Ultracold atoms are powerful platforms through which to study many-body physics. In recent years there has been a marked increase in the number and variety of species (lanthanides, transition metals, dipolar molecules, etc.) brought to ultracold temperatures, providing new ingredients from which to build interesting systems. In this thesis I report on the design and construction of an erbium-lithium mixture apparatus with broad applicability for quantum simulation. The combination of large mass imbalance, competition of interaction length scales, and variable quantum statistics points toward this mixture hosting a rich variety of Bose-Bose and Bose-Fermi phases with applications ranging from condensed matter to astrophysics. Leveraging these desirable properties, however, necessitated the development of a new quantum gas platform. The first section of this thesis is dedicated to describing that process in detail. The second section of this thesis describes the development of experimental sequences through which we synthesize quantum gases. In our experiments we successively laser cool each species before combining them in a suite of optical dipole traps for evaporative cooling to degeneracy. I finally present our creation of the first degenerate Bose-Bose mixture of erbium-166 and lithium-7, as well as a measurement of the interspecies s-wave scattering length.
Ultracold atoms are powerful platforms through which to study many-body physics. In recent years there has been a marked increase in the number and variety of species (lanthanides, transition metals, dipolar molecules, etc.) brought to ultracold temperatures, providing new ingredients from which to build interesting systems. In this thesis I report on the design and construction of an erbium-lithium mixture apparatus with broad applicability for quantum simulation. The combination of large mass imbalance, competition of interaction length scales, and variable quantum statistics points toward this mixture hosting a rich variety of Bose-Bose and Bose-Fermi phases with applications ranging from condensed matter to astrophysics. Leveraging these desirable properties, however, necessitated the development of a new quantum gas platform. The first section of this thesis is dedicated to describing that process in detail. The second section of this thesis describes the development of experimental sequences through which we synthesize quantum gases. In our experiments we successively laser cool each species before combining them in a suite of optical dipole traps for evaporative cooling to degeneracy. I finally present our creation of the first degenerate Bose-Bose mixture of erbium-166 and lithium-7, as well as a measurement of the interspecies s-wave scattering length.
We report the realization of a degenerate mixture of 166Er and 7Li atoms in their energetically lowest spin states. The two species are sequentially laser-cooled and loaded into an optical dipole trap, then transported to a glass cell and simultaneously evaporated to degeneracy. Er serves as the coolant for Li, and we observe efficient sympathetic cooling facilitated by a large interspecies elastic scattering cross section. Three-body losses are found to be small, making this platform promising for the study of interacting mixtures with large mass imbalance.
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.
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.
We have produced a quantum degenerate mixture of fermionic alkali-metal 6Li and bosonic spin-singlet 174Yb gases. This was achieved using sympathetic cooling of lithium atoms by evaporatively cooled ytterbium atoms in a far-off-resonant optical dipole trap. We observe the coexistence of Bose-condensed ($T/{T}_{\mathrm{c}}\ensuremath{\simeq}0.8$) 174Yb with $2.3\ifmmode\times\else\texttimes\fi{}{10}^{4}$ atoms and Fermi degenerate ($T/{T}_{\mathrm{F}}\ensuremath{\simeq}0.3$) 6Li with $1.2\ifmmode\times\else\texttimes\fi{}{10}^{4}$ atoms. Quasipure Bose-Einstein condensates of up to $3\ifmmode\times\else\texttimes\fi{}{10}^{4}$ 174Yb atoms can be produced in single-species experiments. Our results mark a significant step toward studies of few- and many-body physics with mixtures of alkali-metal and alkaline-earth-metal-like atoms, and for the production of paramagnetic polar molecules in the quantum regime. Our methods also establish a convenient scheme for producing quantum degenerate ytterbium atoms in a 1064 nm optical dipole trap.
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.
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