle∣3,+3⟩ states, respectively. Several of these resonances would be ideal for magneto-association of RbCs molecules, prior to transfer to the rovibrational ground-state.
This thesis reports on the tunability of interactions in ultracold Bose-Fermi mixtures of Cesium and Lithium. The \nfirst realization of an optically trapped \n6Li -133Cs mixture enabled to perform trap loss spectroscopy measurements to identify \nmagnetic Feshbach resonances. A total of 19 interspecies Feshbach resonances, all in the magnetic field range between 650 G and 950 G, were observed for the two energetically lowest spin states of each species. Two 5 G broad and especially two 60 G broad s-wave resonances give perspectives to produce a dipolar quantum gas of LiCs ground state molecules as well as to study the Efimov effect in highly mass imbalanced systems. In addition, a unique relative tunability of intra- and interspecies \nscattering lengths was found which makes the 6Li -133Cs system also well suited for the investigation of polarons. Evaporative cooling was performed on optically trapped samples which contained only one of the species. In this way, Bose-Einstein condensates of 6Li molecules as well as 133Cs samples at a phase-space density of p = 4 · 10^2 were prepared. All experiments were performed in a new apparatus, which has been designed and set up during this thesis.
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.)