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Experimental advances toward a compact dual-species laser cooling apparatus

Keith Ladouceur

Published Jan 1, 2008• 1 citation
Publisher

Abstract

This thesis describes the advances made towards a dual-species magneto-optical trap (MOT) of Li and Rb for use in photoassociation spectroscopy, Feshbach resonance studies, and, as long-term aspirations, the formation of ultracold heteronuclear polar molecules. The initial discussion will focus on a brief theoretical overview of laser cooling and trapping and the production of ultracold molecules from a cold atom source. Subsequently, details of the experimental system, including those pertaining to the required laser light, the vacuum chamber, and the computer control system will be presented. Finally, preliminary optimization and characterization measurements showing the performance of a single species Li MOT are introduced. These measurements demonstrated the loading of over 8 x 107 Li atoms directly into a MOT without the need for a Zeeman slower.

DOI
10.14288/1.0066703

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This thesis describes the advances made towards a dual-species magneto-optical trap (MOT) of Li and Rb for use in photoassociation spectroscopy, Feshbach resonance studies, and, as long-term aspirations, the formation of ultracold heteronuclear polar molecules. The initial discussion will focus on a brief theoretical overview of laser cooling and trapping and the production of ultracold molecules from a cold atom source. Subsequently, details of the experimental system, including those pertaining to the required laser light, the vacuum chamber, and the computer control system will be presented. Finally, preliminary optimization and characterization measurements showing the performance of a single species Li MOT are introduced. These measurements demonstrated the loading of over 8 x 107 Li atoms directly into a MOT without the need for a Zeeman slower.
Ultracold mixture experiments hold the promise of providing new insights into many-body quantum systems as well as ultracold chemistry and few-body phenomena. The work presented in this thesis dealt with the construction of a new apparatus for the production and study of ultracold gases of fermionic lithium-6 and bosonic caesium-133. These isotopes offer a wide tunability in their interaction strength, both in inter-species and intra-species collisions, through magnetic Feshbach resonances. Additionally, the widely different resonance frequencies of lithium and caesium enables independent control of each of the species. With this apparatus, Bose-Einstein condensates (BEC) containing 10^4 lithium Feshbach molecules are routinely produced. The cooling system for caesium has been developed in parallel and important steps towards producing ultracold caesium gases have been made. An optical dipole trap has been loaded with 2x10^6 caesium atoms and evaporative cooling towards quantum degeneracy can now be pursued. Laser, vacuum, magnetic and control systems have been developed for the implementation of this experiment. Light produced with this laser system was used to laser cool atoms, create conservative dipole traps as well as to provide means of imaging atomic clouds. Additionally, a system to produce strong magnetic fields of up to 1400 G has been established in order to exploit the wide tunability in the atomic interactions. Software that was developed for the computerised control system facilitated the coordination of all the components involved in the experimental sequence. Measurements and calculations that showcase the functionality of relevant parts of the setup are presented in this thesis. In this experiment, lithium and caesium atoms are obtained from a novel type of Zeeman slower and are loaded into a magneto-optical trap (MOT). The system is capable of doing this independently for each of the atomic species as well as sequentially. After the MOT has been loaded with atoms, they are transferred into a conservative far-off-resonance optical dipole trap. By adjusting the interactions between atoms and lowering the depth of the dipole trap, efficient evaporative cooling of lithium was carried out from which a molecular BEC was obtained. Time-of-flight measurements were used to characterise the condensate and study its expansion dynamics.
There is great interest in the use of ultracold polar molecules confined in op- tical lattices for quantum simulation of complex many-body problems. The objective of our work is to realise a system of ultracold fermionic 40K133Cs molecules in a 1D optical lattice consisting of an array of 2D pancake traps. Such a system will enable a wide range of future investigations, including the study of novel regimes of interlayer superfluidity [1]. To create polar molecules we will employ magnetoassociation on an interspecies Feshbach resonance followed by transfer to the rovibrational ground state using STIm- ulated Raman Adiabatic Passage (STIRAP). This thesis reports the progress towards this ambitious goal focusing on the initial laser cooling stage. We present a simple single cell vacuum system that can be used to locate the interspecies Feshbach resonances essential for magnetoassociation. We give a description of the laser setups for both species, including details of the laser spectroscopy used for frequency stabilisation. We then present results on the optimisation of the magneto optical trap (MOT) for each species (focusing on the most abundant 39K isotope). The effects of the intensities and detun- ings of both the cooling and repump light and the magnetic field gradient are investigated in order to maximise the number of trapped atoms. Finally, we present preliminary observations of simultaneously loading overlapped MOTs for both species.
Heteronuclear polar molecules have recently attracted enormous attention owing to their ground state having a large electric dipole moment. The long range anisotropic dipole-dipole interaction in such systems is the basis for a variety of applications including quantum computing, precision measurements, ultracold chemistry and quantum simulations. Heteronuclear bi-alkali molecules, only a small subset of polar molecules, have received special attention mainly because the constituent alkali atoms are easy to laser cool and can be relatively easily associated to form molecules at ultracold temperatures. Our choice, the LiRb molecule, is motivated by the relatively high dipole moment (4.1 Debye) of the LiRb molecule in its rovibronic ground state. In this thesis, we study the LiRb molecule using laser spectroscopy and report, for the first time, the production of ultracold LiRb molecules by photoassociation (PA). The LiRb molecule is the least studied among all bi-alkali molecules and the first spectroscopic measurements on hot vapor phase LiRb molecules were reported only in 2011. We describe these measurements and their significance in chapter 2, after a brief introduction to the field of ultracold polar molecules in chapter 1. In chapter 3, we discuss our apparatus for simultaneous cooling and trapping of Li and Rb atoms in a dual-species magneto-optical trap (MOT) and report the measurement of interspecies collision-induced losses. In chapter 4, we describe the production of ultracold LiRb molecules in excited electronic states by photoassociation (PA). We report the measurements of the C6 coefficients for the Li (2s 2S1/2) + Rb (5p 2P1/2) and the Li (2s 2S1/2) + Rb (5p 2P3/2) asymptotes. We find a molecule formation rate (PLiRb) as high as 3.5x107 s-1 and a PA rate coefficient (KPA) as high as 1.3x10-10 cm3/s, the highest among heteronuclear bi-alkali molecules. In chapter 5, we discuss results on two-photon PA and we conclude, in chapter 6, with a road roadmap of future experiments for the production and detection of ultracold LiRb molecules in their rovibronic ground state.
Integrating two chemically distinct atomic species into a miniaturized, high-flux cold-atom source is a key challenge for modern experiments with ultracold atomic mixtures and polar molecules. Here, we report a compact setup with in-series two-dimensional magneto-optical traps (2D MOTs) that provides high-flux cold lithium and rubidium atoms. Thanks to the efficient short-distance Zeeman slowing, the maximum 3D MOT loading rate of lithium atoms reaches a record value of 6.6 × 10 9 atoms/s at a moderate lithium-oven temperature of 372 degrees Celsius, which is 44 times higher than that without the Zeeman slowing light. The flux of rubidium is also as high as 2.3 × 10 9 atoms/s with the rubidium oven held at room temperature. Meanwhile, the entire vacuum-chamber system, including an ultra-high-vacuum science cell, is within a small volume of 65 × 55 × 70 cm 3 . Our work represents a substantial improvement over traditional bulky and complex dual-species cold-atom setups. It provides a good starting point for the fast production of a double-degenerate lithium-rubidium atomic mixture and large samples of ultracold lithium-rubidium ground-state molecules.
We demonstrate a two species effusive source and Zeeman slower for lithium 6 and rubidium 85. The fluxes produced by this slower allow for magneto-optical trap loading rates in excess of 10⁸ atoms per second for both species. A detailed model is developed to predict the emission properties of the effusive source along with the flux of cold atoms produced by the slower. Novel to this design is the mating of Zeeman slower magnetic field to the field produced by trapping coils which increases the effective length over which atoms are slowed. This allows for a smaller, more compact slower, without a sacrifice in performance. Details relating to the design and performance of the vacuum system and magnetic field producing coils are also covered. The apparatus can be easily adapted to operate with different atomic species making it well suited for ultracold atomic physics experiments studying mixtures or as starting point for the creation of hetero-nuclear molecules.
This thesis describes the design and construction of a new apparatus to laser cool and trap Yb and Cs atoms with the ultimate aim of creating ultracold molecules with both an electric and magnetic dipole moment, which are of great interest in a range of fields. The Yb and Cs atomic beams are first generated and overlapped in a dual species oven. A collimated atomic beam of the two species is formed by the use of an array of capillary tubes at the exit of the oven. At this stage the atoms have an average velocity of 311m/s or 264m/s for Yb and Cs respectively, therefore it is necessary to reduce their velocity before they can be trapped in a magneto-optical trap (MOT). This initial stage of slowing is carried out with a Zeeman slower. Due to their atomic properties, Yb and Cs atoms are unable to be slowed by the same magnetic field profile therefore they cannot be loaded simultaneously, however, the Zeeman slower can load the atoms into a MOT sequentially. Once their velocities have been reduced to approximately 48m/s (10m/s) the Cs (Yb) atoms can be captured in a MOT. The laser systems for cooling Yb and Cs are also presented. Laser cooling of Cs is achieved on the D2 transition at 852.3nm whereas Yb can either be cooled on the 1S0 to 1P1 or 1S0 to 3P1 transitions at 398.9nm or 555.8nm respectively. Due to the relative linewidths of these transitions (2π×28.0MHz and 2π × 182.2kHz), Zeeman slowing is performed for Yb on the 1S0 to 1P1 transition and magneto-optical trapping on the 1S0 to 3P1 transition. A Cs MOT with 1 × 10^8 atoms and an Yb MOT with the order of 10^8 atoms on the narrow 556nm transition are demonstrated. The mixture can be sequentially loaded into a dual species MOT, paving the way for many further experiments exploring this novel mixture.