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arXiv:0910.0634 · Physical Review Letters

Bose-Einstein Condensation of Strontium

Simon Stellmer, Meng Khoon Tey, Bo Huang, Rudolf Grimm, Florian Schreck

Published Nov 9, 2009• 225 citations
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Abstract

We report on the attainment of Bose-Einstein condensation with ultracold strontium atoms. We use the (84)Sr isotope, which has a low natural abundance but offers excellent scattering properties for evaporative cooling. Accumulation in a metastable state using a magnetic-trap, narrowline cooling, and straightforward evaporative cooling in an optical trap lead to pure condensates containing 1.5 x 10(5) atoms. This puts (84)Sr in a prime position for future experiments on quantum-degenerate gases involving atomic two-electron systems.

DOI
10.1103/physrevlett.103.200401

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Simon Stellmer, Florian Schreck, T. C. Killian

Degenerate quantum gases of alkaline-earth-like elements open new opportunities in research areas ranging from molecular physics to the study of strongly correlated systems. These experiments exploit the rich electronic structure of these elements, which is markedly different from the one of other species for which quantum degeneracy has been attained. Specifically, alkaline-earth-like atoms, such as strontium, feature metastable triplet states, narrow intercombination lines, and a non-magnetic, closed-shell ground state. This review covers the creation of quantum degenerate gases of strontium and the first experiments performed with this new system. It focuses on laser-cooling and evaporation schemes, which enable the creation of Bose-Einstein condensates and degenerate Fermi gases of all strontium isotopes, and shows how they are used for the investigation of optical Feshbach resonances, the study of degenerate gases loaded into an optical lattice, as well as the coherent creation of Sr_2 molecules.

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We report on the attainment of Bose-Einstein condensation with ultracold strontium atoms. We use the (84)Sr isotope, which has a low natural abundance but offers excellent scattering properties for evaporative cooling. Accumulation in a metastable state using a magnetic-trap, narrowline cooling, and straightforward evaporative cooling in an optical trap lead to pure condensates containing 1.5 x 10(5) atoms. This puts (84)Sr in a prime position for future experiments on quantum-degenerate gases involving atomic two-electron systems.
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 describe the construction and characterization of a new apparatus that can produce degenerate quantum gases of strontium. The realization of degenerate gases is an important first step toward future studies of quantum magnetism. Three of the four stable isotopes of strontium have been cooled into the degenerate regime. The experiment can make nearly pure Bose-Einstein condensates containing approximately 1x10^4 atoms, for strontium-86, and approximately 4x10^5 atoms, for strontium-84. We have also created degenerate Fermi gases of strontium-87 with a reduced temperature, T/T_F of approximately 0.2. The apparatus will be able to produce Bose-Einstein condensates of strontium-88 with straightforward modifications. We also report the first experimental and theoretical results from the strontium project. We have developed a technique to accelerate the continuous loading of strontium atoms into a magnetic trap. By applying a laser addressing the 3P1 to 3S1 transition in our magneto-optical trap, the rate at which atoms populate the magnetically-trapped 3P2 state can be increased by up to 65%. Quantum degenerate gases of atoms in the metastable 3P0 and 3P2 states are a promising platform for quantum simulation of systems with long-range interactions. We have performed an initial numerical study of a method to transfer the ground state degenerate gases that we can currently produce into one of the metastable states via a three-photon transition. Numerical simulations of the Optical Bloch equations governing the three-photon transition indicate that >90% of a ground state degenerate gas can be transferred into a metastable state.
We report on the attainment of Bose-Einstein condensation of 86Sr^{86}\mathrm{Sr}86Sr. This isotope has a scattering length of about +800a0+800{a}_{0}+800a0​ and thus suffers from fast three-body losses. To avoid detrimental atom loss, evaporative cooling is performed at low densities around $3\ifmmode\times\else\texttimes\fi{}{10}^{12}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ in a large volume optical dipole trap. We obtain almost pure condensates of $5\ifmmode\times\else\texttimes\fi{}{10}^{3}$ atoms.
We describe the operation and performance of an ultracold strontium apparatus that is capable of generating quantum degenerate gases. The experiment has produced Bose-Einstein condensates (BECs) of 84Sr and 86Sr as well as degenerate Fermi gases (DFGs) of 87Sr with a reduced temperature of T/TF = 0.2 at a Fermi temperature of TF = 55 nK. Straightforward modifications could be made to allow for isotopic mixtures and BECs of the fourth stable isotope, 88Sr. We also report on a technique to improve the continuous loading of a magnetic trap by adding a laser tuned to the 3P1 - 3S1 transition. The method increases atom number in the magnetic trap and subsequent cooling stages by up to 65% for the bosonic isotopes and up to 30% for the fermionic isotope of strontium. We optimize this trap loading strategy with respect to laser detuning, intensity, and beam size. To understand the results, we develop a one-dimensional rate equation model of the system, which is in good agreement with the data. We discuss the use of other transitions in strontium for accelerated trap loading and the application of the technique to other alkaline-earth-like atoms. Finally, we also report on an updated investigation of photoassociation resonances relative to the 1S0 + 3P1 dissassociation limit in bosonic strontium. Multiple new resonances for 84Sr and 86Sr were measured out to binding energies of -5 GHz and several discrepancies in earlier measurements were resolved. These measurements will allow for the development of a more accurate mass-scaled model and a better theoretical understanding of the molecular potentials near the 3P1 state. We also measure the strength of the 84Sr 0u transitions in order to characterize their use as optical Feshbach resonances.
Degenerate quantum gases of alkaline-earth-like elements open new opportunities in research areas ranging from molecular physics to the study of strongly correlated systems. These experiments exploit the rich electronic structure of these elements, which is markedly different from the one of other species for which quantum degeneracy has been attained. Specifically, alkaline-earth-like atoms, such as strontium, feature metastable triplet states, narrow intercombination lines, and a non-magnetic, closed-shell ground state. This review covers the creation of quantum degenerate gases of strontium and the first experiments performed with this new system. It focuses on laser-cooling and evaporation schemes, which enable the creation of Bose-Einstein condensates and degenerate Fermi gases of all strontium isotopes, and shows how they are used for the investigation of optical Feshbach resonances, the study of degenerate gases loaded into an optical lattice, as well as the coherent creation of Sr_2 molecules.