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Feshbach spectroscopy of an ultracold Rb-Cs mixture

Daniel L. Jenkin

Published Jan 1, 2012• 0 citations
OpenAlex

Abstract

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.

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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 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.
This thesis reports the formation of a dual-species Bose-Einstein condensate of 87^{87}87Rb and 133^{133}133Cs in the same trapping potential. Quantum degenerate mixtures exhibit rich physics inaccessible to single species experiments and provide an ideal starting point for the creation of ultracold dipolar molecules. These molecules offer a wealth of new research avenues including precision metrology, quantum simulation and computation. The experimental method exploits the efficient sympathetic cooling of 133^{133}133Cs via elastic collisions with 87^{87}87Rb, initially in a magnetic quadrupole trap and subsequently in a levitated optical trap. Evaporative cooling in the dipole trap must compete against a high interspecies three-body inelastic collision rate \mbox{∼10−25−10−26\sim10^{-25}-10^{-26}∼10−25−10−26~cm6/^{6}/6/s}. The two condensates each contain up to \mbox{2×1042\times10^{4}2×104} atoms and exhibit a striking phase separation, revealing the mixture to be immiscible due to strong repulsive interspecies interactions. Sacrificing all the 87^{87}87Rb during the cooling leads to the creation of single-species 133^{133}133Cs condensates of up to \mbox{6×1046\times10^{4}6×104} atoms. In addition this thesis reports the observation of an interspecies Feshbach resonance at 181.7(5)~G and the creation of a pure sample of Cs2_{2}2​ molecules via magneto-association on the 4(g)4 resonance at 19.8~G. These results represent important steps towards the creation of ultracold polar RbCs molecules.
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.
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.
We report the observation of interspecies Feshbach resonances in an optically trapped mixture of 85{}^{85}85Rb and 133{}^{133}133Cs. We measure nine resonances in the lowest spin channel for a magnetic field range from 0 to 700 G and show that they are in good agreement with coupled-channel calculations. The interspecies background scattering length is close to zero over a large range of magnetic fields, permitting the sensitive detection of Feshbach resonances through interspecies thermalization. Our results confirm the quality of the Rb-Cs potential curves [Phys. Rev. A 85, 032506 (2012)] and offer promising starting points for the production of ultracold polar molecules.
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.