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Journal of the Optical Society of America B

Cooling and trapping of atomic strontium

Xinye Xu, Thomas H. Loftus, J. L. Hall, Alan C. Gallagher, Jun Ye

Published May 1, 2003• 138 citations
Publisher

Abstract

We present a detailed investigation of strontium magneto-optical trap (MOT) dynamics. Relevant physical quantities in the trap, such as temperature, atom number and density, and loss channels and lifetime, are explored with respect to various trap parameters. By studying the oscillatory response of a two-level 1S0–1P1 88Sr MOT, we firmly establish the laser cooling dynamics predicted by Doppler theory. Measurements of the MOT temperature, however, deviate severely from Doppler theory predictions, implying significant additional heating mechanisms. To explore the feasibility of attaining quantum degenerate alkaline-earth samples via evaporative cooling, we also present the first experimental demonstration of magnetically trapped metastable 88Sr. Furthermore, motivated by the goal of establishing the fermionic isotope 87Sr as one of the highest-quality, neutral-atom-based optical frequency standards, we present a preliminary study of sub-Doppler cooling in a 87Sr MOT. A dual-isotope (87Sr and 88Sr) MOT is also demonstrated.

DOI
10.1364/josab.20.000968

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Investigation of sub-Doppler cooling in an ytterbium magneto-optical trap

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We report experimental evidence of Sisyphus cooling in atoms with a ${}^{1}{S}_{0}$ ground state. Since $J=0,$ any cooling mechanism which requires multiple sublevels can only occur in the isotopes which have nuclear spin $I\ensuremath{\ne}0.$ Ytterbium has seven stable isotopes and offers a unique system in which we can study cooling on $F=\stackrel{\ensuremath{\rightarrow}}{0}1$ ${(}^{168,170,172,174,176}\mathrm{Yb}),$ $F=1/\stackrel{\ensuremath{\rightarrow}}{2}3/2$ ${(}^{171}\mathrm{Yb}),$ and $F=5/\stackrel{\ensuremath{\rightarrow}}{2}7/2$ ${(}^{173}\mathrm{Yb}),$ depending on the selection of isotope. We have trapped each of the seven stable isotopes of ytterbium in a magneto-optical trap (MOT) using the strong ${}^{1}{S}_{0}{\ensuremath{-}}^{1}{P}_{1}$ transition, and transferred them into a second MOT which uses the much narrower ${}^{1}{S}_{0}{\ensuremath{-}}^{3}{P}_{1}$ intercombination transition. We have measured the temperature of isotopes ${}^{171}\mathrm{Yb},$ ${}^{173}\mathrm{Yb},$ and ${}^{174}\mathrm{Yb}$ in the ${}^{1}{S}_{0}{\ensuremath{-}}^{3}{P}_{1}$ MOT, as a function of the intensity and detuning of the trapping laser. The temperature of ${}^{174}\mathrm{Yb}$ was found to increase more rapidly with intensity than predicted by Doppler cooling theory, in agreement with earlier work on alkaline-earth atoms. In the odd isotopes the temperature was found to decrease with increasing angular momentum, as observed in earlier experiments and three-dimensional simulations.

Physical Review A•Jul 25, 2003•75 citations
We present a detailed investigation of strontium magneto-optical trap (MOT) dynamics. Relevant physical quantities in the trap, such as temperature, atom number and density, and loss channels and lifetime, are explored with respect to various trap parameters. By studying the oscillatory response of a two-level 1S0–1P1 88Sr MOT, we firmly establish the laser cooling dynamics predicted by Doppler theory. Measurements of the MOT temperature, however, deviate severely from Doppler theory predictions, implying significant additional heating mechanisms. To explore the feasibility of attaining quantum degenerate alkaline-earth samples via evaporative cooling, we also present the first experimental demonstration of magnetically trapped metastable 88Sr. Furthermore, motivated by the goal of establishing the fermionic isotope 87Sr as one of the highest-quality, neutral-atom-based optical frequency standards, we present a preliminary study of sub-Doppler cooling in a 87Sr MOT. A dual-isotope (87Sr and 88Sr) MOT is also demonstrated.
Alkaline-earth-metal atoms present an ideal platform for exploring magneto-optic trap (MOT) dynamics, enabling unique and definitive tests of laser cooling and trapping mechanisms. We have measured the trapping beam intensity, detuning, magnetic-field gradient, trap density, and lifetime dependence of the spring constant $\ensuremath{\kappa}$ and damping coefficient $\ensuremath{\alpha}$ for a ${}^{1}{S}_{0}{\ensuremath{-}}^{1}{P}_{1}$ 88Sr{}^{88}\mathrm{Sr}88Sr MOT by fitting the oscillatory response of the atom cloud to a step-function force. We find that the observed behavior of $\ensuremath{\kappa}$ and $\ensuremath{\alpha}$ provide a unified and consistent picture of trap dynamics that agrees with Doppler cooling theory at the level of 1010%.10 Additionally, we demonstrate that the trapped atom temperature can be determined directly from measured value of $\ensuremath{\kappa}$ and the trap size, in excellent agreement with free-expansion temperature measurements. However, the experimentally determined temperature is much higher than Doppler cooling theory, implying significant additional heating mechanisms.
Summary form only given. The experiment consists of a Sr vapor cell magneto optical trap (MOT). Since for laser-cooled alkaline earth atoms the even isotopes have no hyperfine structure, they provide an ideal system for testing Doppler-cooling theory. (Alkali atoms are subject to sub-Doppler cooling mechanisms.) We have measured the spring constant (k) and damping coefficient (/spl alpha/) in a strontium MOT by observing the damped oscillation response of the trapped atom location to a square-wave chopped pushing beam.
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.
We report on a narrow-line laser cooling and trapping of strontium atoms near quantum degeneracy. Employing a magneto-optical trap (MOT) on the spin-forbidden transition 1S0−3P1 at 689 nm, we have laser-cooled an atomic sample down to the photon recoil temperature of 400 nK with a phase space density of 10−2. The atoms were then compressed into a new type of far-off resonance optical dipole trap that was designed to allow simultaneous Doppler cooling, resulting in a phase space density of 10% to that required for quantum degeneracy. It is shown that this value is finally limited by light-assisted collisions occurring in the optical cooling. To reduce these inelastic losses, we applied evaporative cooling and demonstrated a creation of two-dimensional atomic gases.
ABSTRACT In a ceramic vapor cell we have created a robust Sr magneto-optical trap that stores about 108 atoms with lifetimes>200 ms. We eliminate the 5p 1P1—4d 1D2—*5p 3P2 leak and achieve a 10-fold improvement in trap lifetime byrepumping the 5p 3Po,2 dark states with 679 nm and 707 nm light. The observed lifetime is now limited by coldcollision losses, and we have preliminary measurements of the 2-body loss rate. Direct readout of the trap velocity distribution is possible using the narrow (7 kHz) 52 'S0----5p 3P1 intercombination line at 689 nm. We can also cool with this narrow transition and have achieved a 40-fold 1-D velocity compression for about 5% of the trapped atoms by applying this second-stage cooling. Keywords: Laser cooling, laser trapping, alkaline earths, cold collisions, frequency metrology 1. INTRODUCTION Laser cooling and trapping techniques have advanced to become an important tool of the atomic physics community, and in recent years there has been considerable interest in expanding the range of trapped elements. The alkalineearths, in particular, possess unique characteristics which make them interesting to trap. Studies of long-rangeatom-atom interactions are simplified because they have no hyperfine structure, and their narrow intercombinationtransitions make them excellent candidates for optical frequency standards. Furthermore, Doppler cooling with thesenarrow transitions can reduce temperatures to near the recoil limit.The most abundant alkaline earth isotopes have I =
We report experimental evidence of Sisyphus cooling in atoms with a 1S0{}^{1}{S}_{0}1S0​ ground state. Since J=0,J=0,J=0, any cooling mechanism which requires multiple sublevels can only occur in the isotopes which have nuclear spin $I\ensuremath{\ne}0.$ Ytterbium has seven stable isotopes and offers a unique system in which we can study cooling on $F=\stackrel{\ensuremath{\rightarrow}}{0}1$ (168,170,172,174,176Yb),{(}^{168,170,172,174,176}\mathrm{Yb}),(168,170,172,174,176Yb), $F=1/\stackrel{\ensuremath{\rightarrow}}{2}3/2$ (171Yb),{(}^{171}\mathrm{Yb}),(171Yb), and $F=5/\stackrel{\ensuremath{\rightarrow}}{2}7/2$ (173Yb),{(}^{173}\mathrm{Yb}),(173Yb), depending on the selection of isotope. We have trapped each of the seven stable isotopes of ytterbium in a magneto-optical trap (MOT) using the strong ${}^{1}{S}_{0}{\ensuremath{-}}^{1}{P}_{1}$ transition, and transferred them into a second MOT which uses the much narrower ${}^{1}{S}_{0}{\ensuremath{-}}^{3}{P}_{1}$ intercombination transition. We have measured the temperature of isotopes 171Yb,{}^{171}\mathrm{Yb},171Yb, 173Yb,{}^{173}\mathrm{Yb},173Yb, and 174Yb{}^{174}\mathrm{Yb}174Yb in the ${}^{1}{S}_{0}{\ensuremath{-}}^{3}{P}_{1}$ MOT, as a function of the intensity and detuning of the trapping laser. The temperature of 174Yb{}^{174}\mathrm{Yb}174Yb was found to increase more rapidly with intensity than predicted by Doppler cooling theory, in agreement with earlier work on alkaline-earth atoms. In the odd isotopes the temperature was found to decrease with increasing angular momentum, as observed in earlier experiments and three-dimensional simulations.