This dissertation is about the physics of dilute gaseous Bose-Einstein condensates (BECs) confined to lower dimensions by optical lattices. The central theme of the effects of reduced dimensionality is explored within various one-dimensional (1D) and two-dimensional (2D) systems. We create a 2D BEC by adiabatically increasing the confinement of a trapping potential in one direction to the point where motion in that direction is frozen out. Doing this in two directions, we create a 1D BEC. Two experiments examine the ground state properties of a 1D and 2D system. In the 1D system (Chap. 9), a reduction in three-body recombination signals an increase in correlation resulting in a partial "fermionization" of the Bose gas. In the 2D system (Chap. 8), we measure temperature-dependent condensate phase fluctuations in the vicinity of the Berezinskii-Kosterlitz-Thouless transition. Other experiments investigate dynamic properties of reduced dimension systems. Strongly inhibited transport of a 1D gas in a lattice is observed in one experiment (Chap. 9). Another 2D experiment measures suppressed collisional decay rates due to the reduced dimensionality (Chap. 9). A final experiment (Chap. 7) examines quantum/classical correspondence in the effectively 1D dynamics of a 3D BEC. The dynamics is effectively 1D in the sense that the experiment is over before motion in the radial directions (which are not frozen out) can occur. This dissertation also describes the design and implementation of a novel 1D "accordion lattice" (Chaps. 5-6) which greatly facilitated the Berezinskii-Kosterlitz-Thouless experiment, the quantum/classical correspondence experiment, and a "superlattice" experiment conducted to assist in the calibration of the accordion lattice.
This dissertation is about the physics of dilute gaseous Bose-Einstein condensates (BECs) confined to lower dimensions by optical lattices. The central theme of the effects of reduced dimensionality is explored within various one-dimensional (1D) and two-dimensional (2D) systems. We create a 2D BEC by adiabatically increasing the confinement of a trapping potential in one direction to the point where motion in that direction is frozen out. Doing this in two directions, we create a 1D BEC. Two experiments examine the ground state properties of a 1D and 2D system. In the 1D system (Chap. 9), a reduction in three-body recombination signals an increase in correlation resulting in a partial "fermionization" of the Bose gas. In the 2D system (Chap. 8), we measure temperature-dependent condensate phase fluctuations in the vicinity of the Berezinskii-Kosterlitz-Thouless transition. Other experiments investigate dynamic properties of reduced dimension systems. Strongly inhibited transport of a 1D gas in a lattice is observed in one experiment (Chap. 9). Another 2D experiment measures suppressed collisional decay rates due to the reduced dimensionality (Chap. 9). A final experiment (Chap. 7) examines quantum/classical correspondence in the effectively 1D dynamics of a 3D BEC. The dynamics is effectively 1D in the sense that the experiment is over before motion in the radial directions (which are not frozen out) can occur. This dissertation also describes the design and implementation of a novel 1D "accordion lattice" (Chaps. 5-6) which greatly facilitated the Berezinskii-Kosterlitz-Thouless experiment, the quantum/classical correspondence experiment, and a "superlattice" experiment conducted to assist in the calibration of the accordion lattice.
This dissertation is about the physics of dilute gaseous Bose-Einstein conden-sates (BECs) confined to lower dimensions by optical lattices. The central theme of the effects of reduced dimensionality is explored within various one-dimensional (1D) and two-dimensional (2D) systems. We create a 2D BEC by adiabatically in-creasing the confinement of a trapping potential in one direction to the point where motion in that direction is frozen out. Doing this in two directions, we create a 1D BEC. Two experiments examine the ground state properties of a 1D and 2D system. In the 1D system (Chap. 9), a reduction in three-body recombination signals an increase in correlation resulting in a partial “fermionization ” of the Bose gas. In the 2D system (Chap. 8), we measure temperature-dependent condensate phase fluctuations in the vicinity of the Berezinskii-Kosterlitz-Thouless transition. Other experiments investigate dynamic properties of reduced dimension sys-tems. Strongly inhibited transport of a 1D gas in a lattice is observed in one ex-periment (Chap. 9). Another 2D experiment measures suppressed collisional decay rates due to the reduced dimensionality (Chap. 9). A final experiment (Chap. 7)
Since the experimental achievement of Bose-Einstein condensation (BEC) in dilute gases, ultra-cold atom systems have proven to be an unparalleled test bed for condensed matter phenomena. With this in mind, our laboratory set out to build an apparatus for the study of the effects of disorder on condensed matter phase transitions using a BEC loaded into one, two, and three-dimensional lattices. My thesis is divided into two main sections. In the first section I describe in detail the design and construction of our apparatus. Our system is designed to form <super>87<\super>Rb condensates of approximately 10<super>5<\super> atoms. We have three possible experimental science chambers. 1. One can perform 1D lattice experiments in the chamber where the condensate is formed. There is a mirror located in vacuum, which is suitable for this purpose. 2. In a glass chamber one can do 1D, 2D, or 3D lattice experiments. Atoms are loaded into a dipole trap (optical tweezer) prior to condensation and transported approximately 20 cm to the glass cell where optical evaporation is performed to form a BEC. 3. One can transfer, using the same techniques as (2), to a chamber with a multi-channel plate detector for Rydberg atom experiments. The experimental results described in my thesis pertain to situation (1). However, I describe some details of (2) in the construction section. The first experiment presented in my thesis demonstrates the effect of disorder on the time-dependent dynamics of lattice systems. We observe that a small perturbation produces a dramatic change in the adiabaticity criteria for loading a BEC into one-dimensional optical lattice. I conclude with experiments that we expect to perform on this apparatus.
Cold atoms trapped in optical lattices (crystals of light) provide a pristine platform for exploring quantum many body physics. Motivated by several recent experiments, this thesis examines the equilibrium and non-equilibrium dynamics of a Bose-Einstein condensate (BEC) loaded in a low dimensional optical lattice in order to realize novel quantum phases. There are two main research directions in this thesis. The first one involves the possibility that exotic order spontaneously forms when two-component bosons are trapped in a honeycomb lattice. My studies on this theme is motivated by the observation of a “twisted superfluid” state in Prof. Klaus Sengstock’s group at Hamburg (Soltan-Panahi et al., Nat. Phys. 8, 71 (2012)). A twisted superfluid involves Bose-Einstein condensation into a state whose order parameter has a spatially varying phase. In chapter 3, I study the stability of a Bose-Einstein condensate towards forming a twisted superfluid within the framework of mean field theory. Despite a exhaustive numerical search I do not find a parameter regime with a twisted superfluid. This search involved all experimentally relevant parameter regimes and therefore mean field theory predicted that the experimentalists should not observe a twisted superfluid. I conclude that the experimental observations were either a manifestation of counter superfluidity or due to interactions during time-of-flight. Subsequent experiments showed that the observations were an artifact of the measurement process. The second research direction in this thesis is an exploration of the stability of periodically driven quantum systems (also known as Floquet systems). Floquet systems can be used to realize exotic non-equilibrium quantum phases which do not have a counterpart in static systems. However, the driving can cause these systems to heat up which presents a major obstacle to creating exotic states. To explore this issue in a concrete example, I model an experiment (Parker, Ha, and Chin, Nat. Phys. 9, 769 (2013)) where a Bose-Einstein condensate loaded in an optical lattice is subjected to periodic shaking. I investigate the stability of this Floquet BEC to interactions. This research direction consists of 3 studies. In chapter 4, I first do this analysis for a purely one-dimensional system and identify a large parameter regime where the BEC is stable. In the next two chapters, I go beyond 1D and consider the role of transverse degrees of freedom. This is because the shaken lattice experiments that I model involves a 1D array of pancakes. I find that this geometry leads to much more dissipation than a purely 1D system. This extra dissipation arises because interactions can transfer energy between different directions. In chapter 5, I consider the extreme case where there is no transverse confinement. I find that in the absence of transverse confinement, a one-dimensional Floquet BEC is generically unstable. Finally, in chapter 6, I consider harmonic transverse confinement modeling the crossover between chapters 4 and 5. I find that as the transverse confinement is made stronger, the atom loss rate initially increases, but beyond a critical transverse confinement, the atom loss disappears due to unavailability of phase space for scattering. I also predict that if the transverse confinement is tuned to the vicinity of certain magic values, the heating rate exhibits a sharp drop. I perform similar analyses for a shaken square lattice and find that generically a low-dimensional Floquet BEC can be stabilized by suitably designing the transverse confinement.
This thesis manuscript presents experimental results on Bose-Einstein condensates in optical lattices and in the regime of fast rotation. In an optical lattice an array of inde-pendent 2-dimensional Bose-Einstein condensates is produced from an initially 3-dimen-sional cloud. The number of 2D BECs in this array can be controlled by superimposing a magnetic field gradient along the axis of the lattice and then selectively evaporating the sites. When producing a small number of sites and letting them interfere, dislocations in the interference pattern were observed, which must be due to nontrivial phase defects, as for example free vortices. Such vortices and vortex-antivortex pairs play an important role in the superfluid to normal phase transition in a 2-dimensional gas, the Kosterlitz-Thouless transition. With 3-dimensional condensates the limit of fast rotation is studied in a hybrid potential contain-ing a small quartic term. In this trap rotation frequencies larger than the harmonic trapping fre-quency can be reached. These experiments are a first step to a possible observation of strongly correlated states close to the regime of the fractional quantum Hall effect.
I have been one of the main responsible for building a new laboratory for Bose-Einstein condensation with 87Rb. In particular, the experimental setup has been designed for performing experiments with Bose-Einstein condensates load into optical lattices of variable geometries. All parts essential for Bose-Einstein condensation are in place. Atoms are collected in a magneto-optical trap, transferred to another vacuum chamber, with better vacuum, and trapped in another magneto-optical trap. Atoms are successfully transferred to a dark magnetic trap, and system for diagnostics with absorption imaging has been realized. We have not yet been able to form a Bose-Einstein condensate, due to a range of technical difficulties. Equipment for alignment of optical lattices with flexible geometry has been designed, built, and tested. This tool has been proven to work as desired, and there is a great potential for a range of unique experiments with Bose-Einstein condensates in optical lattices of various geometries, including superlattices and quasi-periodic lattices. Numerical studies have been made on anisotropic optical lattices, and the existence of a transition between a 2D superfluid phase and a 1D Mott-insulating phase has been confirmed. We have shown that the transition is of Berezinskii-Kosterlitz-Thouless type. In another numerical study it has been shown that using stimulated Raman transitions is a practical method for transferring atoms between states in a double optical lattice. Thus, it will be possible to transfer populations between the lattices, with further applications in qubit read/write operations.
We analyze the results of a recent experiment with bosonic rubidium atoms harmonically confined in a quasi-two-dimensional (2D) geometry. In this experiment a well-defined critical point was identified, which separates the high-temperature normal state characterized by a single component density distribution, and the low-temperature state characterized by a bimodal density distribution and the emergence of high-contrast interference between independent 2D clouds. We first show that this transition cannot be explained in terms of conventional Bose–Einstein condensation of the trapped ideal Bose gas. Using the local density approximation (LDA), we then combine the mean-field (MF) Hartree–Fock theory with the prediction for the Berezinskii–Kosterlitz–Thouless (BKT) transition in an infinite uniform system. If the gas is treated as a strictly 2D system, the MF predictions for the spatial density profiles significantly deviate from those of a recent quantum Monte Carlo (QMC) analysis. However, when the residual thermal excitation of the strongly confined degree of freedom is taken into account, excellent agreement is reached between the MF and the QMC approaches. For the interaction strength corresponding to the experiment, we predict a strong correction to the critical atom number with respect to the ideal gas theory (factor ∼2). Quantitative agreement between theory and experiment is reached concerning the critical atom number if the predicted density profiles are used for temperature calibration.