Abstract We report a new apparatus for the study of two-species quantum degenerate mixture of 41 K and 6 Li atoms. We develop and combine several advanced cooling techniques to achieve both a large atom number and high phase space density of the two-species atom clouds. Furthermore, we build a high efficiency two-species magnetic transport system to transfer atom clouds from the 3D magneto-optical-trap chamber to a full glass science chamber of an extreme high vacuum environment and good optical access. We perform a forced radio-frequency evaporative cooling for 41 K atoms while the 6 Li atoms are sympathetically cooled in an optically plugged magnetic trap. Finally, we achieve the simultaneous quantum degeneracy for the 41 K and 6 Li atoms. The Bose–Einstein condensate of 41 K has 1.4 × 10 5 atoms with a condensate fraction of about 62%, while the degenerate Fermi gas of 6 Li has a total atom number of 5.4 × 10 5 at 0.25 Fermi temperature.
Abstract We report a new apparatus for the study of two-species quantum degenerate mixture of 41 K and 6 Li atoms. We develop and combine several advanced cooling techniques to achieve both a large atom number and high phase space density of the two-species atom clouds. Furthermore, we build a high efficiency two-species magnetic transport system to transfer atom clouds from the 3D magneto-optical-trap chamber to a full glass science chamber of an extreme high vacuum environment and good optical access. We perform a forced radio-frequency evaporative cooling for 41 K atoms while the 6 Li atoms are sympathetically cooled in an optically plugged magnetic trap. Finally, we achieve the simultaneous quantum degeneracy for the 41 K and 6 Li atoms. The Bose–Einstein condensate of 41 K has 1.4 × 10 5 atoms with a condensate fraction of about 62%, while the degenerate Fermi gas of 6 Li has a total atom number of 5.4 × 10 5 at 0.25 Fermi temperature.
We report a compact experimental setup for producing a quantum degenerate mixture of Bose 23Na and Fermi 40K gases. The atoms are collected in dual dark magneto–optical traps (MOT) with species timesharing loading to reduce the light-induced loss, and then further cooled using the gray molasses technique on the D 2 line for 23Na and D 1 line for 40K. The microwave evaporation cooling is used to cool 23Na in | F = 2,mF = 2〉 in an optically plugged magnetic trap, meanwhile, 40K in | F = 9/2,mF = 9/2〉 is sympathetically cooled. Then the mixture is loaded into a large volume optical dipole trap where 23Na atoms are immediately transferred to |1,1〉 for further effective cooling to avoid the strong three-body loss between 23Na atoms in |2,2〉 and 40K atoms in |9/2,9/2〉. At the end of the evaporation in optical trap, a degenerate Fermi gas of 40K with 1.9 × 105 atoms at T/TF = 0.5 in the |9/2,9/2〉 hyperfine state coexists with a Bose–Einstein condensate (BEC) of 23Na with 8 × 104 atoms in the |1,1〉 hyperfine state at 300 nK. We also can produce the two species mixture with the tunable population imbalance by adjusting the 23Na magneto–optical trap loading time.
Simulating highly correlated quantum systems with classical computers exceed their capa- bilities as the system size increases. In analog quantum simulators, the system of interest is mimicked by another quantum system in a highly controllable and accessible environ- ment. Even though quantum simulations with ultracold atoms have been demonstrated successfully, ultra-low temperature regimes are experimentally challenging to achieve and still remain to be explored. New approaches to realizing ever lower temperatures, especially in Fermi systems, have been and continue to be a constant pursuit in Atomic, Molecular, and Optical Physics (AMO). Within this thesis, a new quantum gas experiment of fermionic 6Li and bosonic 166Er is presented and first results on sympathetic cooling of Li to quantum degeneracy by Er are demonstrated. The experimental apparatus is described in detail with a focus on the Li subsystem of the dual-species experiment. Standard and further developed laser cooling techniques used to produce a Magneto-Optical Trap (MOT) of Li are characterized, along with their associated vacuum system stages. In this context, a combined Optical Pumping / Transversal Cooling (OP/TC) technique of the atomic beam on the D1-line is presented, and an enhancement of the MOT loading rate by one order of magnitude is demonstrated. The architecture and building blocks of the laser system for providing resonant light at 671 nm for laser cooling of Li are detailed, and laser locking techniques for frequency stabilization and light amplification by injection locking of high-power laser diodes are presented. Special focus is placed on the introduction and characterization of a new laser- diode-independent injection locking technique, keeping spectrally pure amplified light by a periodic slave current optimization routine executed by a microcontroller. The system has been submitted for patent approval. In further steps of optical cooling of Li, a non-standard free-space Gray Molasses (GM) technique based on intensity ramps, providing temperatures as low as 13 µK, is presented. The non-standard technique is characterized and compared to the standard pulse GM approach, which typically achieves 50 µK only. Cooling steps for preparing an ultracold Li cloud in the glass cell start from an Optical Dipole Trap (ODT) loaded by a compressed Magneto-Optical Trap (cMOT) and consecutive evaporation into a 1D transport lattice of counter propagating 1064 nm laser beams. By detuning one of the lattice beams, optical transport over 1 m to the glass cell with an efficiency of 74% is demonstrated. In a combined ErLi sequence, the two species are transported to the glass cell together and loaded to 1064 nm ODTs. Sympathetic cooling of Li by forced evaporation of Er to T/TF = 0.77 ± 0.10 next to the production of an Er BEC are presented. To conclude, this work introduces a new promising dual-species quantum gas experiment and its ability for providing sympathetic cooling of Li by Er to quantum degeneracy is demon- strated. These findings are an encouraging foundation for achieving higher degeneracies by refined experimental control in pursuit of penetrating unexplored temperature regimes in quantum simulators.
We achieve a highly degenerate and strongly interacting Fermi gas in a mixture of the two lowest hyperfine states of 6 Li by direct evaporative cooling in a high power crossed optical dipole trap. The trap is loaded from a large atom number magneto-optical trap realized by a laser system of 2.5-W intracavity-frequency-doubled light output at 671 nm. With this system, we also demonstrate the production of a molecular Bose-Einstein condensate (mBEC) of 6 Li 2 , and observe the anisotropic expansion of Fermi gases in the so-called BEC-Bardeen-Cooper-Schrieffer crossover regime.
During my PhD, I participated in the full setup from scratch of an experiment that aims at cooling down and manipulating two fermionic alkaline atomic species, 6Li and 40K. Our goal is to study ultracold Fermi mixtures with mass imbalance, and to realize a flexible quantum analog simulator. In fact, for certain quantum many-body problems, such as high critical temperature superconductivity and frustrated anti-ferromagnetism, there is neither complete analytical nor numerical solution. Therefore, cold atoms systems, due to their purity and their high degree of tunability, even on their interaction itself, offer an interesting complementary point of view in the study of those phenomena. As far as the experimental setup is concerned, we built an ultra-high vacuum system, a complete and stable laser source for each species, as well as two performing atomic sources, a 6Li Zeeman slower and a 40K bidimensionnal magneto-optical trap; most of the optical and electrical quantities, as well as the imaging diagnostics, being computer controlled. Once those preliminary steps have been performed and optimized, we could obtain an efficient double magneto-optical trap, containing typically 5x10^9 atoms of 6Li and 8x10^9 atoms of 40K. In this configuration, we produced the first 6Li40K* heteronuclear molecules by photoassociation, for which we identified 70 rovibrational lines. In a second part, I describe in detail the magnetic transport of the atomic mixture, between the magneto-optical trap chamber and a science cell, located in an ultra-high vacuum region with large optical access. The complete setup, from its design to its experimental implementation and optimization, as well as the development of the program and the useful numerical diagnostics, are the heart of my work. The transport efficiency could be tested and optimized, allowing for an efficient transfer of the mixture in the science cell. Thus, after this thesis, all the experimental tools are operational in order to allow for evaporative cooling of the mixture in an optically plugged magnetic trap. Therefore, the field is open for quantum simulation and understanding of many-body problems in ultracold Fermi mixtures.
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 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