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Experimental realization of two-isotope collision-assisted Zeeman cooling

Mathew Hamilton, Roberts, Jacob, Lundeen, Stephen, Gelfand, Martin, Bartels, Randy

Published Jan 3, 2007• 0 citations
Publisher

Abstract

The work presented in this thesis focuses on the demonstration and initial evaluation of a novel non-evaporative cooling method called collision-assisted Zeeman cooling. For this realization, an ultracold gas consisting of a mixture of 87Rb and 8Rb was used. Cooling was accomplished through interisotope inelastic spin-exchange collisions that converted kinetic energy into magnetic energy. Continual optical pumping spin polarized the 85Rb which ensured that only kinetic energy reducing collisions occurred and the scattered pump photons carried entropy out of the system. Thus, cooling of the ultracold gas can be achieved without requiring the loss of any atoms in order to do so. This represents a theoretical advantage over forced evaporative cooling, which is the current state-of-the-art cooling technique in most experiments. This thesis discusses the details of collision-assisted Zeeman cooling, as well as how the theory of the technique has been extended from cooling a single species to cooling with two species. There are many predicted advantages from using two rather than one species of atom in this type of cooling: greater flexibility in finding favorable spin-exchange collision rates, easier requirements on the magnetic fields that must be used, and an additional means to mitigate reabsorption (the primary limitation in many if not most non-evaporative cooling techniques). The experimental considerations needed to prepare a system that simultaneously trapped two isotopes to be able to perform collision-assisted Zeeman cooling are discussed. Because this cooling scheme is highly reliant on the initial conditions of the system, a focused experiment examining the loading of the optical trap with both isotopes of Rb was conducted and the results of that experiment are described here. The first experimental observations of spin-exchange collisions in an ultracold gas mixture of Rb are described as a part of this work. The experiments where collision-assisted Zeeman cooling were demonstrated are then described and evaluated. In this first implementation of the cooling technique the initial densities were too low and optical-pump-induced heating and loss too high for achieving the full predicted performance of the cooling technique. Through additional modeling, these limitations were understood and the necessary improvements for the next iteration of CAZ cooling experiments are laid out at the end of this work.

DOI
10.25675/3.021563

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•Jan 1, 2000•1 citation
The work presented in this thesis focuses on the demonstration and initial evaluation of a novel non-evaporative cooling method called collision-assisted Zeeman cooling. For this realization, an ultracold gas consisting of a mixture of 87Rb and 8Rb was used. Cooling was accomplished through interisotope inelastic spin-exchange collisions that converted kinetic energy into magnetic energy. Continual optical pumping spin polarized the 85Rb which ensured that only kinetic energy reducing collisions occurred and the scattered pump photons carried entropy out of the system. Thus, cooling of the ultracold gas can be achieved without requiring the loss of any atoms in order to do so. This represents a theoretical advantage over forced evaporative cooling, which is the current state-of-the-art cooling technique in most experiments. This thesis discusses the details of collision-assisted Zeeman cooling, as well as how the theory of the technique has been extended from cooling a single species to cooling with two species. There are many predicted advantages from using two rather than one species of atom in this type of cooling: greater flexibility in finding favorable spin-exchange collision rates, easier requirements on the magnetic fields that must be used, and an additional means to mitigate reabsorption (the primary limitation in many if not most non-evaporative cooling techniques). The experimental considerations needed to prepare a system that simultaneously trapped two isotopes to be able to perform collision-assisted Zeeman cooling are discussed. Because this cooling scheme is highly reliant on the initial conditions of the system, a focused experiment examining the loading of the optical trap with both isotopes of Rb was conducted and the results of that experiment are described here. The first experimental observations of spin-exchange collisions in an ultracold gas mixture of Rb are described as a part of this work. The experiments where collision-assisted Zeeman cooling were demonstrated are then described and evaluated. In this first implementation of the cooling technique the initial densities were too low and optical-pump-induced heating and loss too high for achieving the full predicted performance of the cooling technique. Through additional modeling, these limitations were understood and the necessary improvements for the next iteration of CAZ cooling experiments are laid out at the end of this work.
In this thesis I describe two non-evaporative cooling schemes for cooling Rb atoms. The first is a Sisyphus-like ultracold gas cooling scheme called Spatially-selecTive Optical Pumping (STOP) cooling. In principle, STOP cooling has wide applicability to both atoms and molecules. STOP cooling works by exploiting the fact that atoms or molecules in a confining potential can be optically pumped out of an otherwise dark state in a spatially-selective way. Selecting atoms or molecules for optical pumping out of a dark state in a region of high potential energy and then waiting a fixed time after the optical pumping allows for the creation of a group of high kinetic energy atoms or molecules moving in a known direction. These can then be slowed using external fields (such as the scattering force from a resonant laser beam) and optically pumped back into the dark state, cooling the gas and closing the cooling cycle. I present theoretical modeling of the STOP cooling technique, including predictions of achievable cooling rates. I have conducted an experimental study of the cooling technique for a single cooling cycle, observing one dimensional cooling rates in excess of 100 micro-K per second in an ultracold gas of 87Rb atoms. I will also comment on the prospects for improving the cooling performance beyond that presented in this work. The second cooling scheme I investigated is called Two-Isotope Collision Assisted Zeeman (2-CAZ) cooling. Through a combination of spin-exchange collisions in a magnetic field and optical pumping, it is possible to cool a gas of atoms without requiring the loss of atoms from the gas. I investigated 2-CAZ cooling using 85Rb and 87Rb. I was able to experimentally confirm that the measured 2-CAZ cooling rate agreed with a cooling rate predicted though a simple analytic model. As part of the measured cooling rate, I quantitatively characterized the heating rates associated with our actual implementation of this cooling technique and found hyperfine-changing collisions to be a significant limitation for the 85/87Rb gas mixture. Possible improvements to this experiment will be discussed as well as the prospects for improved cooling performance using an atom without hyperfine structure as the optically pumped atom.
Through a combination of spin-exchange collisions in a magnetic field and optical pumping, it is possible to cool a gas of atoms without requiring the loss of atoms from the gas. We investigated this technique, Collision Assisted Zeeman Cooling, using 85Rb and 87Rb. We experimentally confirmed that our measured two isotope CAZ cooling rate agreed with a cooling rate predicted though a simple analytic model. As part of the measured cooling rate, we quantitatively characterized the heating rates associated with our actual implementation of this cooling technique and found hyperfine-changing collisions to be a significant limitation for the 85/87Rb gas mixture. We comment on the prospects for improving the cooling performance beyond that presented in this work.
In this thesis, I study optical pumping as a powerful cooling tool for trapped ultra-cold atoms in a highly collisional regime. First application of optical pumping is a continuous loading scheme used to transfer atoms from a guided beam into a hybrid trap. Further, I introduce a Sisyphus cooling scheme based on radio-frequency transitions and optical pumping, operating simultaneously to the accumulation of atoms in the trap. The combined scheme of continuous loading and Sisyphus cooling is demonstrated for a large range of initial conditions of the guided atoms. Thereby, I show that collisional thermalization occurs in a steady-state for almost arbitrary initial conditions, provided that the first dissipative step is able to prevent the atom from leaving the trap during its first passage. On the one hand, this scheme could be applied to a wide range of atomic or molecular beams. On the other hand, phase-space density of 4*10^-4 is reached in a continuous operation mode with chromium atoms. In the second part, I investigate demagnetization cooling based on dipolar relaxation collisions driving the thermalization of the internal (spin) and the external (motional) degrees of freedom. In the case of a gas, one has the advantage that the spin degree of freedom can be cooled very efficiently using optical pumping. It is shown, that demagnetization cooling of a gas is more efficient than evaporation cooling in terms of phase-space density gain versus loss of atoms. This allows reaching a temperature of 6uK at a phase-space density of 0.03. It is observed, that both, continuous Sisyphus cooling and demagnetization cooling are limited by a density dependent loss mechanism. I present circumstantial evidence for excited-state collisions as the dominant limiting process. Finally, I discuss possible extensions to the current experimental procedures, possibly allowing reaching quantum degeneracy by optical means only.
The purpose of our collaboration is to study ultracold collisions in a gas sample of atomic lithium. The associated experimental context is a scheme for producing such a gas sample in conditions where Bose-Einstein condensation (BEC) should be observable in the case of the {sup 7}Li isotope or Fermi degenerate behavior for the {sup 6}Li isotope. The plan is to load 10{sup 5}-10{sup 9} Li atoms from a laser cooled atomic beam into a magnetic trap with a diameter of about 1 mm. The fundamental limitation to achieving high density will be collisions of ground state atoms. We are interested in the loss of atomic density in a magnetic trap due to two-body dipolar collisions and spin-exchange collisions. Lithium atoms offer important advantages relative to other alkali atoms from the theoretical point of view. In the first place there is the greater accuracy of the available long-range potential parameters. Secondly, one avoids a complication of the heavier alkalis connected with the unknown spin-spin term following in second order from the electronic spin-orbit couplings In this contribution we present preliminary results for the above loss-mechanisms.
This thesis describes trapping and evaporative cooling of ultracold 87Sr and 88Sr mixtures in an optical dipole trap to produce the first Bose-Einstein condensate (BEC) of 88Sr. Furthermore, this work presents thermalization studies that characterize the scattering properties of these ultracold strontium samples. Such ultracold atomic gases have become an important area of research because of their potential for improving optical frequency standards and for realizing quantum computation using neutral atoms. A BEC of 88Sr is particularly interesting because the small value of its background s-wave scattering length may enable the use of optical Feshbach resonances to create two-dimensional solitons. However, the small scattering length for 88Sr also hinders efficient evaporative cooling in the optical dipole trap, a necessary step to producing a BEC. Experiments with other ultracold gases have successfully overcome this hurdle by mixing in a second atomic species which, by introducing stronger interactions with the weakly interacting species, enables evaporation to colder temperatures via sympathetic cooling. For this work, we use 87Sr to sympathetically cool 88Sr during forced evaporation to quantum degeneracy.\nPrevious experiments in the Killian Lab characterized 88Sr in detail. Here, I emphasize the new or improved aspects that have allowed trapping and cooling of the mixtures of 87Sr and 88 Sr: trapping of 87Sr by itself, spectroscopic measurements of all the stable strontium isotopes to guide the trapping of isotopic mixtures, imaging of both 87Sr and 88Sr, and the various trade-offs necessary to simultaneously trap 87Sr and 88Sr. Finally, I discuss how the thermalization studies of the scattering properties of the isotopes guide the forced evaporation of mixed isotope samples. These efforts result in the production of the BEC of 88Sr, but they also point the way to future studies of fermionic quantum degeneracy in 87Sr and to the rich physics of mixed species ultracold atomic systems.
A cold atomic gas formed by two different species represents an intriguing system for a deeper understanding of atom-atom interactions at ultralow temperatures. We present experiments on a mixture of atomic lithium and cesium which are of particular interest regarding the formation of heteronuclear molecules on the one hand, and the prospects for sympathetic cooling of atomic gases through mutual thermalization on the other hand. A first series of experiments on interaction in presence of a near-resonant light field is performed in a two-species magneto-optical trap. The collisional properties of the lithium-cesium mixture are investigated through detailed analysis of trap-loss processes induced by the trap light. Photoassociation in an additional near-resonant laser field yields high-resolution spectra of the excited Cs_2 dimers, but shows no unambiguous indication of LiCs molecule formation. A second series of experiments on pure ground-state collisional properties utilizes an optical dipole trap formed by light that is detuned extremely far below atomic resonance (quasi-electrostatic trap). Storage times of many minutes are achieved in a particularly simple and versatile setup for both atomic species. Cooling of cesium through evaporation and thermalization by elastic collisions is observed. The evolution of temperature and particle number is compared with model simulations of evaporative cooling. Direct laser cooling of trapped cesium in the absolute energetic ground state is demonstrated. Homonuclear spin-changing collisions of ground-state cesium and lithium atoms are analyzed, and first evidence for pure ground-state collisions between atoms of different species is found. Based on the current achievements, prospects for future experiments are discussed. (orig.)