This thesis describes experiments with ultracold diatomic molecules associated from ultracold 87Rb atoms using magnetically-induced scattering resonances (Feshbach resonances). The association method is based on a slow magnetic-field ramp across a Feshbach resonance which converts an atom pair into a bound molecule. The reaction is reversible and coherent and no latent heat is released. A Bose-Einstein condensate serves as a source of ultracold atoms. In one of the experiments reported here, the association technique is used to create a quantum state with exactly one molecule at each site of a three-dimensional optical lattice. This state opens up interesting possibilities with respect to quantum simulations and quantum information processing.
This thesis describes experiments with ultracold diatomic molecules associated from ultracold 87Rb atoms using magnetically-induced scattering resonances (Feshbach resonances). The association method is based on a slow magnetic-field ramp across a Feshbach resonance which converts an atom pair into a bound molecule. The reaction is reversible and coherent and no latent heat is released. A Bose-Einstein condensate serves as a source of ultracold atoms. In one of the experiments reported here, the association technique is used to create a quantum state with exactly one molecule at each site of a three-dimensional optical lattice. This state opens up interesting possibilities with respect to quantum simulations and quantum information processing.
Ultracold molecules are associated from an atomic Bose-Einstein condensate by ramping a magnetic field across a Feshbach resonance. The reverse ramp dissociates the molecules. The kinetic energy released in the dissociation process is used to measure the widths of four Feshbach resonances in 87Rb. This method to determine the width works remarkably well for narrow resonances even in the presence of significant magnetic-field noise. In addition, a quasimonoenergetic atomic wave is created by jumping the magnetic field across the Feshbach resonance.
This thesis reports on experiments with ultracold atomic and molecular quantum gases. Magnetically tunable Feshbach resonances are used to associate diatomic molecules from atomic 87Rb in a three-dimensional optical lattice. In a first experiment, molecules are formed by a sudden jump of the magnetic field to the resonance. This induces coherent atom-molecule oscillations with large amplitude. In another experiment, a dissipative analog of the one-dimensional Tonks-Girardeau gas is realized with molecules in an optical lattice. Inelastic collisions between the molecules lead to particle loss. This interaction is so strong that the system reaches the strongly correlated regime, and the molecules, which are bosons, are forced to behave much like fermions. The strong correlations suppress the molecule loss rate by up to three orders of magnitude.
In this thesis two experiments with heteronuclear Bose-Bose mixtures are discussed. The goal of the first experiment is a controlled doping of a rubidium condensate with single caesium atoms. These undertake the task of a non-destructive probe to investigate quantum mechanical phenomena time- and spatially resolved. In this thesis the necessary methods to produce, store, and detect both components, single atoms and the condensate, are realized. In a first experiment up to 10 caesium atoms are stored as a probe in contact with a cold rubidium atomic cloud. The interaction parameters are extracted from the dynamics of the single atoms. This is an important step towards the controlled doping of a condensate. The aim of the second experiment is the production and spectroscopy of ultracold heteronuclear potassium-rubidium molecules with universal properties. Close to two magnetic s-wave Feshbach resonances weakly bound molecules in high vibrational states are created, and their binding energy and the position of the associated Feshbach resonance are determined. These results in combination with two narrow d-wave Feshbach resonances provide the basis for a more precise parametrization of the potassium-rubidium molecular potential. The knowledge of this is important to identify a proper scheme to transfer the molecules via coherent coupling into their rovibrational ground state. These molecules offer a permanent dipole moment and thus are of particular interest for e.g. quantum information processing due to their anisotropic, long-range dipole-dipole interaction.
This thesis reports on the first observation of optical tuning of interactions in an ultracold atomic gas. To control the elastic interactions, we use optically induced scattering resonances. They are realized by coupling pairs of atoms to a bound molecular state with the help of laser light. In comparison to the well-known magnetic Feshbach resonances, these optical Feshbach resonances allow a very fast switching of interactions in a spatially resolved way. This is because laser light can be switched on and off much faster than magnetic fields and because the light intensity can be spatially structured. Our experiments are performed with a Bose-Einstein condensate (BEC) of Rb atoms, which we produce in our new BEC setup. This apparatus has been constructed in the framework of this thesis and offers excellent optical access to the ultracold atom cloud. To investigate the tunability of atomic interactions with single-frequency laser light, we optically couple pairs of free atoms to an electronically excited molecular state. The coupling is achieved with a laser that is tuned close to the respective free-bound photoassociation transition. By changing the laser detuning we are able to adjust the atomic scattering length to any value between 10 a0 and 190 a0, where a0 denotes the Bohr radius. The atomic scattering length is a single quantity that fully describes ultracold elastic interactions. Since laser induced atom losses restrict the duration of our experiments, Bragg spectroscopy is employed as a fast method to measure the scattering length. We also demonstrate that optical Feshbach resonances can be realized with a two-color Raman coupling scheme. To implement this scheme, a second laser is added that couples the excited molecular state to a weakly bound molecular level in a ground state potential. Tuning the second laser results in a modification of the atomic scattering length similar to the one observed with a single laser. In addition, this scheme allows to control the width of the resonance via the detuning of the first laser. For the one-color and two-color coupling schemes, we investigate how the elastic scattering properties depend on the different available parameters, i.e. the laser detunings and intensities. Our experimental findings can successfully be described with an available theoretical model, which treats ultracold collisions in the presence of laser light. Optical Feshbach resonances have the possibility of becoming a valuable tool for fast switching of interactions. This is of special interest in quantum computation schemes with neutral atoms, where interactions between different atomic qubits need to be controlled.
The research described in this thesis is part of an international effort with the aim of studying ultracold quantum degenerate gas samples like Bose-Einstein condensates and Fermi degenerate systems, consisting of atoms confined in magnetic or optical traps. The behavior of such samples is governed by the inter-atomic interactions and the resulting properties of the atom-atom scattering. At the relevant temperatures of 1nK to 10µK a key property is the atom-atom scattering length a. In chapter 2 a theoretical method is presented which enables one to describe the interaction and scattering of (ultra)cold atoms to unprecedented precision. It is also unparalleled in comprehensiveness: it allows the prediction of a large and varied set of experimental data for all isotopes of the same element. The method relies on the extraction from experiments of a few (phase) parameters which completely summarize the behavior of the atoms in the (ultra)cold regime. In chapter 3 the method is applied to the "workhorses" of cold-atom physics: the atomic species 85Rb and 87Rb. We extract the foregoing parameters to a very high precision from several recent high precision experiments, allowing us to predict e.g. the 87Rb spinor condensate to be ferromagnetic: a prediction for which the scattering length has to be calculated with a precision better than 1%. We also predict Feshbach resonances at experimentally accessible magnetic field strengths; resonances searched for and found by the experimental group of Rempe. In close collaboration with his group we "fine-tune" the interaction parameters found previously, by making use of only one of the observed resonances. We then obtain agreement with 42 out of the observed 43 resonance field strengths and are able to identify bound states inducing the Feshbach resonances at these locations. Chapter 4 describes the results of this research. With a thorough understanding of the rubidium interactions, we then switch to lithium which has a fermionic (6Li) and a bosonic (7Li) isotope. Both are being used in cold-atom experiments. In chapter 5 we evaluate the interaction parameters for lithium allowing us to predict magnetic field strengths for which a sample of fermionic 6Li atoms can be regarded as strongly interacting. Furthermore, a three-level method for measuring mean-field shifts, based on radio-frequency techniques, is introduced. For weak interactions we find proportionality of resonance shifts to interaction strengths. In the strongly interacting regime, however, these shifts become very small reflecting the quantum unitarity limit and many-body effects. Most interesting is the fact that in this regime the shifts are small both for large positive a and for large negative a, likely reflecting the universality of the interaction energy. In chapters 6 and 7 the interactions between lithium atoms are reinvestigated, making use of newly available experimental data and with the updated interaction parameters special attention is paid to locating field strengths at which magnetically tunable Feshbach resonances occur in the scattering of lithium atoms. In chapter 6 scattering events in a gas of (fermionic) 6Li atoms are studied. In chapter 7 we show that a consistent description of the 6Li+6Li system can be given. We discuss theoretical uncertainties for the position of the wide 6Li Feshbach resonance and present an analytic scattering model for this resonance, based on the inclusion of a field-dependent virtual open-channel state. We predict new Feshbach resonances for the 6Li-7Li system, and their importance for different types of crossover superfluidity models is discussed. Molecules created by magnetically sweeping over these resonances will have a fermionic character. One magnetic field strength is predicted at which two different fermionic molecules can be created simultaneously.
We report the creation of ultracold bosonic 23 Na 87 Rb Feshbach molecules via magneto-association. By ramping the magnetic field across an interspecies Feshbach resonance (FR), at least 4000 molecules can be produced out of the near degenerate ultracold mixture. Fast loss due to inelastic atom–molecule collisions is observed, which limits the pure molecule number, after residual atoms removal, to 1700. The pure molecule sample can live for 21.8(8) ms in the optical trap, long enough for future molecular spectroscopy studies toward coherently transferring to the singlet ro-vibrational ground state, where these molecules are stable against chemical reaction and have a permanent electric dipole moment of 3.3 Debye. We have also measured the Feshbach molecule's binding energy near the FR by the oscillating magnetic field method and found these molecules have a large closed-channel fraction.