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arXiv:physics/0601086 · Physical Review A

Cooling of Sr to high phase-space density by laser and sympathetic cooling in isotopic mixtures

G. Ferrari, Robert E. Drullinger, N. Poli, Fiodor Sorrentino, Guglielmo Maria Tino

Published Feb 10, 2006• 36 citations
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Abstract

Based on an experimental study of two-body and three-body collisions in ultracold strontium samples, a novel optical-sympathetic cooling method in isotopic mixtures is demonstrated. Without evaporative cooling, a phase-space density of $6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$ is obtained with a high spatial density that should allow us to overcome the difficulties encountered so far to reach quantum degeneracy for Sr atoms.

DOI
10.1103/physreva.73.023408

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Based on an experimental study of two-body and three-body collisions in ultracold strontium samples, a novel optical-sympathetic cooling method in isotopic mixtures is demonstrated. Without evaporative cooling, a phase-space density of 6\ifmmode×\else\texttimes\fi10\ensuremath−26\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}6\ifmmode×\else\texttimes\fi10\ensuremath−2 is obtained with a high spatial density that should allow us to overcome the difficulties encountered so far to reach quantum degeneracy for Sr atoms.
We report on the realization of quantum degenerate gas mixtures of the alkaline-earth-metal element strontium with the alkali-metal element rubidium. A key ingredient of our scheme is sympathetic cooling of Rb by Sr atoms that are continuously laser cooled on a narrow-linewidth transition. This versatile technique allows us to produce ultracold gas mixtures with a phase-space density of up to 0.06 for both elements. By further evaporative cooling we create double Bose-Einstein condensates of 87{}^{87}87Rb with either 88{}^{88}88Sr or 84{}^{84}84Sr, reaching more than $1\ifmmode\times\else\texttimes\fi{}{10}^{5}$ condensed atoms per element for the 84{}^{84}84Sr-87{}^{87}87Rb mixture. These quantum gas mixtures constitute an important step towards the production of a quantum gas of polar, open-shell RbSr molecules.
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.
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.
Employing a far-off resonance optical-dipole trap (FORT), we attained a phase-space density exceeding 0.1, or an order to quantum degeneracy. Strontium atoms were magneto-optically cooled and trapped using the spin-forbidden ${}^{1}{S}_{0}{\ensuremath{-}}^{3}{P}_{1}$ transition and then compressed into a FORT that was designed to allow simultaneous Doppler cooling. We discussed that the phase-space density was finally limited by the light-assisted collisions occurring in the optical cooling.
Sympathetic cooling of two atomic isotopes is experimentally investigated. Using forced evaporation of a bosonic 7Li{}^{7}\mathrm{Li}7Li gas in a magnetic trap, a sample of $1.3\ifmmode\times\else\texttimes\fi{}{10}^{5}$ 6Li{}^{6}\mathrm{Li}6Li fermions has been sympathetically cooled to $9(3) \ensuremath{\mu}\mathrm{K},$ corresponding to 2.2(0.8)2.2(0.8)2.2(0.8) times the Fermi temperature. The measured rate constant for two-body inelastic collisions of the 7Li{}^{7}\mathrm{Li}7Li ∣2,2〉|2,2〉∣2,2〉 state at low magnetic field is ${1.0}_{\ensuremath{-}0.5}^{+0.8}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14} {\mathrm{cm}}^{3}{\mathrm{s}}^{\ensuremath{-}1}.$
We report Bose-Einstein condensation of 88Sr^{88}\mathrm{Sr}88Sr, which has a small, negative sss-wave scattering length (${a}_{88}=\ensuremath{-}2{a}_{0}$). We overcome the poor evaporative cooling characteristics of this isotope by sympathetic cooling with 87Sr^{87}\mathrm{Sr}87Sr atoms. 87Sr^{87}\mathrm{Sr}87Sr is effective in this role despite the fact that it is a fermion because of the large ground-state degeneracy arising from a nuclear spin of I=9/2I=9/2I=9/2, which reduces the impact of Pauli blocking of collisions. We observe a limited number of atoms in the condensate (${N}_{\mathrm{max}}\ensuremath{\approx}{10}^{4}$) that is consistent with the value of a88{a}_{88}a88​ and the optical dipole trap parameters.