This thesis describes the design and realization of a novel experimental setup for the investigation of ultracold Bose-Fermi mixtures. The new setup combines a mixture experiment of 40K and 87Rb with one-dimensional trapping geometries that are accessible in the miniaturized wire traps of an atom chip. By additionally coupling the internal states of magnetically trapped atoms with radio-frequency (rf) fields, versatile state-selective and species-selective potentials can be created. One special case is the deformation of the static one-dimensional trap to a novel two-dimensional geometry in which the atoms are confined to the walls of a straight tube. An extensive numerical and experimental analysis of this trapping geometry with the aim of minimizing the variations in the trapping potential was performed. This leads to a new improved spatial arrangement for the wires that create the rf fields. The optimized wire layout was implemented in the new experimental setup and will allow the first realization of such a two-dimensional trap configuration with periodic boundary conditions. Results of experiments performed with a simplified arrangement of the rf wires allowed the observation of the two-dimensional regime with thermal atoms.
This thesis describes the design and realization of a novel experimental setup for the investigation of ultracold Bose-Fermi mixtures. The new setup combines a mixture experiment of 40K and 87Rb with one-dimensional trapping geometries that are accessible in the miniaturized wire traps of an atom chip. By additionally coupling the internal states of magnetically trapped atoms with radio-frequency (rf) fields, versatile state-selective and species-selective potentials can be created. One special case is the deformation of the static one-dimensional trap to a novel two-dimensional geometry in which the atoms are confined to the walls of a straight tube. An extensive numerical and experimental analysis of this trapping geometry with the aim of minimizing the variations in the trapping potential was performed. This leads to a new improved spatial arrangement for the wires that create the rf fields. The optimized wire layout was implemented in the new experimental setup and will allow the first realization of such a two-dimensional trap configuration with periodic boundary conditions. Results of experiments performed with a simplified arrangement of the rf wires allowed the observation of the two-dimensional regime with thermal atoms.
In order to take quantum mechanical systems out of the lab and produce practical devices, a method of miniaturizing and integrating apparatus components is needed. In the context of ultracold atomic gas experiments, the ideal method of achieving this miniaturization is the implementation of atom chip technology. By depositing micro-scale current carrying wires onto planar substrates, it is possible to create a magnetic trap capable of confining a cloud of neutral atoms in sufficient proximity to the chip surface for solid state devices to interact with the cloud. This thesis presents our lab’s first steps towards creating an atom chip capable of confining a cloud of rubidium-87. The chip prototypes were developed with the intent to incorporate them into the hybrid system apparatus within our lab. The chips are intended to serve as a platform for facilitating the interaction between the atomic gas ensemble and mesoscopic solid-state devices, allowing for the hybridization of the two systems. In this work I describe the design of the trapping wire configuration and the fabrication of the first two generations of chip prototypes. Additionally, I provide numerical simulations of the produced magnetic field, from which estimates of the trap frequency and depth can be calculated. Finally, I introduce some basic characterization tests of the absolute current limitations and resistive behavior of the chip wires, and the dimensional uniformity of the fabricated wire profiles.
Ultracold atomic gases have proven to be an excellent tool for research in quantum systems. A Bose gas can be trapped on an atom chip using very well defined and tunable spatially-dependent potentials. The proximity of the atoms to the chip permits the use of low currents allowing for highly accurate temporal changes. Excellent experimental apparatus is needed to achieve Bose-Einstein condensation with a sufficient atom number to study low-dimensional physics. The setup described in this document utilises a set of current-carrying structures on top of which an atom chip sits. For improved atom loading rate, a two-dimensional loading stage was added, extending the lifetime of the magnetic trap. From this loading stage to the atom chip, Bose-Einstein condensation of 105 Rubidium-87 atoms was achieved in less than 30 s, allowing for a large rate of experimental cycles. The high spatial and temporal tunability of this setup results in the ability to split the atomic cloud and quench the trapping potential geometry. Maximising the ratio between trapping frequencies for different spatial directions leads to the system presenting features of a one-dimensional gas. Manipulating the coherence dynamics of a one-dimensional Bose-Einstein condensate creates fluctuations in the phase properties of the wavefunction. These fluctuations are observed as atom density perturbations after releasing the trapping potentials, and are a tool for temperature measurements. When the cloud of atoms is positioned at a few tens of micrometres from the surface of the atom chip, corrugations in the microstructures of the chip affect the trapping potentials at very low temperatures 1 µK. This effect is simulated and quantified in the thesis, with the aim of improving future setups. Additionally, the effect is explored for microscopy purposes. The behaviour of a Bose-Einstein condensate, especially the expansion rate, has long been studied. In this thesis, the Gross-Pitaevskii Equation is introduced, finding its numerical solutions to the two-dimensional and three-dimensional forms, using the Split-Step Fourier Method. The results show very good agreement with the experimental results, as well as with other well- established theories of condensates. The creation of such a toolbox opens up the opportunity to further investigate the coherence dynamics of low-dimensional systems.
During this thesis a novel trapping and manipulation technique for neutral atoms on atom chips has been developed and experimentally implemented. Radio-frequency (rf) coupling of internal states of magnetically trapped atoms results in versatile microscopic potentials, which overcome various limitations of static magnetic traps. An extensive experimental analysis of this new technique has been carried out, exploring the state-dependency of the rf potentials and the possible realization of non-trivial trapping geometries. Based on these results, the first coherent matter wave beam splitter on an atom chip has been realized, demonstrating for the first time the all-magnetic coherent splitting of a condensate. This new atom optical tool has then been applied to investigate the phase-properties of one-dimensional Bose-Einstein condensates. The potential configuration developed during this thesis provides direct access to the phase fluctuations in the system and has allowed the first time-resolved study of equilibrium properties and non-equilibrium dynamics in a one-dimensional Bose gas.
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.
An ultracold Bose-Fermi gas mixture of 40K and 87Rb atoms has tunable interspecies interactions and therefore provides a fantastic platform for exploring not only few-body physics such as Feshbach molecule formation and Efimov trimers, but also many-body physics including Bose polarons, quantum Hall physics and so on. In this thesis, I present experimental evidence of Bose polarons in cold atoms obtained using radio-frequency spectroscopy to measure the excitation spectrum of fermionic 40K impurities resonantly interacting with a BEC of 87Rb atoms. These Bose polarons originate from the dressing of an impurity coupled to its environment, which is an important paradigm in quantum many-body physics. I also present initial work that launches an exciting new direction for our experiment, which is exploring rotating quantum gases. Goals for this work include studying both rotating Bose and Fermi superfluids with tunable interactions as well as working toward rapidly rotating quantum gases in the quantum Hall regime. For these goals, a new all-optical trap for rotating gases was designed, implemented, and tested using a 87Rb Bose-Einstein condensate.
Atom chips are an excellent tool for studying ultracold degenerate quantum gases, due to the high degree of controllability afforded by the precise potentials generated from the current-carrying microfabricated wires on the chip surface. The geometries of the trapping potentials are inherently capable of realising extreme aspect ratios, and therefore creating model systems with effectively reduced dimensionality, particularly the theoretically-tractable one-dimensional Bose gas. In addition, the temporal tunability makes it possible to impart non-adiabatic changes on the trapping potentials, allowing experimental investigation of samples which have been brought out of equilibrium - a situation which is not fully theoretically understood. This thesis describes the implementation, development and characterisation of an experimental system for producing the first Bose-Einstein condensates of atomic rubidium 87 gas trapped on the surface of an atom chip in Nottingham. Such an apparatus is very complex and requires careful characterisation in order to run in a stable and reliable way. Details of the experimental setup are thoroughly outlined, including the vacuum system, lasers, electronics, computer control and timing, and the optical imaging system. A newly installed compact two-dimensional magneto-optical trap provides an loading rate of 5e7 atoms per second for loading a three-dimensional mirror-magneto-optical trap with 1.5e8 atoms, at a temperature of 300uK within 10s. The cloud is then sub-Doppler cooled to 50uK, and spin-polarised with 96% purity into the |F=2,mF=+2> ground state within 5ms, in preparation for loading a purely magnetic trap. A millimeter sized copper Z-shaped conductor located beneath the atom chip surface creates a Ioffe-Pritchard magnetic trap, into which the laser cooled cloud is loaded with 70% efficiency, and can be held with a vacuum-limited lifetime of 40s. Evaporative cooling then pre-cools the sample to below 20uK within 10s, to allow the subsequent loading into potentials created by the atom chip with 100% efficiency. A final evaporation stage then cools the cloud below the phase transition temperature of 800nK, resulting finally in pure BECs with 105 atoms confined using the atom chip. Key measurements of various properties of the trapped condensates are presented, which are important in order to characterise the system fully, and to compare with theoretical expectations. In particular, included are the variation of condensate fraction with temperature, the BEC expansion dynamics, and the condensate lifetime in the trap, for example. Finally, it is demonstrated how BECs can be produced on the atom chip without the use of external macroscopic coils, achieved by using novel, integrated sheet structures located beneath the chip surface - unique to this experimental system - to create the necessary bias fields.