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.
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.
This work describes experiments with quantum-degenerate atomic mixtures at ultracold temperatures, where quantum statistics determine macroscopic system properties. The first heteronuclear molecules at ultracold temperatures are formed in a quantum degenerate two-species Fermi-Fermi mixture on the repulsive side of a narrow s-wave Feshbach resonance. Elastic collisions in this mixture are investigated with the method of cross-dimensional relaxation. Long-lived two-body bound states on the atomic side of the resonance are detected due to a many-body effect at the crossover of the narrow Feshbach resonance. In addition, atom scattering with fermionic 40K on a light field grating in the Bragg and Kapitza-Dirac regimes is realized for the first time. The versatile experimental platform, where the investigations are done, offers the possibility to perform studies on mixtures involving the bosonic species 87Rb and the two fermionic species 6Li and 40K. Within this work, mainly interactions between the two fermionic species are considered. A quantum-degenerate mixture of 6Li and 40K can be used to create heteronuclear bosonic molecules close to an interspecies s-wave Feshbach resonance. By an adiabatic magnetic field sweep, up to 4 × 10^4 molecules are produced with conversion efficiencies close to 50%. A direct and sensitive molecule detection method is developed to probe molecule properties. The lifetime of the molecules in an atom-molecule mixture exhibits a strong magnetic field dependence. Close to resonance, lifetimes of more than 100ms are observed what offers excellent starting conditions for further investigation and manipulation of the molecular cloud. The interspecies Feshbach resonance, which serves for the production of molecules, is further characterized. The method of cross-dimensional relaxation is applied for the first time to a Fermi-Fermi mixture. For this method, a non-equilibrium state is created, which rethermalizes by pure interspecies collisions due to the fermionic nature of the two species. The lighter atomic species, 6Li, relaxes faster in the mixture than the heavier one, 40K. This is verified by an analytical model, Monte-Carlo simulations, and measurements. With this technique, elastic scattering cross sections are measured over a wide range of magnetic field strengths across the Feshbach resonance. The position (B0 = 154.71(5)G) and the magnetic field width of the Feshbach resonance (Delta = 1.02(7)G) are determined. By comparison of the several measurements, long-lived bound states exist on the atomic side of the resonance due to a many-body effect in the crossover regime of the resonance. In addition, atomic scattering with ultracold 40K on a light field crystal is studied for the first time. The light grating is generated by two counter-propagating laser beams. Suitable pulse parameters for the realization of atom scattering in the Bragg and Kapitza-Dirac regime are found. The momentum spread of the cloud determines the efficiency of the scattering process, which is increased by lowering the temperature of the system.
This thesis deals with dilute fermionic and bosonic quantum gases in the nK temperature regime.In the work presented here, the first quantum-degenerate mixture of two different fermionic atomic species and the first triple-degenerate Fermi-Fermi-Bose mixture were produced.The quantum-degenerate mixtures were realized using sympathetic cooling of the fermionic species 6 Li and 40 K by an evaporatively cooled gas of bosonic 87 Rb atoms in a magnetic trap.The apparatus, which was designed and built from scratch in this thesis work, provides a very versatile platform for a broad range of experiments.As the first important step in the production of the quantum-degenerate three-species mixture, simultaneous magneto-optical trapping of 6 Li, 40 K, and 87 Rb was achieved, thereby demonstrating the first three-species magneto-optical trap ("triple MOT") and the first two-fermion MOT.The triple MOT is loaded with lithium from a Zeeman slower and with potassium and rubidium from the background vapor that is maintained by atomic vapor dispensers.For this purpose, dispensers for potassium enriched in the isotope 40 K were produced.The triple MOT was characterized and studied with respect to light-assisted interspecies losses.Furthermore, a sequence combining a compressed MOT and a temporal dark MOT is presented that, in spite of the differing requirements of the three species, allows us to achieve favorable starting conditions for further cooling in the QUIC-type magnetic trap.A magnetic transport system is used to transfer the precooled atomic clouds from the MOT position into the QUIC trap.The transport scheme was adapted to the specific requirements of the three-species mixture, in particular to the temperature of laser-cooled lithium, which is high in comparison to other species.The quantum-degenerate regime is reached by evaporative and sympathetic cooling in the QUIC trap.Species-selective evaporation of rubidium is achieved by driving a 87 Rb hyperfine transition to an untrapped state.In order to avoid undesired atom losses and heating due to spin-exchange collisions, the atoms are prepared in the maximally polarized internal ground states.Careful state cleaning during the cooling process was found crucial to attain quantum degeneracy.Since evaporative and sympathetic cooling both rely on energy transfer by elastic collisions between atoms, the efficiency of the cooling process depends on the relevant elastic scattering cross sections.The 87 Rb-87 Rb and 40 K-87 Rb scattering cross sections are quite favorable for this purpose.The 6 Li-87 Rb scattering cross section, however, was unknown at the beginning of the project and was later found to be comparatively small, making sympathetic cooling of 6 Li by 87 Rb rather challenging.In this thesis work, an experimental sequence was developed that nevertheless allows the production of a quantum-degenerate 6 Li gas with large atom number in this mixture.Furthermore, it is shown that in the three-species mixture, the efficiency of the cooling process for 6 Li is significantly increased by the presence of 40 K through catalytic cooling.The quantum-degenerate Fermi-Fermi and Fermi-Fermi-Bose mixtures realized in this thesis work serve as a starting point for a broad range of possible future experiments, including the creation of heteronuclear Fermi-Fermi dimers and the investigation of the BEC-BCS cross-over regime.In this context, the possibilities opened by the mass difference and the differing internal structures of the two fermionic species are of particular interest.v Zusammenfassung Die vorliegende Arbeit befaßt sich mit verdünnten fermionischen und bosonischen Quantengasen im nK-Temperaturbereich. Im Rahmen der Arbeit wurden sowohl die erste quantenentartete Mischung zweier verschiedener fermionischer Atomspezies als auch die erste dreifach entartete Fermi-Fermi-Bose-Mischung erzeugt.Die quantenentarteten Mischungen wurden durch sympathetisches Kühlen der fermionischen Spezies 6 Li and 40 K mit einem mittels Verdampfungskühlung aktiv gekühlten Gas bosonischer 87 Rb-Atome in einer Magnetfalle realisiert.Die Apparatur, die hierfür in dieser Arbeit entworfen und von Grund auf aufgebaut wurde, bietet eine äußerst vielseitige Plattform für ein breites Spektrum an Experimenten.Als erster wichtiger Schritt für die Produktion der quantenentarteten Drei-Spezies-Mischung konnten 6 Li, 40 K und 87 Rb simultan magneto-optisch gefangen werden.Dies stellt die erste Realisierung einer magneto-optischen Falle für drei Spezies ("Dreifach-MOT") und einer MOT für zwei fermionische Spezies dar.Die Dreifach-MOT wird mit Lithium aus einem Zeeman-Abbremser und mit Rubidium und Kalium aus dem Hintergrund-Gas geladen, das durch Atomgas-Dispenser aufrechterhalten wird.Zu diesem Zweck wurden Dispenser mit im Isotop 40 K angereichertem Kalium hergestellt.Die Dreifach-MOT wurde charakterisiert und bezüglich lichtunterstützter Interspezies-Verluste studiert.Desweiteren wird eine aus einer komprimierten MOT und einer zeitlichen Dunkel-MOT kombinierte Sequenz vorgestellt, die es ermöglicht, trotz der sich unterscheidenden Anforderungen der drei Atomsorten günstige Ausgangsbedingungen für das folgende Kühlen in der QUIC-Magnetfalle zu erzielen.Für den Transfer der vorgekühlten Atome von der MOT-Position in die QUIC-Falle wird ein magnetisches Transport-System verwendet.Das Transport-Schema wurde an die besonderen Anforderungen der Drei-Spezies-Mischung angepaßt, insbesondere auch an die im Vergleich zu anderen Spezies hohe Temperatur von lasergekühltem Lithium.Das quantenentartete Regime wird durch Verdampfungskühlung und sympathetisches Kühlen in der QUIC-Falle erreicht.Speziesselektive Verdampfungskühlung von Rubidium wird durch Treiben eines 87 Rb-Hyperfein-Übergangs in einen ungefangenen Zustand erzielt.Um unerwünschte Atomzahlverluste und Heizen durch Spinaustausch-Stöße zu verhindern, sind die Atome in den maximal polarisierten Grundzuständen präpariert.Eine sorgfältige Zustandsreinigung während des Kühlprozesses hat sich als entscheidend zum Erreichen der Quantenentartung erwiesen.Da sowohl Verdampfungskühlung als auch sympathetisches Kühlen auf Energieübertragung durch elastische Stöße zwischen den Atomen beruhen, hängt die Effizienz des Kühlprozesses von den relevanten elastischen Streuquerschnitten ab.Die Streuquerschnitte für 87 Rb-87 Rb-und 40 K-87 Rb-Stöße sind hierfür recht günstig.Der 6 Li-87 Rb-Streuquerschnitt hingegen war zu Beginn des Projektes noch unbekannt und stellte sich später als vergleichsweise klein heraus, so dass sympathetisches Kühlen von 6 Li durch 87 Rb erheblich erschwert ist.In dieser Arbeit konnte jedoch eine experimentelle Sequenz entwickelt werden, die in dieser Mischung trotzdem die Erzeugung eines quantenentarteten 6 Li-Gases mit großer Atomzahl gestattet.Desweiteren wird gezeigt, dass die Kühleffizienz für 6 Li in der Drei-Spezies-Mischung durch die Anwesenheit von 40 K aufgrund von katalytischem Kühlen signifikant erhöht ist.Die im Rahmen dieser Arbeit erzeugten quantenentarteten Fermi-Fermi-und Fermi-Fermi-Bose-Mischungen dienen als Startpunkt für ein breites Spektrum möglicher zukünftiger Experimente, wie beispielsweise der Erzeugung heteronuklearer Fermi-Fermi-Dimere oder der Erforschung des BEC-BCS-Übergangs.Besonders interessant sind dabei die Möglichkeiten, die sich durch die Massendifferenz und die unterschiedlichen internen Strukturen der zwei fermionischen Spezies ergeben.viii
We report on the generation of a quantum degenerate Fermi-Fermi mixture of two different atomic species. The quantum degenerate mixture is realized employing sympathetic cooling of fermionic 6Li and 40K gases by an evaporatively cooled bosonic 87Rb gas. We describe the combination of trapping and cooling methods that proved crucial to successfully cool the mixture. In particular, we study the last part of the cooling process and show that the efficiency of sympathetic cooling of the 6Li gas by 87Rb is increased by the presence of 40K through catalytic cooling. Because of the differing physical properties of the two components, the quantum degenerate 6Li-40K Fermi-Fermi mixture is an excellent candidate for a stable, heteronuclear system allowing the study of several so far unexplored types of quantum matter.
The work described in this thesis can be divided into two parts. The first part is concerned with the production of degenerate Fermi gases and Bose-Fermi mixtures. The second part of the thesis describes studies of strongly interacting spin-mixtures of ultracold fermions. We have cooled a magnetically trapped spin-polarized 6Li Fermi gas into quantum degeneracy by sympathetic cooling with bosonic 23Na. In order to do this, we have converted our previously developed apparatus for Bose-Einstein condensation of 23Na into a two-species cooling facility. We have produced 6Li samples with up to 7 × 107 atoms at temperatures below 0.5TF , and up to 3× 107 atoms at temperatures as low as 0.05TF . Further, we have also produced stable quantum degenerate mixtures of 23Na and 6Li, with several million atoms of each species. In a second set of experiments, we have studied strongly interacting spin-mixtures of 6Li. The strength of interactions was resonantly controlled by applying a magnetic field and tuning to “Feshbach resonances”, which occur when the total energy of the colliding particles coincides with the energy of a bound molecular state. These studies are relevant for the continuing quest for a superfluid transition in a Fermi gas. By studying the inelastic decay of the cloud, we have experimentally identified two Feshbach resonances for the inter-state interactions of the two lowest ground states of 6Li. Most importantly, we found that the lifetime of the strongly interacting system is long enough to observe a superfluid transition in the future. The effects of the coherent collisions in 6Li spin-mixtures were studied using radiofrequency (RF) spectroscopy. We demonstrated the absence of mean field “clock shifts” in a two-state mixture of fermions, and have introduced a three-level method to observe the interaction energy shifts. In the strongly interacting regime, near a Feshbach resonance, we have observed universal behavior of the interaction energy, independent of the value of the scattering length. Finally, we have theoretically studied the effects of Pauli blocking during a free expansion of a degenerate Fermi gas. We have concluded that a strongly interacting normal gas will undergo a hydrodynamic expansion even at zero temperature, due to inefficiency of the Pauli suppression of collisions. Therefore, the already observed hydrodynamic expansion cannot be conclusively interpreted as a signature of superfluidity. Thesis Supervisor: Wolfgang Ketterle Title: John D. MacArthur Professor of Physics
Ultracold mixture experiments hold the promise of providing new insights into many-body quantum systems as well as ultracold chemistry and few-body phenomena. The work presented in this thesis dealt with the construction of a new apparatus for the production and study of ultracold gases of fermionic lithium-6 and bosonic caesium-133. These isotopes offer a wide tunability in their interaction strength, both in inter-species and intra-species collisions, through magnetic Feshbach resonances. Additionally, the widely different resonance frequencies of lithium and caesium enables independent control of each of the species. With this apparatus, Bose-Einstein condensates (BEC) containing 10^4 lithium Feshbach molecules are routinely produced. The cooling system for caesium has been developed in parallel and important steps towards producing ultracold caesium gases have been made. An optical dipole trap has been loaded with 2x10^6 caesium atoms and evaporative cooling towards quantum degeneracy can now be pursued. Laser, vacuum, magnetic and control systems have been developed for the implementation of this experiment. Light produced with this laser system was used to laser cool atoms, create conservative dipole traps as well as to provide means of imaging atomic clouds. Additionally, a system to produce strong magnetic fields of up to 1400 G has been established in order to exploit the wide tunability in the atomic interactions. Software that was developed for the computerised control system facilitated the coordination of all the components involved in the experimental sequence. Measurements and calculations that showcase the functionality of relevant parts of the setup are presented in this thesis. In this experiment, lithium and caesium atoms are obtained from a novel type of Zeeman slower and are loaded into a magneto-optical trap (MOT). The system is capable of doing this independently for each of the atomic species as well as sequentially. After the MOT has been loaded with atoms, they are transferred into a conservative far-off-resonance optical dipole trap. By adjusting the interactions between atoms and lowering the depth of the dipole trap, efficient evaporative cooling of lithium was carried out from which a molecular BEC was obtained. Time-of-flight measurements were used to characterise the condensate and study its expansion dynamics.
The field of ultracold matter has developed rapidly over the course of the past twenty to thirty years. One outstanding achievement has been the realisation of Bose-Einstein condensation (BEC) only 23 years ago. Many other fascinating observations of quantum phenomena have been made at an incredibly fast pace using ultracold atomic samples with contact force interactions as a testbed. Due to the rapid successes of all these experiments on atomic ensembles, researchers began to aim for more elaborate inter-particle interactions. A possible additional one is the dipole-dipole interaction (DDI), the long-range character of which stands out in comparison to the contact interactions. Moreover, DDI is anisotropic and the possibility to tune the direction of the dipoles in the experiment as well as the interaction strength promises the possibility to study a plethora of new quantum phenomena. In recent years, several experiments using different combinations of alkali atoms to create ground-state hetero-nuclear molecules in the ultracold regime have been set up, following the pioneering work on KRb molecules. Of all possible combinations of two alkali atoms, NaK combines the advantages of a high induced electric dipole moment of 2.72 Debye, chemical stability of the ground-state molecules, the possibility to realise bosonic as well as fermionic molecules and a large knowledge base for cooling and trapping of the two atomic species. This thesis reports on the development and construction of an experiment apparatus for the production of ultracold bosonic dipolar ground state 23Na39K molecules. This includes the setup of an ultra-high vacuum (UHV) system, several laser light sources, microwave (MW) and radio frequency (RF) antennas and electronics to control the various components of the experiment. The system pre-cools the atomic samples of 23Na and 39K and loads them into a dual-species three-dimensional magneto-optical trap (3D MOT). After a transfer to a magnetic quadrupole trap, the mixture is cooled by forced MW evaporation until three-body losses limit dual-species operation at high densities and zero magnetic field. Using the pure optical trapping potential of a crossed optical dipole trap (cODT) and RF- state-transfer, the scattering properties of the bosonic mixture of 23Na+39K are investigated in extensive atom loss measurements for a magnetic field range from 0 to 750 G. Several Feshbach resonances, zero crossings of the interspecies scattering length and inelastic loss features in many different scattering channels are identified. The results of this study are used to refine the molecular potentials of NaK and to produce a dual-species quantum-degenerate mixture of two BECs in the state (f = 1, mf = −1) of 23Na + (f = 1, mf = −1) of 39K. The measurements and refined molecular potentials will considerably aid in the future realisation of 23Na39K molecules in their absolute ground state.