We have added an optical potential to a conventional time-averaged orbiting potential (TOP) trap to create a highly anisotropic hybrid trap for ultracold atoms. Axial confinement is provided by the optical potential; the maximum frequency currently obtainable in this direction is 2.2 kHz for rubidium. The radial confinement is independently controlled by the magnetic trap and can be a factor of 700 times smaller than in the axial direction. This large anisotropy is more than sufficient to confine condensates with ∼10 5 atoms in a quasi-2D (Q2D) regime, and we have verified this by measuring a change in the free expansion of the condensate; our results agree with a variational model.
We have added an optical potential to a conventional time-averaged orbiting potential (TOP) trap to create a highly anisotropic hybrid trap for ultracold atoms. Axial confinement is provided by the optical potential; the maximum frequency currently obtainable in this direction is 2.2 kHz for rubidium. The radial confinement is independently controlled by the magnetic trap and can be a factor of 700 times smaller than in the axial direction. This large anisotropy is more than sufficient to confine condensates with ∼10 5 atoms in a quasi-2D (Q2D) regime, and we have verified this by measuring a change in the free expansion of the condensate; our results agree with a variational model.
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 time-averaged 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 1.5–2.0×106 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 feed forward 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 2.5–3.0×106 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 sound, and the chemical potential, found here to be cs =1.34 mm.s-1, and μ=kB×18nK respectively.
We report on the realization of a stiff magnetic trap with independently adjustable trap frequencies, ω z and ω r , in the axial and radial directions, respectively. This has been achieved by applying an axial modulation to a time-averaged orbiting potential (TOP) trap. The frequency ratio of the trap, ω z /ω r , can be decreased continuously from the original TOP trap value of 2.83 down to 1.6. We have transferred a Bose-Einstein condensate (BEC) into this trap and obtained very good agreement between its observed anisotropic expansion and the hydrodynamic predictions. Our method can be extended to obtain a spherical trapping potential, which has a geometry of particular theoretical interest.
Since its first observation in 1995, Bose-Einstein condensation (BEC) experiments in dilute gases have been performed with only a few different kinds of magnetic traps. We have performed a BEC experiment with /sup 87/Rb atoms in a Ioffe-Pritchard magnetic potential, which opens up the possibility to study, e.g., Bose-Einstein condensates released in only one dimension. Our trap consists of a clip-like coil, which generates a pure two-dimensional (2D) quadrupole field for radial confinement. The axial confinement is provided by two plane pinch coils inside the clip-coil. These two coils create a bottle field. Two additional Helmholtz coils on the same symmetry axis are used to compensate the homogeneous part of the bottle field. By continuously varying the currents through the coils, the trapping potential can be changed from a 3D spherical harmonic to a 2D linear potential. The change can be done adiabatically or nonadiabatically by switching off the axial confinement slowly, or instantaneously respectively. This opens up the possibility to study 1D nonlinear atom optics. In combination with, e.g., far detuned light sheets, a Fabry-Perot type interferometer for a condensate might be realizable. Our experiments are performed in a vacuum system with two magneto-optical traps, which serves as the source of cold atoms for the magnetic trap. 10/sup 9/ atoms are captured in less than one minute. RF-induced evaporative cooling is used to increase the phase space density of the trapped atomic cloud. The BEC phase transition occurs at a temperature of 550 nK with 10/sup 6/ particles in the trap. Pure condensates with more than 10/sup 5/ atoms were produced.
Summary form only given.Since its first observation in 1995, Bose-Einstein condensation (BEC) experiments in dilute gases have been performed with only a few different kinds of magnetic traps. We have performed a BEC experiment with /sup 87/Rb atoms in a Ioffe-Pritchard magnetic potential, which opens up the possibility to study, e.g., Bose-Einstein condensates released in only one dimension. Our trap consists of a clip-like coil, which generates a pure two-dimensional (2D) quadrupole field for radial confinement. The axial confinement is provided by two plane pinch coils inside the clip-coil. These two coils create a bottle field. Two additional Helmholtz coils on the same symmetry axis are used to compensate the homogeneous part of the bottle field. By continuously varying the currents through the coils, the trapping potential can be changed from a 3D spherical harmonic to a 2D linear potential. The change can be done adiabatically or nonadiabatically by switching off the axial confinement slowly, or instantaneously respectively. This opens up the possibility to study 1D nonlinear atom optics. In combination with, e.g., far detuned light sheets, a Fabry-Perot type interferometer for a condensate might be realizable. Our experiments are performed in a vacuum system with two magneto-optical traps, which serves as the source of cold atoms for the magnetic trap. 10/sup 9/ atoms are captured in less than one minute. RF-induced evaporative cooling is used to increase the phase space density of the trapped atomic cloud. The BEC phase transition occurs at a temperature of 550 nK with 10/sup 6/ particles in the trap. Pure condensates with more than 10/sup 5/ atoms were produced.
The final stage in the creation of a Bose-Einstein condensate (BEC) is trapping and compressing in a strong magnetic trap. When the spin of an atom changes, it is lost from this magnetic trap. To be able to do experiments with varying atom spin, one can use an optical dipole trap, which traps atoms independent of their spin. This thesis presents a far red-detuned optical dipole trap, which is characterized by trapping BEC's. Furthermore, an approach to rapidly scan this trap is presented, creating a 'quasi-2D' potential. With this potential we attempt to create a horizontal sheet of condensed atoms, where the depth of the sheet of atoms is smaller than the range of the spin dynamics. This would enable a unique visualization of the spin dynamics using spin-dependent imaging.