Manifestations of Quantum Physics at the thermodynamical level are found in a broadrange of physical systems. A famous example is superfluidity, discovered at the beginningof the 20th century and found in many different situations, from liquid helium to neutronstars. Dilute ultracold gases offer a unique versatility to engineer quantum many-bodysystems, which can be directly compared with theory thanks to the controllability of theirenvironment. In this thesis we present several experimental investigations led on ultracoldlithium gases. Lithium provides the possibility to study ensembles of bosons andfermions, with controllable interactions between the constituents. We present experimentaltechniques for preparation and studies of degenerate gases of lithium, with prospects forimprovement of the existing methods. We first report on an investigation of three-bodyrecombination of bosons under a resonant two-body interaction. This study, quantitativelycompared with theory constitutes a benchmark for further studies of the unitary Bose gas.Finally, we present the first experimental realization of a mixture of a Bose superfluid witha Fermi superfluid. We demon- strate that both components are in the superfluid regime,and that the counter-flow motion between them possesses the characteristics of superfluidflow, with the absence of viscosity below a critical velocity, and an onset of friction above.Using collective oscillations of the mixture, we measure the coupling between the twosuperfluids in close agreement with a theoretical model.
Manifestations of Quantum Physics at the thermodynamical level are found in a broadrange of physical systems. A famous example is superfluidity, discovered at the beginningof the 20th century and found in many different situations, from liquid helium to neutronstars. Dilute ultracold gases offer a unique versatility to engineer quantum many-bodysystems, which can be directly compared with theory thanks to the controllability of theirenvironment. In this thesis we present several experimental investigations led on ultracoldlithium gases. Lithium provides the possibility to study ensembles of bosons andfermions, with controllable interactions between the constituents. We present experimentaltechniques for preparation and studies of degenerate gases of lithium, with prospects forimprovement of the existing methods. We first report on an investigation of three-bodyrecombination of bosons under a resonant two-body interaction. This study, quantitativelycompared with theory constitutes a benchmark for further studies of the unitary Bose gas.Finally, we present the first experimental realization of a mixture of a Bose superfluid witha Fermi superfluid. We demon- strate that both components are in the superfluid regime,and that the counter-flow motion between them possesses the characteristics of superfluidflow, with the absence of viscosity below a critical velocity, and an onset of friction above.Using collective oscillations of the mixture, we measure the coupling between the twosuperfluids in close agreement with a theoretical model.
Superconductivity and superfluidity of fermionic and bosonic systems are remarkable many-body quantum phenomena. In liquid helium and dilute gases, Bose and Fermi superfluidity has been observed separately, but producing a mixture in which both the fermionic and the bosonic components are superfluid is challenging. Here we report on the observation of such a mixture with dilute gases of two lithium isotopes, lithium-6 and lithium-7. We probe the collective dynamics of this system by exciting center-of-mass oscillations that exhibit extremely low damping below a certain critical velocity. Using high-precision spectroscopy of these modes, we observe coherent energy exchange and measure the coupling between the two superfluids. Our observations can be captured theoretically using a sum-rule approach that we interpret in terms of two coupled oscillators.
The quantum many-body problem is at the heart of some of the most formidable open problems in modern physics, such as high-Tc superconductivity or the behaviour of neutron stars. Ultracold atomic systems can now be used to simulate model Hamiltonians of condensed matter or nuclear physics, in very well-controlled environnement. In this thesis, we have developed a general method to probe the thermodynamics of homogeneous quantum systems using trapped atomic gases. These measurements are directly compared to the predictions of theories of the quantum many-body problem. We have applied this technique to the spin-1/2 Fermi gas and the Bose with short-range interactions. Using fermionic 6Li, we explored a part of the wide parameter space by changing the interaction strength, the spin-population imbalance or the temperature of the gas. This system exhibits remarkably rich physics, such as normal/superfluid phase transitions (that can be of thermal or quantum character) or a Fermi liquid-type behaviour of the normal phase. We have also used this method to probe a Bose gas of 7Li atoms close to a Feshbach resonance. We have measured the Equation of State of the Bose gas as a function of interactions at very low temperature. For the first time, we measured quantitatively the Lee-Huang-Yang beyond mean-field correction to the ground-state energy of the system, first predicted in 1957. We compared the experimental results to Quantum Monte-Carlo calculations. We have extended this study using out-of-equilibrium measurements of the Bose gas in the strongly interacting regime, which gives a first hint on the properties of the hypothetical unitary Bose gas.
Summary form only given. Superfluidity is amongst the most impressive macroscopic manifestations of quantum behavior. Frictionless flow has been observed in several low-temperature quantum systems, such as liquid helium [1, 2], or more recently ultracold gases [3-5]. Several hallmarks of superfluidity such as critical velocity, quantized vortices, and second sound have been observed [6-11], in both liquid helium and ultracold gases, and in both Bose and Fermi systems.Here, we use the high level of tunability offered by ultracold gases to control their interactions and obtain a double Bose-Fermi superfluid of fermionic 6Li and bosonic ,Li. We have typically Nf = 3.5 . 105 fermionic atoms and Nb = 4 . 104 bosonic atoms at a temperature of about 100 nK, which is below the critical temperature for superfluidity for both the Bose and the Fermi cloud [13].
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
We study ultracold gases of alkali-metal atoms in the quantum degenerate regime. The interatomic interactions in these type of systems can be tuned using resonances induced by magnetic or electric fields. The tunability of the interactions, together with the possibility of confining the atoms with several kinds of external potentials, allows for a completely novel approach to study basic problems in many-body physics, and moreover, allows to enter regimes which have never been accessible in condensed matter or nuclear physics. For example, this has led to the experimental demonstration of an intimate relation between two types of superfluidity: the crossover from Bose-Einstein condensation of tightly bound molecules to the superfluid behavior related to weakly bound BCS-like pairs. Another experimental landmark was reached when the existence of universal Efimov three-body bound states was proven in experiments with ultracold bosonic cesium atoms. In this thesis, we study several aspects of these strongly interacting and ultracold atomic gases. We develop an analytical model that encapsulates all of the relevant scattering physics in atomic systems where open-channel shape resonances and closedchannel Feshbach resonances give rise to complicated and non-trivial scattering properties. This model provides lots of physical insight and is shown to describe important quantities, such as the molecular energies and scattering phase shifts, with a high level of accuracy. The model is compared to full numerical coupled-channel calculations in two atomic systems: rubidium and lithium. We study the BCS-BEC crossover using a many-body description of the ultracold gas that includes the non-trivial energy dependence of the scattering model. We show that it gives rise to superfluid behavior associated with the formation of BCS-like pairs while the low-energy interactions are repulsive in character. The energy dependence of the interactions is crucial, as it gives rise to attractive interactions at the Fermi energy, necessary for the formation of Cooper pairs. We demonstrate new ways of controlling the interatomic interactions using a combination of magnetic and electric fields. This leads to experimental control of, for instance, the three-body parameter in the context of Efimov physics and of non-universal behavior in the BCS-BEC crossover in fermionic gases. Using a four-body method based on first principles, we solve the molecule-molecule scattering problem to calculate several important properties of bosonic and fermionic Summary 145 molecules that consist of light and heavy atoms. These type of molecules are of current experimental interest, and we predict several exciting relations between three- and four-body observables in these type of systems.
The thesis reports on the design and implementation of a novel experimental apparatus for the realization and investigation of ultracold Fermi-Fermi mixtures of Cr-53 and Li-6 atoms. After a brief introduction to the physics of ultracold Fermi gases with tunable interactions, the manuscript also reports on the exceptional peculiarities and unprecedented potentiality of the Cr-Li mixture. The thesis describes the first realization and investigation of magneto-optically trapped cold clouds of Cr and Li atoms worldwide. Furthermore, the thesis also reports on the study of normal and superfluid fermionic currents of Li-6 atoms through a thin Josephson junction across the BEC-BCS crossover. The study led to the extraction of the critical values of particle imbalance across the junction beyond which dissipative dynamics is established.