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arXiv:0808.4147 · Journal of Physics B Atomic Molecular and Optical Physics

Heteronuclear quantum gas mixtures

C. Ospelkaus, S Ospelkaus

Published Oct 1, 2008• 9 citations
arXivHTMLPDF

Abstract

This PhD tutorial article is a review of our experiments on heteronuclear quantum gas mixtures at the University of Hamburg. We introduce basic properties of trapped Fermi-Bose mixtures and demonstrate the achievement of large quantum degenerate mixtures of 40K and 87Rb. Using heteronuclear Feshbach resonances, we show how the heteronuclear interaction can be tuned, allowing us to induce phase separation and collapse for large repulsive and attractive interactions, respectively. We realize Fermi-Bose mixtures in 3D optical lattices as a novel quantum many-body system and study coherence properties of the mixture. Combining our experiments on lattices and Feshbach resonances, we present the first realization of ultracold heteronuclear Feshbach molecules. The molecules are created at individual sites of a 3D optical lattice. We discuss lifetime, binding energy and rf association efficiency in terms of a universal model and give an outlook for possible future developments.

DOI
10.1088/0953-4075/41/20/203001

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Quantum-Atom Optics Downunder (QAO)•Jan 1, 2007•1 citation
This PhD tutorial article is a review of our experiments on heteronuclear quantum gas mixtures at the University of Hamburg. We introduce basic properties of trapped Fermi-Bose mixtures and demonstrate the achievement of large quantum degenerate mixtures of 40K and 87Rb. Using heteronuclear Feshbach resonances, we show how the heteronuclear interaction can be tuned, allowing us to induce phase separation and collapse for large repulsive and attractive interactions, respectively. We realize Fermi-Bose mixtures in 3D optical lattices as a novel quantum many-body system and study coherence properties of the mixture. Combining our experiments on lattices and Feshbach resonances, we present the first realization of ultracold heteronuclear Feshbach molecules. The molecules are created at individual sites of a 3D optical lattice. We discuss lifetime, binding energy and rf association efficiency in terms of a universal model and give an outlook for possible future developments.
Recent years have seen spectacular progress towards the realization of novel strongly correlated systems with homonuclear ultracold quantum gases. The strongly correlated regime has been demonstrated to be accessible both by means of strong, periodic confinement as demonstrated in experiments with optical lattices and through strong resonant interactions which can be produced by means of Feshbach resonances. Given the impressive development seen in homonuclear systems, heteronuclear systems composed of particles with different quantum-statistical behavior (Fermi-Bose mixtures) offer an even wider spectrum of possibilities for quantum simulation and strongly correlated phases, in particular when loaded into an optical lattice and combined with the possibility of almost arbitrary interactions through Feshbach resonances. In this thesis, experiments with Fermi-Bose mixtures in 3-dimensional optical lattices and under the influence of tunable heteronuclear interactions are presented. An experiment allowing for the creation of large mixtures of quantum degenerate fermionic 40K and bosonic 87Rb gases has been set up together with C. Ospelkaus. The onset of degeneracy in both components is demonstrated, and effects of the heteronuclear attractive background interaction are studied. Heteronuclear Feshbach resonances have been used for the first time in order to change the interaction properties of the mixture in a controlled fashion. This has allowed the study of the complete phase diagram of harmonically trapped mixtures, ranging from stable attractively interacting mixtures over a Feshbach-driven controlled mean field collapse of the system for strong attractive interactions, the onset of phase separation for strong repulsive interactions to stable, repulsively interacting mixtures. The studies of Feshbach resonances have also led to the identification of the first p-wave heteronuclear resonance. The periodic potential of an optical lattice is an ideal environment for simulation of quantum many-body Hamiltonians and systems of reduced dimensionality. Theoretical results for fermionic and bosonic atoms in optical lattices are presented, both in a single particle picture, highlighting the role of the external trapping potential, as well as in the interacting many-particle limit. This thesis documents the first realization of a Fermi-Bose quantum many-body system in a 3D optical lattice. In these experiments, the effect of a small fermionic fraction on the coherence properties of the bosonic sample is investigated. Already a small fermionic impurity fraction strongly affects the coherence properties of the bosonic cloud. The quantitative analysis shows that in the presence of the fermionic component, decoherence of the bosonic ensemble occurs at much lower lattice depths than for a pure bosonic sample. The surprisingly large effect has triggered an intense discussion on possible explanations in terms of thermodynamic properties, mean field models and disorder-related localization scenarios. Combining tunable interactions with 3-dimensional optical lattices, formation of heteronuclear Feshbach molecules in single wells of the lattice is demonstrated for the first time, along with a determination of the molecular binding energy and lifetime. By subsequent two-color photoassociation schemes, these (externally) ground state molecules may be transferred into their internal ground state where they exhibit a permanent electric dipole moment. This may result in the realization of novel quantum gases with anisotropic interactions. Das Feld ultrakalter homonuklearer Quantengase wurde in den letzten Jahren durch fundamentale Fortschritte in der Realisierung stark korrelierter Systeme gepragt. Das Regime starker Korrelation wurde sowohl durch starke aussere Einschlusspotentiale in optischen Gittern als auch durch starke resonante Wechselwirkung an einer Feshbach-Resonanz erreicht. Heteronukleare Systeme gemischter Quantenstatistik (Fermi-Bose Mischungen), kombiniert mit dreidimensionalen optischen Gittern und einstellbarer Wechselwirkung, eroffnen die Perspektive, fundamental neue Quantenphasen stark korrelierter Systeme zu realisieren und in einer extrem reinen und kontrollierbaren Umgebung zu studieren. In dieser Arbeit wurde zusammen mit C. Ospelkaus ein Experiment zum Studium ultrakalter Fermi-Bose Mischungen aus 40K und 87Rb in dreidimensionalen optischen Gittern mit einstellbarer Wechselwirkung aufgebaut. Ausgehend von quantentarteten Mischungen groser Teilchenzahl wurden Effekte der attraktiven Hintergrundwechselwirkung zwischen den Konstituenten untersucht. Die Durchstimmbarkeit der Wechselwirkung an einer heteronuklearen Feshbach-Resonanz wurde erstmalig demonstriert und die grundlegenden Phasen harmonisch gefangener Fermi-Bose Mischungen untersucht. Dies reicht von einem wechselwirkungsinduzierten Kollaps im Falle attraktiver Wechselwirkung uber stabile Mischungen attraktiver und repulsiver Wechselwirkung bis hin zur Phasenseparation im Falle starker repulsiver Wechselwirkung. Periodische optische Potentiale bieten eine ideale Umgebung zur Simulation von Vielteilchen-Hamiltonoperatoren und niederdimensionalen Systemen. Grundlegende theoretische Ergebnisse fur fermionische und bosonische Systeme in optischen Gittern werden sowohl aus der Perspektive eines Einteilchenbildes als auch eines Vielteilchenbildes mit und ohne Wechselwirkung vorgestellt. Im Rahmen dieser Arbeit wurde erstmalig ein Vielteilchensystem gemischter Statistik in einem dreidimensionalen optischen Gitter realisiert. In ersten Experimenten wurden die Koharenzeigenschaften der bosonischen Komponente unter dem Einfluss der fermionischen Atome untersucht. Bereits eine kleine Beimischung fermionischer Atome beeinflusst die Koharenzeigenschaften der Bosonen signifikant und induziert einen Koharenzverlust in der bosonischen Wolke bei wesentlich flacheren Gittern als im rein bosonischen System. Die Ursache dieses erstaunlich grosen Effektes wird derzeitig kontrovers diskutiert. Als mogliche Ursachen werden thermodynamische Eigenschaften des Ladeprozesses der Mischung in das Gitter, eine Verschiebung des Ubergangs einer superfluiden in eine Mott-isolierende Phase unter dem Einfluss der heteronuklearen Wechselwirkung sowie unordnungsinduzierte Lokalisierungsphanomene diskutiert. Schlieslich wird die erstmalige Erzeugung langlebiger heteronuklearer Feshbach-Molekule in einem optischen Gitter vorgestellt. In diesen Experimenten wird die Durchstimmbarkeit der Wechselwirkung an einer Feshbach Resonanz mit dreidimensionalen optischen Gittern kombiniert. Die Effizienz des Assoziationsprozesses wird charakterisiert und die Bindungsenergie und Lebensdauer der Molekule gemessen. Fur heteronukleare Molekule aus 40K und 87Rb wird ein permanentes elektrisches Dipolmoment in ihrem absoluten Grundzustand vorhergesagt. Diese heteronuklearen Molekule bieten daher weitreichende Perspektiven zum Studium von Quantengasen anisotroper dipolarer Wechselwirkung.
We report on the creation of ultracold heteronuclear molecules assembled from fermionic 40K and bosonic 87Rb atoms in a 3D optical lattice. Molecules are produced at a heteronuclear Feshbach resonance on both the attractive and the repulsive sides of the resonance. We precisely determine the binding energy of the heteronuclear molecules from rf spectroscopy across the Feshbach resonance. We characterize the lifetime of the molecular sample as a function of magnetic field and measure lifetimes between 20 and 120 ms. The efficiency of molecule creation via rf association is measured and is found to decrease as expected for more deeply bound molecules.
This thesis reports on experiments with quantum degenerate atomic mixtures and molecular gases at ultracold temperatures.The work describes the first realization of a quantum degenerate Fermi-Fermi mixture of two different species with unequal masses.In addition, the first quantum degenerate three-species mixture is realized.Furthermore, the first heteronuclear bosonic molecules with temperatures close to quantum degeneracy are created from a two-species mixture.Within this work a new and very versatile experimental platform to study quantum degenerate two-species Fermi mixtures is presented.The experimental concept relies on sympathetic cooling of two fermionic species ( 40 K and 6 Li) by a large bosonic gas ( 87 Rb).It is shown that large atom numbers and reliable operation are guaranteed by careful choice of the experimental components and parameters, which is essential to deal with the complexity of such an experimental platform with three atomic species.The first important milestone towards quantum degeneracy is realized by simultaneous trapping 87 Rb, 40 K and 6 Li in a magneto-optical trap.This marks the first realization of magneto-optical trapping of two fermionic species and also of three species.To achieve quantum degeneracy, different experimental challenges and difficulties of the three species with very different initial temperatures, scattering cross sections and masses were overcome.A combined compressed MOT and temporal dark MOT phase, a careful state cleaning process during the cooling process, species selective evaporative cooling of rubidium and removing of high energetic lithium atoms turn out to be essential for the successful realization of a quantum degenerate Fermi-Fermi-Bose mixture.Furthermore, at the end of the sympathetic cooling process, the cooling efficiency of lithium is improved by catalytic cooling through potassium.A quantum degenerate Fermi-Fermi mixture can be realized in the strongly interacting regime at a s-wave Feshbach resonance.By an adiabatic magnetic field sweep across the Feshbach resonance ultracold bosonic heteronuclear molecules are created with high efficiencies of up to 50 %.The associated number of molecules are up to 4 × 10 4 molecules, which is the largest number of heteronuclear molecules produced so far.Furthermore, close to resonance an increased molecular lifetime of more than 100 ms is observed.Therefore, this is the first system that may be used to explore many-body physics of a heteronuclear mixture in the strongly interacting regime across a Feshbach resonance.The heteronuclear Fermi mixture adds the mass ratio and the different internal structure as new parameters to quantum degenerate Fermi gases.The molecular cloud being close to quantum degeneracy marks the first starting point for the realization of a heteronuclear molecular BEC and to investigate the BEC-BCS crossover with unequal masses.Moreover, the molecules can be transferred into the rovibrational ground state to create a polar BEC with an anisotropic, long-range interaction.