The physics of quantum degenerate cold gases in optical lattices has rapidly grown to an extremely dynamic field over the past few years. In the experimental realizations, however, mainly cubic symmetries (or their two- and one-dimensional projections) are considered. We have implemented an optical lattice with an underlying triangular symmetry in order to investigate strongly correlated ultra-cold atoms in a novel experimental geometry. Exhibiting an explicit polarization dependence (compare figure 1) the optical lattice realized here should allow for the creation and analysis of thus far unexplored magnetic phases. Experiments attending to the quantum phase transition from a superfluid to a Mott-insulating state in a three- as well as in a two-dimensional system with triangular symmetry have been performed. Similarities as well as differences to the findings obtained in cubic lattices will be discussed and can be attributed to the inherent differences in the crucial lattice parameters such as tunneling energy J and on-site interaction U.
The physics of quantum degenerate cold gases in optical lattices has rapidly grown to an extremely dynamic field over the past few years. In the experimental realizations, however, mainly cubic symmetries (or their two- and one-dimensional projections) are considered. We have implemented an optical lattice with an underlying triangular symmetry in order to investigate strongly correlated ultra-cold atoms in a novel experimental geometry. Exhibiting an explicit polarization dependence (compare figure 1) the optical lattice realized here should allow for the creation and analysis of thus far unexplored magnetic phases. Experiments attending to the quantum phase transition from a superfluid to a Mott-insulating state in a three- as well as in a two-dimensional system with triangular symmetry have been performed. Similarities as well as differences to the findings obtained in cubic lattices will be discussed and can be attributed to the inherent differences in the crucial lattice parameters such as tunneling energy J and on-site interaction U.
Over recent years, exciting developments in the field of ultracold atoms confined in optical lattices have led to numerous theoretical proposals devoted to the quantum simulation of problems e.g. known from condensed matter physics. Many of those ideas demand experimental environments with non-cubic lattice geometries. In this paper, we report on the implementation of a versatile three-beam lattice allowing for the generation of triangular as well as hexagonal optical lattices. As an important step, the superfluid–Mott insulator (SF–MI) quantum phase transition has been observed and investigated in detail in this lattice geometry for the first time. In addition to this, we study the physics of spinor Bose–Einstein condensates (BEC) in the presence of the triangular optical lattice potential, especially spin changing dynamics across the SF–MI transition. Our results suggest that, below the SF–MI phase transition, a well-established mean-field model describes the observed data when renormalizing the spin-dependent interaction. Interestingly, this opens up new perspectives for a lattice-driven tuning of a spin dynamics resonance occurring through the interplay of the quadratic Zeeman effect and spin-dependent interaction. Finally, we discuss further lattice configurations that can be realized with our setup.
During the last decade the physics of ultracold quantum gases has matured into a highly active and versatile field of research. Within the framework of this thesis experiments dedicated to the physics of Bose-Einstein condensates have been performed and diverse phenomena, which are distinguished by fundamentally different regimes of interaction, could be investigated. For the first time the dynamical evolution of long-lived dark solitons has been studied as a paradigm of non-linear physics. Solitons, characterized as non-spreading wave packets, are stabilized against dispersion by a suitable non-linear interaction. It has been possible to observe oscillations of dark solitons in elongated Bose-Einstein condensates and good agreement with the theoretically predicted oscillation frequency has been obtained. Moreover experiments have been conducted which address the collision of two dark solitons distinguished by different depths. This particular feature enabled the identification of the individual solitons beyond the actual collision process and as a central result it could be shown, that these peculiar entities interpenetrate without significantly influencing each other. The theoretical description of solitons as weakly interacting quasi particles is in good agreement with these findings. Continuative studies on vectorial dark-bright solitons and their dynamical properties complement the investigations on dark solitons. A central goal of this dissertation has been the design and implementation of an optical lattice with an underlying triangular symmetry in order to investigate strongly correlated ultracold atoms in a novel experimental geometry. Exhibiting an explicit polarization dependence the optical lattice realized here should allow for the creation and analysis of thus far unexplored magnetic phases. Experiments attending to the quantum phase transition from a superfluid to a Mott-insulating state in a three- as well as in a two-dimensional system with triangular symmetry have been performed. Similarities as well as differences to the findings obtained in cubic lattices have been worked out and could be attributed to the inherent differences in the crucial lattice parameters such as tunneling energy J and onsite interaction U. Devoted to the dynamics of spinor condensates, a third part of this thesis concerns the physics of multi component quantum gases. Employing suitable spin-independent trapping potentials the physics of Bose-Einstein condensates may be extended to the investigation of the static and dynamical properties of internal degrees of freedom, in this case the spin of the atoms. The measurements presented here concentrate on the coherent dynamical evolution of the vectorial order parameter of a spinor condensate. Rb condensates with F=1 and F=2 have been studied and the existence of a spin dynamics resonance, caused by the competition between spin-dependent non-linear interaction and quadratic Zeeman effect was demonstrated for the first time.
Die Physik ultrakalter Quantengase stellt sich heutzutage als ein hochaktives und vielseitiges Forschungsfeld dar. Im Rahmen dieser Dissertation konnten fundamentale Erkenntnisse zu unterschiedlichen Phanomenen in Bose-Einstein Kondensaten gewonnen werden, welche vollig verschiedenen Wechselwirkungsbereichen zuzuordnen sind. Erstmals wurden Experimente durchgefuhrt, die als eines der Paradigmen nichtlinearer Physik die Dynamik extrem langlebiger dunkler Solitonen zeigen. Solitonen, die sich als formstabile Wellenpakete auszeichnen, resultieren aus einer Kompensation der Dispersion durch eine entgegengerichtet wirkende nichtlineare Wechselwirkung. Oszillationen von dunklen Solitonen in elongierten Bose-Einstein Kondensaten wurden beobachtet und eine sehr gute ubereinstimmung mit der theoretisch erwarteten Oszillationsfrequenz wurde gefunden. Desweiteren konnten im Verlauf dieser Arbeit Experimente durchgefuhrt werden, bei denen die Kollisionen unterschiedlich tiefer dunkler Solitonen untersucht wurden. Es war dadurch moglich die Solitonen uber den Kollisionszeitpunkt hinaus zu verfolgen. Als ein zentrales Ergebnis wurde gezeigt, dass Solitonen sich im Verlauf der Kollision durchdringen ohne einen nennenswerten Einfluss aufeinander auszuuben, was in ubereinstimmung mit der theoretischen Beschreibung von Solitonen als schwach wechselwirkende Quasiteilchen steht. Daruber hinaus konnten diese Studien durch Untersuchungen an 'gefullten' Solitonen abgerundet werden. Als ein wesentliches Forschungsziel wurde im Verlauf dieser Dissertation ein optisches Gitter mit einer Dreieckssymmetrie aufgebaut, um die experimnetelle Grundlage fur die Untersuchung stark korrelierter ultrakalter Atome in einer neuartigen Geometrie zu schaffen. Es konnte eine Polarisationsabhangigkeit des periodischen Potentials herausgestellt werden, welche die Untersuchung neuartiger magnetischer Phasen ermoglichen sollte. Experimente zum Quantenphasenubergang zwischen Superfluid und Mott-Isolator in drei- als auch zweidimensionalen Systemen mit Dreieckssymmetrie wurden durchgefuhrt und analysiert. Unterschiede als auch Gemeinsamkeiten zu fruheren Ergebnissen, die in kubischen Gittern gewonnen wurden, konnten nachgewiesen und auf Unterschiede der entscheidenden Gitterparameter, wie etwa Tunnelenergie und Wechselwirkung, zuruckgefuhrt werden. Ein weiterer Teil der vorliegenden Arbeit widmet sich Untersuchungen zur Dynamik von mehrkomponentigen Spinor Bose-Einstein Kondensaten. In entsptrechend spinunabhangigen Fallenpotentialen, wie sie durch optische Dipolfallen realisiert werden konnen, lasst sich die Physik Bose-kondensierter Materie auf eine zusatzliche Dynamik der inneren Freiheitsgrade, in diesem Fall des Spins, erweitern. Die im Rahmen dieser Dissertation durchgefuhrten Experimente zur Physik mehrkomponentiger Quantengase konzentrieren sich auf die koharente zeitliche Entwicklung des vektoriellen Ordnungsparameters eines Spinor Kondensates, gemeinhin als Spindynamik bezeichnet. Rb Kondensate mit Spin-1 als auch Spin-2 wurden untersucht und als ein zentrales Ergebnis konnte erstmals eine Spindynamikresonanz vermessen werden, welche aus der Konkurrenz zwischen spinabhangiger Wechselwirkung and quadratischem Zeemaneffekt heraus entsteht.
In recent years the quantum simulation of condensed-matter physics problems has resulted from ex- citing experimental progress in the realm of ultracold atoms and molecules in optical lattices. In this thesis we analyze theoretically a spinor Bose gas loaded into a three-dimensional cubic optical lattice. In order to account for different superfluid phases of spin-1 bosons with a linear Zeeman effect, we work out a Ginzburg-Landau theory for the underlying spin-1 Bose-Hubbard model. To this end we add artificial symmetry- breaking currents to the spin-1 Bose-Hubbard Hamiltonian in order to break the global U (1) symmetry. With this we determine a diagrammatic expansion of the grand-canonical free energy up to fourth order in the symmetry-breaking currents and up to the leading non-trivial order in the hopping strength which is of first order. As a cross-check we demonstrate that the resulting grand- canonical free energy allows to recover the mean-field theory. Applying a Legendre transfor- mation to the grand-canonical free energy, where the symmetry-breaking currents are transformed to order parameters, we obtain the effective Ginzburg-Landau action. With this we calculate in detail at zero temperature the Mott insulator-superfluid quantum phase boundary as well as condensate and particle number density in the superfluid phase. We find that both mean-field and Ginzburg-Landau theory yield the same quantum phase transition between the Mott insulator and superfluid phases, but the range of validity of the mean-field theory turns out to be smaller than that of the Ginzburg-Landau theory. Due to this finding we expect that the Ginzburg-Landau theory gives better results for the superfluid phase and, thus, we restrict ourselves to extremize only the effective Ginzburg-Landau action with respect to the order parameters. With- out external magnetic field the superfluid phase is a polar (ferromagnetic) state for anti-ferromagnetic (ferromagnetic) interactions, i.e. only the hyperfine spin 0 (1) is macroscopically occupied, in accor- dance with previous mean-field results. On the other hand, in the presence of the external magnetic field for ferromagnetic interaction, the superfluid phase does not change as the minimization of the energy implies the maximal spin value. However, when an anti-ferromagnetic interaction competes with the linear Zeeman effect, we can distinguish various ferromagnetic and anti-ferromagnetic su- perfluid phases within the range of validity of the Ginzburg-Landau theory. Increasing the external magnetic field yields a breaking of spin singlet pairs and a subsequent alignment of spins, thus anti- ferromagnetic phases decrease until only a ferromagnetic superfluid phase prevails. In addition, we find that the superfluid-Mott insulator phase transition is always of second order for both ferromag- netic and anti- ferromagnetic interactions. However, the transitions between different superfluid phases for an anti-ferromagnetic interaction can be both of first and second order depending on whether the respective macroscopic occupation of hyperfine spin states changes discontinuously or continuously. The established Ginzburg-Landau theory for spin-1 bosons in optical lattices will certainly be the basis for many further applications as, for instance, time-of-flight absorption pictures or collective excitations, which are of experimental importance.
An important new development in the field of ultracold atomic gases is the study of the properties of these gases in a so-called optical lattice. An optical lattice is a periodic trapping potential for the atoms that is formed by the interference pattern of a few laser beams. A reason for the interest in these systems is that the effects of the interatomic interactions can be strongly enhanced. More specifically, it has been shown in a beautiful experiment by Greiner et al. in 2002 that by loading a Bose-Einstein condensate into an optical lattice it is possible for the system to undergo a quantum phase transition to a new quantum phase of matter, the so-called Mott insulator phase. Within this Mott insulator phase each lattice site is occupied by exactly one atom. This makes the Mott insulator phase especially well suited for applications in the field of quantum computation and quantum information processing. We have theoretically investigated the above mentioned quantum phase transition and our formalism allows for a description of the Mott insulator phase at nonzero temperatures. Another important experimental development in the field of ultracold atomic gases is the use of Feshbach resonances to control the interatomic interactions. Such a resonance occurs whenever two colliding atoms form a long-lived molecule for some time. The crucial point of a Feshbach resonance is that the above mentioned molecule has a magnetic moment that is not equal to twice the magnetic moment of the atom. As a consequence the energy difference between the two atoms and the molecule and hence the interactions between the atoms can be controlled by using an external magnetic field. By combining these two techniques, i.e, by trapping ultracold atomic gases in an optical lattice and by tuning a magnetic field near a Feshbach resonance there can be a new quantum phase transition between two superfluid phases. We gave derived the the theory for the description of these Feshbach resonances in optical lattices and applied it to various systems. To be a bit more precise, if we tune the external magnetic field such that the energy difference between a molecule and two atoms is sufficiently negative, then the gas consists of a Bose-Einstein condensate of molecules. In contrast, if the energy difference is large enough and positive we have a gas that consists primarily of a Bose-Einstein condensate of atoms. It turns out that these two limits are separated by an Ising-like quantum phase transition. By using atomic Bose gases near a Feshbach resonance detailed experimental studies which test the theoretical predictions of the statistical and dynamical properties of these quantum phase transitions can be made.
Quantum simulation is the study of one quantum mechanical system via analog with another. In this thesis we explore a simple model of interacting bosons on a lattice, known as the Bose-Hubbard model, by experimental investigation of ultracold rubidium atoms in an optical lattice made from laser light. We describe the construction and stabilization of an optical superlattice with threefold symmetry, and its use in studying the Bose-Hubbard model on triangular and trimerized kagome lattices. We study the short range phase coherence of a Mott insulator on the triangular lattice, and develop a scheme to mitigate out-of-equilibrium effects arising from the state preparation. We show the first experimental realization of an optical trimerized kagome lattice for cold atoms, and discuss experiments characterizing this lattice. Finally, we provide evidence for a Mott insulating state with fractional average particle number per site with measurements of the nearest-neighbor phase coherence of strongly interacting atoms in the trimerized kagome lattice.
In this thesis I report on experiments that enter a new regime in the many body physics of ultracold atomic gases. A Bose-Einstein condensate is loaded into a three-dimensional optical lattice potential formed by a standing wave laser light field. In this novel quantum system we have been able to both realize a quantum phase transition from a superfluid to a Mott insulator, and to observe the collapse and revival of a macroscopic matter wave field. Quantum phase transitions are driven by quantum fluctuations and occur, even at zero temperature, as the relative strength of two competing energy terms in the underlying Hamiltonian is varied across a critical value. In the first part of this work I report on the observation of such a quantum phase transition in a Bose-Einstein condensate with repulsive interactions, held in a three-dimensional optical lattice potential. In the superfluid ground state, each atom is spread-out over the entire lattice, whereas in the Mott insulating state, exact numbers of atoms are localized at individual lattice sites. We observed the reversible transition between those states and detected the gap in the excitation spectrum of the Mott insulator. A Bose-Einstein condensate is usually described by a macroscopic matter wave field. However, a quantized field underlies such a "classical" matter wave field of a Bose-Einstein condensate. The striking behavior of ultracold matter due to the field quantization and the nonlinear interactions between the atoms is the focus of the second part of this work. The matter wave field of a Bose-Einstein condensate is observed to undergo a series of collapses and revivals as time evolves. Furthermore, we show that the collisions between individual pairs of atoms lead to a fully coherent collisional phase shift in the corresponding many-particle state, which is a crucial cornerstone of proposed novel quantum computation schemes with neutral atoms. With these experiments we enter a new field of physics with ultracold quantum gases. In this strongly correlated regime, interactions between atoms dominate the behavior of the many-body system such that it can no longer be described by the usual theories for weakly interacting Bose gases. This novel quantum system offers the unique possibility to experimentally address fundamental questions of modern solid state physics, atomic physics, quantum optics, and quantum information.