This thesis describes the construction of a versatile apparatus to study ultracold quan-tum mixtures capable of simultaneously cooling fermionic 6Li and 40K, as well as the bosonic 41K. The main features of the experimental setup are presented, in particular the addition of a new species 23Na, which has enabled the study of the Bose-Fermi mixture 23Na-40K. Three main experimental benchmarks are outlined: first, the production of a Bose-Einstein Condensate of 41K is discussed and an evaluation of its properties as a coolant are analysed. Secondly, the creation of triply degenerate Bose-Fermi-Fermi of 41K-40K-6Li gas is presented. Simultaneous observation of Pauli Pressure and Bose Condensation in the triply degenerate gas is reported. In addition, interspecies Fes-hbach resonances between 41K-40K and 6Li-41K are observed, opening the way to the study of a strongly interacting isotopic Bose-Fermi mixture of 41K-40K, which have similar mass. Thirdly, the creation of a quantum degenerate Bose-Fermi mixture of 23Na-40K is discussed and over thirty Feshbach resonances are identified.
This thesis describes the construction of a versatile apparatus to study ultracold quan-tum mixtures capable of simultaneously cooling fermionic 6Li and 40K, as well as the bosonic 41K. The main features of the experimental setup are presented, in particular the addition of a new species 23Na, which has enabled the study of the Bose-Fermi mixture 23Na-40K. Three main experimental benchmarks are outlined: first, the production of a Bose-Einstein Condensate of 41K is discussed and an evaluation of its properties as a coolant are analysed. Secondly, the creation of triply degenerate Bose-Fermi-Fermi of 41K-40K-6Li gas is presented. Simultaneous observation of Pauli Pressure and Bose Condensation in the triply degenerate gas is reported. In addition, interspecies Fes-hbach resonances between 41K-40K and 6Li-41K are observed, opening the way to the study of a strongly interacting isotopic Bose-Fermi mixture of 41K-40K, which have similar mass. Thirdly, the creation of a quantum degenerate Bose-Fermi mixture of 23Na-40K is discussed and over thirty Feshbach resonances are identified.
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
Two sets of studies are described in this thesis. In the first set, an atom interferometry technique was developed for the measurement of the fine structure constant using a BoseEinstein Condensate. In the second set, degenerate Fermi gases were prepared and their properties explored in a regime of strong interactions. We have developed an atom interferometer which is capable of measuring with high precision the “photon recoil frequency” (ωrec). ωrec corresponds to the kinetic energy of an atom recoiling due to absorption of a photon. ωrec can be used to determine the quantity h/matom and the fine structure constant, α. A preliminary measurement using a 23Na BoseEinstein Condensate yielded ωrec with a precision of 7 × 10−6 which deviated by 2 × 10−4 from the currently accepted value. Plausible upgrades to the apparatus should produce a precision of 10−9 which would bring within reach a measurement of h/matom and α in the 10−9 range. Such accuracy would be of considerable scientific and metrological import. A quantum degenerate gas of 6Li fermions was produced by sympathetic cooling with 23Na bosons in a two-species atom trapping apparatus. The cooling strategy was optimized to enable production of fermions with atom numbers up to 7 × 107 at half the Fermi temperature (TF ), or temperatures down to 0.05TF ∼ 100 nK with 3× 107 atoms. We can also produce degenerate Bose-Fermi mixtures with several million atoms in each species. We studied the behavior of mixtures of fermi gases in regimes of strong interactions near “Feshbach” resonances. A study of system stability enabled the experimental observation of two such Feshbach resonances. We carried out a theoretical study to interpret the observation (by many experimental groups) of hydrodynamic behavior of fermi gases during expansion out of an atom trap in a strongly-interacting regime. The study concluded that this behavior was not a qualitative signature of fermionic superfluidity and could arise from classical collisions. Finally, radio-frequency (RF) spectroscopy was used to probe interaction in 6Li. We demonstrated the absence of mean field “clock” shifts of RF transitions in a two-(spin)state fermion system. Using a three-state system, we measured the interaction strength between different spin states. The measurements near Feshbach resonances indicate a saturation of the interaction strength at a large negative value. This result is of relevance in the continuing quest for fermionic superfluidity in atomic gases. Thesis Supervisor: David E. Pritchard Title: Cecil and Ida Green Professor of Physics Thesis Supervisor: Wolfgang Ketterle Title: John D. MacArthur Professor of Physics
This thesis presents studies of quantum degenerate atomic gases of fermionic 6Li and bosonic 7Li. Degeneracy is reached by evaporative cooling of 7Li in a strongly confining magnetic trap. Since at low temperatures direct evaporative cooling is not possible for a polarized fermionic gas, 6Li is sympathetically cooled by thermal contact with 7Li. In a first series of experiments both isotopes are trapped in their low-field seeking higher hyperfine states. A Fermi degeneracy of is achieved for 105 fermions. For more than ~300 atoms, the 7Li condensate collapses, due to the attractive interatomic interaction in this state. This limits the degeneracy reached for both species. To overcome this limit, in a second series of experiments 7Li and 6Li atoms are transferred to their low field seeking lower hyperfine states, where the boson-boson interaction is repulsive but weak. The inter-isotope collisions are used to thermalize the mixture. A 7Li Bose-Einstein condensate (BEC) of 104 atoms immersed in a Fermi sea is produced. The BEC is quasi-one-dimensional and the thermal fraction can be negligible. The measured degeneracies are . The temperature is measured using the bosonic thermal fraction, which vanishes at the lowest temperatures, limiting our measurement sensitivity. In a third series of experiments, the bosons are transferred into an optical trap and their internal state is changed to , the lowest energy state. A Feshbach resonance is detected and used to produce a BEC with tunable atomic interactions. When the effective interaction between atoms is tuned to be small and attractive, we observe the formation of a matter-wave bright soliton. Propagation of the soliton without spreading over a macroscopic distance of 1.1~mm is observed.
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.
The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. iii Goldwin, Jonathan Michael (Ph.D., Physics) Quantum Degeneracy and Interactions in the 87Rb-40K Bose-Fermi Mixture Thesis directed by Associate Professor Deborah S. Jin An apparatus for producing dilute-gas quantum degenerate Bose-Fermi mix-tures is presented. The experiment uses forced evaporative cooling to obtain a nearly pure 87Rb Bose-Einstein condensate immersed in a gas of sympathetically cooled 40K atoms at 0.2 times the Fermi temperature. The design, construction, and operation of the apparatus are described in detail. The onset of quantum degeneracy is characterized and contrasted between species, revealing the effects of quantum statistics on the bulk properties of the gases. The apparatus is then used to study the interactions between species in ultracold non-degenerate mixtures. Measurements of rethermalization rates are used to determine the elastic scattering cross-section between species. From these measurements the magnitude of the inter-species s-wave scattering length is de-termined. The scattering length is central to characterizing a variety of static and dynamic properties of the mixture. The apparatus is additionally used to study for the first time Feshbach resonances in the collisions between 87Rb and 40K atoms. These resonances present a means for experimentally tuning the scattering length between species to any desired value. Prospects for future experiments exploiting this real-time control over the interactions are discussed. iv
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.