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An Atom Waveguide for Interferometry with a Bose-Einstein Condensate of 87-Rb

J. M. Reeves

Published May 1, 2006• 4 citations
Publisher

Abstract

A Bose-Einstein condensation (BEC) production machine has been assembled and operated, using two vacuum chambers isolated from each other by a thin tube. A magnetooptical trap (MOT) is operated in the first chamber, where atoms are captured out of a thermal vapor and cooled to about 200 µK. The atoms are then transferred to a magnetic trap which is mounted on to a movable stage. A programmable motor moves the stage about half a meter, carrying the atoms to the second vacuum chamber where they are evaporatively cooled in a time-orbiting potential trap. We successfully used this apparatus to observe the first BEC's at the University of Virginia. We have also implemented a novel atom trap for BEC's of 87 Rb to be used in atom interferometry experiments. The trap is based on a time-orbiting potential waveguide. It supports the atoms against gravity while providing weak confinement to minimize interaction effects. We have loaded a condensate into the waveguide, and removed all other confinement fields. Up to 2 ×10 4 condensate atoms have been loaded into the trap, at estimated temperatures as low as 850 pK. We expect this novel type of trap will be useful for a variety of applications in condensate interferometry. Finally, we have characterized our trap by perturbing the atomic cloud with a sudden change in the confinement field. We subsequently obtain harmonic oscillation frequencies vi (ω x ,ω y ,ω z ) as low as 2π×(6.0, 1.2, 3.3) Hz. We have developed a mathematical description of the waveguide fields to account for the residual fields from the trap leads, obtaining good agreement between the measured and predicted trap behavior. The weak confinement of our guide should greatly reduce the limiting effects of atomic interactions. We anticipate that interferometer measurement times of 1 s or more should be achievable in this device. With suitable modifications, our waveguide could be used to precisely measure electric polarizability, gravitational forces, rotations, and other phenomena. We expect that the trap design presented here will play an essential role in allowing condensate interferometry to realize its potential. Note: Abstract extracted from PDF text

DOI
10.18130/v3r004

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•Jan 1, 2019•2 citations
A Bose-Einstein condensation (BEC) production machine has been assembled and operated, using two vacuum chambers isolated from each other by a thin tube. A magnetooptical trap (MOT) is operated in the first chamber, where atoms are captured out of a thermal vapor and cooled to about 200 µK. The atoms are then transferred to a magnetic trap which is mounted on to a movable stage. A programmable motor moves the stage about half a meter, carrying the atoms to the second vacuum chamber where they are evaporatively cooled in a time-orbiting potential trap. We successfully used this apparatus to observe the first BEC's at the University of Virginia. We have also implemented a novel atom trap for BEC's of 87 Rb to be used in atom interferometry experiments. The trap is based on a time-orbiting potential waveguide. It supports the atoms against gravity while providing weak confinement to minimize interaction effects. We have loaded a condensate into the waveguide, and removed all other confinement fields. Up to 2 ×10 4 condensate atoms have been loaded into the trap, at estimated temperatures as low as 850 pK. We expect this novel type of trap will be useful for a variety of applications in condensate interferometry. Finally, we have characterized our trap by perturbing the atomic cloud with a sudden change in the confinement field. We subsequently obtain harmonic oscillation frequencies vi (ω x ,ω y ,ω z ) as low as 2π×(6.0, 1.2, 3.3) Hz. We have developed a mathematical description of the waveguide fields to account for the residual fields from the trap leads, obtaining good agreement between the measured and predicted trap behavior. The weak confinement of our guide should greatly reduce the limiting effects of atomic interactions. We anticipate that interferometer measurement times of 1 s or more should be achievable in this device. With suitable modifications, our waveguide could be used to precisely measure electric polarizability, gravitational forces, rotations, and other phenomena. We expect that the trap design presented here will play an essential role in allowing condensate interferometry to realize its potential. Note: Abstract extracted from PDF text
We describe an atom trap for Bose-Einstein condensates of 87Rb^{87}\mathrm{Rb}87Rb to be used in atom interferometry experiments. The trap is based on a time-orbiting potential waveguide. It supports the atoms against gravity while providing weak confinement to minimize interaction effects. We observe harmonic oscillation frequencies $({\ensuremath{\omega}}_{x},{\ensuremath{\omega}}_{y},{\ensuremath{\omega}}_{z})$ as low as $2\ensuremath{\pi}\ifmmode\times\else\texttimes\fi{}(6.0,1.2,3.3)\phantom{\rule{0.3em}{0ex}}\mathrm{Hz}$. Up to $2\ifmmode\times\else\texttimes\fi{}{10}^{4}$ condensate atoms have been loaded into the trap, at estimated temperatures as low as 850pK850\phantom{\rule{0.3em}{0ex}}\mathrm{pK}850pK. We anticipate that interferometer measurement times of 1s1\phantom{\rule{0.3em}{0ex}}\mathrm{s}1s or more should be achievable in this device.
This dissertation documents a second Bose-Einstein Condensate production apparatus at the University of Virginia. The apparatus produces condensates of 87 Rb atoms to be used as the wave source for atom interferometry experiments. Additionally, a new magnetic trap was developed which provides a harmonic potential with cylindrical symmetry and also supports the atoms against gravity. This trap is based on a time-orbiting potential. To characterize the trapping potential, a condensate was loaded into the trap and perturbed by suddenly changing the confinement field during loading. This had the effect of inducing harmonic oscillations which were measured at varying trap parameters. We expect that this trap will be useful for the implementation of a compact atom interferometer-based gyroscope. Finally, an asymmetric Bragg splitting pulse was developed which allowed for the implementation of two simple interferometers. These interferometers developed a phase which depended on the recoil frequency of 87 Rb and served as a proof of principle for this new apparatus.
The construction of an apparatus capable of producing Bose-Einstein condensates marks a significant milestone in every experimental cold atom laboratory. In this thesis I describe the development of a system to create a Bose-Einstein condensate of 87Rb^{87}Rb87Rb in a Time-Orbiting Potential trap.\n\nI review the optical and magnetic techniques required to trap and cool an atomic sample under vacuum, motivating our decision to build a double MOT system comprised of a high-pressure (10−910^{-9}10−9 torr) chamber to gather atoms and a low-pressure (10−1110^{-11}10−11 torr) chamber to cool atoms to degeneracy.\n\nBy theoretically modeling the atom number and temperature inside the magnetic trap during evaporative cooling I demonstrate a simple approach to determining a cooling path that reaches the transition temperature. By making use of the condensates produced under these non-optimized conditions I determine the heating rate of the condensate in the TOP trap to be 300300300 nK/s. I further use the condensates to make a more precise measurement of the TOP trap bias field.\n\nI improve upon the conventional evaporation path used in TOP trap experiments by introducing and optimizing additional bias field compression stages in between RF evaporation ramps. I demonstrate how, by adding these additional stages, the system is capable of reaching the BEC phase transition with a final atom number of 2times1052\\times 10^{5}2times105. In contrast, RF evaporation after only a single bias field ramp has yielded condensates with only 30times10330\\times 10^330times103 atoms.
We report on the design and implementation of a compact apparatus for the production of Bose-Einstein condensation and the investigation of atom interferometry in a time-orbiting potential trap. The apparatus fits in a volume of about 30 L, and includes a vacuum chamber, magnetic trapping coils, beam delivery optics, and an imaging camera. It supports testing of in-air atom chips through a silicon membrane that serves as a thin vacuum wall and a mirror for a magneto-optical trap. The apparatus could reliably trap atoms near the membrane, but the number of atoms was limited and too low to support evaporative cooling. In future work, the cell will be loaded with optical tweezers from a separate cold-atom source.
In this thesis I discuss work that has been carried out on the dynamics of a Bose Einstein condensate of Rb 87 produced near an atom chip. A Bose Einstein Condensate (BEC) is a quantum state of matter where a single quantum state becomes occupied by a macroscopic number of identical Bosons. In our case this is achieved by cooling a system of trapped identical rubidium 87 atoms to its ground state. To reach temperatures of condensation we initially laser cool atoms from room temperature, before loading them into a magnetic trap. The magnetic trap is produced through a combination of uniform magnetic fields from coils outside our vacuum chamber and currents running through wires on an atom chip. The atom chip is a microfabricated device, produced by a coating a silicon chip with a thin layer of gold and etching wires into it. Together, these fields create a magnetic field minimum 120μm from the surface of the chip which can be used to confine low field seeking hyperfine states of the atom in an elongated harmonic trap. Once the atoms are confined in the magnetic trap we used force evaporative cooling out to reach the phase space densities required for Bose Einstein condensation. The BEC is used to investigate the relative dynamics between the fraction of the atoms in the condensate to those not in the condensate. Our atom chip provided a suitable environment to investigate this due to fragmentation of the magnetic potential close to the chip. Small imperfections in the wires on our atom chip mean that the trapping potential isn’t smooth. Small regions of higher trapping frequency - or fragments - are formed. Due to the small size of these fragments it is possible to find a position where a condensate can form in the fragment, and see a potential of high frequency, whereas a non-condensed atom will see a lower frequency potential. We exploit this to set the condensed fraction moving relative to the non condensed part and investigate the subsequent damping of their motion relative to each other.
The quantum gravimeter QG-1 is designed to perform local gravity measurements in field with residual uncertainties surpassing state-of-the-art absolute gravimeters. It will use delta-kick collimated Bose-Einstein condensates (BEC) of 87 Rb atoms as the source for matter-wave interferometry and aims for an uncertainty level below 3 nm/s 2 with a repetition rate of 0.5 Hz. In the frame of this thesis, the atom-chip source has been set up and characterized. In order to meet the requirements for transportability, the apparatus needs to be compact and robust. Additionally, a high BEC flux is desired for the targeted statistical uncertainty. To meet these requirements a double magneto-optical trap (MOT) configuration based on a 3-layer atom chip has been implemented. Furthermore, a compact high power fiber-based laser system has been developed to provide the light for atomic manipulation. To power and control this laser system and generate the magnetic fields necessary to trap atoms, a miniaturized electronic system with low power consumption has been utilized and characterized. The steps towards trapping the atoms inside a magnetic trap on which the evaporation will be implemented have been realized and optimized. A two-dimensional MOT has been employed and an optimized atomic beam flux of 2.2 × 10 9 atoms/s with a mean longitudinal velocity of 17.2 m/s and a narrow longitudinal velocity distribution of 2.2 m/s has been used to load a 3D-chip MOT. The resulting 3D-MOT has been characterized and a loading rate of 8.1×10 8 atoms/s has been attained. After loading for 300 ms, 2.4×10 8 atoms are trapped in the 3D-MOT. Applying a subsequent compressed MOT stage, the cloud density has been increased by a factor of 10. This denser atomic ensemble with 2.9 × 10 10 atoms/cm 3 has been further cooled in a polarization gradient cooling (PGC) stage to 5.6 μK in order to increase the mode match with the subsequent magnetic trap. In the initial large volume magnetic trap generated by the wire structures on the mesoscopic and the base layers of the atom-chip assembly, 3.6 × 10 7 atoms have been captured. This trap features a temperature of 44.3 μK, a trapping frequency of 9.1 Hz in its weak confining axis and a phase-space density of 4.5 × 10 −7 . The process of transferring the atoms into the final magnetic trap with higher trapping frequency for an efficient BEC evaporation, which is formed by the wire structures on the base and science chip has been tested. With this, all the necessary functionalities of a high-flux BEC source setup have been demonstrated.