Revision with unchanged content. The weak interactions inherent in dilute Bose Einstein Condensates make it an ideal testing ground for basic many body quantum theories; to an extent that is not possible in traditional condensed matter systems. Moreover, the use of Sodium as the atom of choice for BEC experiments yields immense advantages, the primary being the large number of atoms that can be placed in the BEC state and the low final temperatures of this state. This present work has been an exploration of the production and use of BECs of Na atoms for the study of superfluidity. A major portion of this work deals in the actual science of producing and manipulating large, cold samples of Na BECs using a distinct trapping scheme, the Optically Plugged Trap. Afterwards, the work focuses on the unique metastable nature of BEC states; from the very existence of a BEC state which is metastable with respect to the formation of molecules, to quantum vortices, to a metastability which is unique to the Optically Plugged Trapped detailed here. Also, a novel optical diagnostic scheme to probe one of these metastable states, a Vortex Lattice, is presented.
Revision with unchanged content. The weak interactions inherent in dilute Bose Einstein Condensates make it an ideal testing ground for basic many body quantum theories; to an extent that is not possible in traditional condensed matter systems. Moreover, the use of Sodium as the atom of choice for BEC experiments yields immense advantages, the primary being the large number of atoms that can be placed in the BEC state and the low final temperatures of this state. This present work has been an exploration of the production and use of BECs of Na atoms for the study of superfluidity. A major portion of this work deals in the actual science of producing and manipulating large, cold samples of Na BECs using a distinct trapping scheme, the Optically Plugged Trap. Afterwards, the work focuses on the unique metastable nature of BEC states; from the very existence of a BEC state which is metastable with respect to the formation of molecules, to quantum vortices, to a metastability which is unique to the Optically Plugged Trapped detailed here. Also, a novel optical diagnostic scheme to probe one of these metastable states, a Vortex Lattice, is presented.
Since Feynman's realization of using quantum systems to investigate quantum dynamics, interest in creating controllable quantum systems to simulate condensed matter phenomenon has been high. With the realization of BECs in 1995, the realization of a relatively clean testbed for simulating some of these phenomenon became a reality. \n My PhD research has been an exploration of the production and use of Bose-Einstein Condensates for the study of superfluidity. The first 3 years have been spent in the actual building of a Na BEC apparatus. During this time, we’ve implemented a distinct technique to trap ultra cold Na atoms, i.e. the Optically Plugged Trap. In the process, we have shown how atoms in a linear trap can show spin metastability and thus maintain a nonequilibrium state for long periods of time.\n In studying the interaction of ultra-cold atoms with light, we have developed a technique to measure the velocity distribution of atoms using a standing optical wave (Bragg Spectroscopy). Alongside this, we have also created optical traps for atoms in which we can change to shape of the trap itself to probe different condensed matter systems. The eventual goal being the investigation of condensed matter physics, specifically superfluidity, using ultra-cold atoms.
For many years, 4He typified Bose-Einstein superfluids, but recent advances in dilute ultra-cold alkali-metal gases have provided new neutral superfluids that are particularly tractable because the system is dilute. This chapter starts with a brief review of the physics of superfluid 4He, followed by the basic ideas of Bose-Einstein condensation (BEC), first for an ideal Bose gas and then considering the effect of interparticle interactions, including time-dependent phenomena. Extensions to more exotic condensates include magnetic dipolar gases, mixtures of two components, and spinor condensates that require a focused infrared laser for trapping of all the various hyperfine magnetic states in a particular hyperfine F manifold of mF states. With an applied rotation, the trapped BECs nucleate quantized vortices. Recent theory and experiment have shown that laser coupling fields can mimic the effect of rotation. The resulting synthetic gauge fields have produced vortices in a nonrotating condensate.
Research on degenerate quantum gases of dilute atomic gases has become a well established field in modern physics and revealed many astonishing phenomena. The first experiments on Bose and Fermi gases at low temperatures aimed to prove the predictions of quantum mechanics on a macroscopic scale. Since then we make use of Bose and Fermi gases and mixtures of any combination to model other many body systems. A property of Bose Einstein condensates is superfluidity which we want to investigate further in toroidal potentials with a new apparatus which can produce an all-optical BEC. This means trapping thermal atoms in an optical dipole trap and cooling the sample by evaporation into the quantum degenerate regime. This new technique was first successfully demonstrated by M.D. Barrett et al. in 2001 and promises to be faster and simpler than the well established and common way of evaporation in magnetic traps. A new apparatus was built by the author and we report on the experimental details of the trapping of 87Rb atoms from a hot vapour and cooling them in a magneto-optical trap, and further evaporation to quantum degeneracy in a crossed optical dipole trap operating at a wavelength of l = 1064nm, with accurate control of the power in both beams via a feedback loop. The evaporation of neutral atoms of 87Rb in far red detuned optical dipole traps using linearly polarised laser light is spin independent, and mixed spinor condensates of the F=1 manifold can be formed. As an empirical technique we found by applying a magnetic gradient field during the final evaporation that we can selectively populate mF spin states or prepare mixtures. This intriguing mechanism was found earlier as well by M. S. Chang, but is yet not fully understood and subject of our future research. We can now routinely prepare an almost pure condensate containing up to 7000 atoms in the condensed phase purely in the mF = 0 spin state. This thesis contains the theoretical framework of Bose-Einstein condensation with in mean-field theory, the concepts of cooling of atoms in magneto-optical traps, the trapping and evaporation in optical dipole traps and the experimental realisation. In the future this experiment will be extended by a fast scanning dipole trap to apply arbitrary time averaged potentials of more complex geometries, such as toroidal traps to study the effects of superfluidity in a periodic potential. The production of toroidal traps and cold atoms in such a trap are outlined in this thesis.
The experimental realization in 1995 of Bose–Einstein condensation in dilute atomic gases marked the beginning of a very rapid development in the study of quantum gases. The initial experiments were performed on vapours of rubidium, sodium, and lithium. So far, the atoms 1 H, 7 Li, 23 Na, 39 K, 41 K, 52 Cr, 85 Rb, 87 Rb, 133 Cs, 170 Yb, 174 Yb and 4 He * (the helium atom in an excited state) have been demonstrated to undergo Bose–Einstein condensation. In related developments, atomic Fermi gases have been cooled to well below the degeneracy temperature, and a superfluid state with correlated pairs of fermions has been observed. Also molecules consisting of pairs of fermionic atoms such as 6 Li or 40 K have been observed to undergo Bose–Einstein condensation. Atoms have been put into optical lattices, thereby allowing the study of many-body systems that are realizations of models used in condensed matter physics. Although the gases are very dilute, the atoms can be made to interact strongly, thus providing new challenges for the description of strongly correlated many-body systems. In a period of less than ten years the study of dilute quantum gases has changed from an esoteric topic to an integral part of contemporary physics, with strong ties to molecular, atomic, subatomic and condensed matter physics. The dilute quantum gases differ from ordinary gases, liquids and solids in a number of ways, as we shall now illustrate by giving values of physical quantities.
The study of BEC in weakly interacting system holds the promise of revealing new macroscopic quantum phenomena that can be understood from first principles, and may advance present understanding of superconductivity and superfluidity. By employing several new experimental techniques, the dark SPOT trap, rf induced evaporation, and the optically plugged magnetic trap, the authors were able to observe BEC in a gas of sodium atoms. The striking signature of Bose condensation was the sudden appearance of a bimodal velocity distribution below the critical temperature of {approximately} 2 {mu}K. The distribution consisted of an isotropic thermal distribution and an elliptical core attributed to the expansion of a dense condensate. Condensates contained up to 5 x 10{sup 5} atoms at densities exceeding 10{sup 14} cm{sup {minus}3}. Such high densities are unprecedented in cold atomic gases and open up new possibilities for studying a weakly interacting Bose gas over a broad range of densities and therefore strengths of interaction.
Superfluidity has become a topic of active interest in the field of atom trap Bose-Einstein condensates (BECs). We motivate this line of research by placing the BEC developments against the general backdrop of the theory of superfluidity and by highlighting the advantages offered by the atom trap technology. We review the standard description of the BEC dynamics, as well as basic concepts of superfluidity and we present theoretical arguments for expecting the BECs to exhibit superfluid behaviour. In atom traps, as in condensed and nuclear matter, superfluidity can manifest itself in a manifold of guises. Moreover, some of its manifestations can even be undesirable, as we illustrate by the poignant example of sympathetic cooling with condensates. We also review recent experimental evidence for superfluid BEC behaviour and we suggest further experiments to probe superfluidity in atom traps. The low densities of the BECs allow optical imaging and imply that the relevant physics occurs on time and length scales that are much slower and larger than in the traditional 4He superfluid. Consequently, BEC experiments can study known superfluid phenomena and structures in a different and, perhaps, more detailed manner. In addition, the flexibility offered by existing techniques in atomic, molecular and optical physics is unprecedented in the field of low-temperature physics and suggests the prospect of creating superfluids of fundamentally novel structures. Finally, it is conceivable that new applications of superfluidity may arise from these studies.