Ultracold atomic gas systems provide a remarkably versatile platform for studying a wide range of physical phenomena, from analogue particle physics and gravity, to the emergence of subtle and profound order in many body and condensed matter systems. In addition, ultracold atomic gas systems can be used to perform a range of precision measurements, from time keeping to variations in the fine structure constant. In this dissertation, I describe our efforts to build a new apparatus capable testing a range of techniques for performing precision measurements in a magnetic storage ring for cold, possibly Bose-condensed, lithium and rubidium atoms. Next, I briefly touch upon our explorations of spin vortices in a ferromagnetic rubidium Bose-Einstein condensate before presenting an exhaustive account of our work using free-particle-like magnon excitations of the ferromagnetic gas to cool it and measure its temperature in a never-before-seen regime of low entropy. Using magnons as a thermometer, we measure temperatures as low as one nanokelvin in gases with an entropy per particle of about one thousandth of the Boltzmann constant, 0.001 k_B. I conclude by presenting the details of our procedure for calculating the entropy of our coldest, lowest entropy gases in the regime where the local density approximation does not apply.
Ultracold atomic gas systems provide a remarkably versatile platform for studying a wide range of physical phenomena, from analogue particle physics and gravity, to the emergence of subtle and profound order in many body and condensed matter systems. In addition, ultracold atomic gas systems can be used to perform a range of precision measurements, from time keeping to variations in the fine structure constant. In this dissertation, I describe our efforts to build a new apparatus capable testing a range of techniques for performing precision measurements in a magnetic storage ring for cold, possibly Bose-condensed, lithium and rubidium atoms. Next, I briefly touch upon our explorations of spin vortices in a ferromagnetic rubidium Bose-Einstein condensate before presenting an exhaustive account of our work using free-particle-like magnon excitations of the ferromagnetic gas to cool it and measure its temperature in a never-before-seen regime of low entropy. Using magnons as a thermometer, we measure temperatures as low as one nanokelvin in gases with an entropy per particle of about one thousandth of the Boltzmann constant, 0.001 k_B. I conclude by presenting the details of our procedure for calculating the entropy of our coldest, lowest entropy gases in the regime where the local density approximation does not apply.
At low temperatures, the thermodynamic properties of a many-body system are determined by the low energy excursions from the ground state. In quantum degenerate rubidium, the low energy excitations are phonons—phase and density modes—and magnons—spin modes. This thesis reports on the construction of an ultracold rubidium apparatus and techniques to create and image phonons and magnons. We propose and study matter wave interferometry of phonons in a ring trap as a potential rotation sensor in a compact sensor. Precision measurements of coherent magnons allow us to determine the magnon dispersion relation, including a gap induced by magnetic dipole-dipole interactions.
Ultracold atomic system provides us a versatile platform to address problems ranging from quasi-particle excitations supported by a variety of ground states, to cold collisions which could lead to a precise determination of interatomic molecular potential, and to atomtronics focusing on atomic analogues of electronic components. In this dissertation, I describe our efforts in exploring a wide topic range by our \\Rb Bose Einstein condensate (BEC) machine and \\Li - \\Rb mixture machine, including magnons, which are collective spin excitations supported by Rb F = 1 ferromagnetic BEC ground state, cold elastic collisions between Li and Rb co-trapped in a spherical quadrupole trap, and Li spin dynamics in a spin bath formed by Rb.
Under suitable experimental conditions, collective spin-wave excitations, magnons, form a Bose–Einstein condensate (BEC), where the spins precess with a globally coherent phase. Bose–Einstein condensation of magnons has been reported in a few systems, including superfluid phases of 3He, solid state systems, such as yttrium-iron-garnet films, and cold atomic gases. The superfluid phases of 3He provide a nearly ideal test bench for coherent magnon physics owing to experimentally proven spin superfluidity, the long lifetime of the magnon condensate, and the versatility of the accessible phenomena. We first briefly recap the properties of the different magnon BEC systems, with focus on superfluid 3He. The main body of this review summarizes recent advances in the application of magnon BEC as a laboratory to study basic physical phenomena connecting to diverse areas from particle physics and cosmology to vortex dynamics and new phases of condensed matter. This line of research complements the ongoing efforts to utilize magnon BECs as probes and components for potentially room-temperature quantum devices. In conclusion, we provide a roadmap for future directions in the field of applications of magnon BEC to fundamental research.
Ultra-cold atoms have become indispensable in quantum sensing, quantum computing, and quantum communication applications, as well as in fundamental sciences. In quantum sensing, they enable precision measurements with outstanding sensitivities that can be of great benefit in navigation, geodesy, and fundamental physics applications. The sensitivity of the atom interferometers used herefore typically scales with the square of the interrogation time. Therefore, in atom interferometric measurements, one seeks ensembles that feature a particularly low internal kinetic energy and can thus be detected after a long time-of-flight. The lowest values are achieved using Bose-Einstein condensates in conjunction with delta-kick collimation, a technique for the collimation of the atomic wave function. Bose-Einstein condensation can be achieved using a multi-stage laser cooling scheme, followed by evaporative cooling, which finally brings the temperature down to the critical temperature, typically in the nanokelvin regime. In addition to the low thermal spread, microgravity enables particularly long observation times, as it avoids limitations of the interrogation time due to vacuum chamber constraints, which are present when falling freely under the influence of gravity. Additionally, microgravity enables measurements that are not possible under normal gravity conditions, such as buoyancy-free miscibility studies, truly uniform potentials, and advanced cold atom quantum memories, which are projected to feature unprecedented time-bandwidth products. For the preparation of a Bose-Einstein condensate, two techniques are widely used in science, namely magnetic traps and optical dipole traps. For microgravity missions, magnetic traps on atom chips have been used in nearly all setups so far due to their particularly low SWAP budget and high cold atom flux. In contrast, ground-based experiments often prefer optical traps due to their higher versatility, full optical access, and the absence of surfaces in proximity to the atoms, which is fundamental for precision measurements. Additionally, optical traps enable the manipulation of intra- and inter-species scattering lengths using Feshbach resonances. Compared to atom chip traps, they also feature an intrinsically better symmetry of the trapping potential, in theory leading to fewer aberrations in matter-wave optics. The latter could enable the generation of ensembles in the yet unreached femtokelvin regime using delta-kick collimation with fewer perturbations. This thesis is about the realization of a 1064nm optical dipole trap set up for the microgravity environment of the Bremen Drop Tower. It aims to highlight the advantages of optical traps in conjunction with microgravity and prepare optical trapping technology for use in future microgravity and space missions. The thesis describes the experimental setup, the direct loading process, the characterization of the optical potential, and the optimization and investigation of the evaporative cooling in the optical dipole trap. For the evaporation process, the influence of gravity is comprehensively investigated through analytical calculations and microgravity tests. Bose-Einstein condensation is demonstrated, proving the achievement of condensation through absorption imaging of the spatial distribution, tests on the free evolution of the ensemble, and investigation of the phase-space densities achieved. Up to (74 ± 4)% condensation of an ensemble featuring (30 000 ± 13 000) atoms at a kinetic temperature of (37 ± 3) nK is demonstrated. The evaporation shows high efficiency down to a 3 s evaporation time, while a critical phase-space density can be achieved within (1.29 ± 0.07) s using an even faster evaporation ramp. This opens the door to fundamental physics experiments on Bose-Einstein condensates in microgravity, as well as applications pursuing atomic ensembles with exceptionally low expansion velocities.
Lately there has been rapid progress in techniques for trapping samples of neutral atoms at elevated densities and extremely low temperatures. Now a group at the Joint Institute for Laboratory Astrophysics in Boulder, Colorado, has used a new type of magnetic trap to confine rubidium atoms at the lowest kinetic temperature yet recorded for an atomic gas. Compelling motivations for this effort include the use of such ultra-cold gases in high-precision spectroscopy, low-energy tests of the "standard model" of particle physics, and studies of atomic collisions at very low relative velocities. Particularly exciting is the prospect of studying quantum collective phenomena such as Bose-Einstein condensation (p37).
At sufficiently high phase space densities, the many-body state of a degenerate Bose gas can be described by a single macroscopic wavefunction that undergoes nonlinear dynamics. Ultracold Bose gases are a prominent platform in the field of nonlinear physics, enabling the study of stable stationary wavepackets, known as solitons. In these platforms, atom interactions are often weak and treated within a mean-field approach. However, interactions play a crucial role in the physics of these Bose gases and explain for example the formation mechanism of solitons or the emergence of a remarkable phase of matter: superfluidity. This thesis describes investigations on peculiar dynamics of ultracold Bose gases, which are approximated by zero-temperature physics. After introducing our experimental setup and its theoretical characterization, we discuss the survival of the superfluid property in spatially modulated systems, in connection with Leggett's seminal work in 1970 and the spectacular supersolid phase of matter. We then explore the physics of a specific type of 1D soliton that emerges in a weakly-immiscible two-component mixture, known as the magnetic soliton. Its name originates from its appearance in 1D ferromagnetic spin chains. Magnetic solitons exhibit a captivating periodic motion under a uniform force field, similar to the Bloch oscillations of electrons in solids. Finally, we introduce a new experimental setup for studying Rydberg excitations and the realization of an all-optical Bose-Einstein-Condensate.