Creation and applications of large toroidal time-averaged optical potentials for BECs

Abstract

This thesis reports the final stages of construction and performance of a high atom number rubidium-87 Bose-Einstein condensate (BEC) apparatus, and the generation of timeaveraged optical potentials geared towards interferometric applications. A detailed description of the hybrid trap used in our experiment is given, which comprises magnetic confinement from a quadrupole field, and optical trapping from a focussed 1064 nm laser beam. We present the typical temperatures of atom clouds at all stages of cooling, from the 3D magneto-optical trap, through to the onset of condensation in the hybrid trap.Our trap typically realises nearly-pure BECs of 1.5-2.0 × 10 6 atoms, at temperatures of (217 ± 6) nK, over duty cycles of 16-25 s.The implementation of a time-averaged optical potential is also discussed, as a means of generating versatile, planar potentials for BECs. In this work, we focus on the generation of large, ring-shaped potentials, which have trapping frequencies in the ranges 2π × (110-140, 35-55) Hz vertically and radially, respectively. The details of a feedforward algorithm are presented, which allows for the realisation of smooth ring structures up to 150 µm in radius, which enclose areas ten times larger than optical ring traps created previously. These larger rings are promising candidates for interferometric measurements of the rotation-induced Sagnac phase, with the device sensitivity proportional to the enclosed area. We typically retain 2.5-3.0 × 10 6 atoms in these large traps, after evaporatively cooling to temperatures of (44 ± 9) nK. The measured lifetimes in the trap are on the order of 16 s.These ring condensates are expected to be fully phase coherent, despite their reduced dimensionality, as a result of high atom numbers, weak confinement, and low temperatures. We present preliminary data demonstrating these coherence properties, and outlay future plans for a comprehensive study of this coherence, probing both fully coherent rings, and those expected to fall within the phase-fluctuating regime.The control afforded using our time-averaged optical potentials is demonstrated through the generation of acoustic waves propagating around the ring condensates. The dispersion is linear for low driving frequencies, and allows for a determination of the speed of Publications included in this thesisNo publications included. Contributions by others to this thesisThe experimental data presented in Figure 5.18 were collected and analysed by ThomasBell. In addition, a number of CAD renderings of the apparatus shown in chapter 4 were also his work. Appropriate mention has been made in the captions of relevant figures. Statement of parts of thesis submitted to qualify for the award of another degreeNone. To the other experimentalists in the lab, both on my experiment, and on the other side:Guillaume Gauthier, and Nick McKay Parry -thanks for your input and advice along the way. To Tom Bell, who has spent many a late night fixing electronics, upgrading essential components of the experiment, as well...

Engineering time-averaged optical potentials for Bose-Einstein condensates

“…Since these methods cannot control azimuthal intensity variations, localised non-uniformities are impossible to correct. This constraint has limited condensate radii to R < 25 µm, where interaction energies exceed the non-uniformities [123,124]. Toroidal potentials formed using the reconfigurable AOD and DMD technologies should overcome this limitation.…”

“…For large waveguide radii the potential energy modulations eventually exceed the condensate chemical potential, impeding the formation of connected toroidal densities. Sheet modulations, among other aberrations, have previously constrained the achievable radii R < 25 µm [123,124].…”