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New Journal of Physics

Species-selective microwave cooling of a mixture of rubidium and caesium atoms

M Haas, Vanessa Y. F. Leung, Daniel Frese, D. Haubrich, Shincy John, Claudia Weber, Arno Rauschenbeutel, Dieter Meschede

Published May 23, 2007• 30 citations
Publisher

Abstract

We have sympathetically cooled a small sample of 133 Cs atoms with 87 Rb to below 1 μK. Evaporative cooling was realized with microwave radiation driving the Rb ground-state hyperfine transition. By analysing the sympathetic cooling dynamics, we derive a lower limit of the modulus of the Rb–Cs interspecies triplet s -wave scattering length of 200 a 0 . For temperatures below 5 μK we observe strong non-exponential losses of the Cs sample in the presence of the Rb sample.

DOI
10.1088/1367-2630/9/5/147

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We have sympathetically cooled a small sample of 133 Cs atoms with 87 Rb to below 1 μK. Evaporative cooling was realized with microwave radiation driving the Rb ground-state hyperfine transition. By analysing the sympathetic cooling dynamics, we derive a lower limit of the modulus of the Rb–Cs interspecies triplet s -wave scattering length of 200 a 0 . For temperatures below 5 μK we observe strong non-exponential losses of the Cs sample in the presence of the Rb sample.
Binary mixtures of ultracold atoms are of great interest in the research field of quantum optics and are studied by several groups aiming at different applications. This paper works with rubidium and caesium, which are simultaneously stored in a magnetic trap. Species-selective microwave cooling is used on the rubidium groundstate hyperfine transition. Caesium is sympathetically cooled via elastic collisions with rubidium. When cooling down the mixture to temperatures below 1 muK, below 4 muK we observe strong losses of caesium. Analysing the dynamics of sympathetic cooling, lower limit for the modulus of the rubidium-caesium triplet s-wave scattering length is estimated.
We report on measurements of the collisional properties of a mixture of 133Cs^{133}\mathrm{Cs}133Cs and 87Rb^{87}\mathrm{Rb}87Rb atoms in a magnetic trap at $\ensuremath{\mu}\mathrm{K}$ temperatures. By selectively evaporating the Rb atoms using a radio-frequency field, we achieved sympathetic cooling of Cs down to a few $\ensuremath{\mu}\mathrm{K}$. The interspecies collisional cross section was determined through rethermalization measurements, leading to good agreement with a theoretical prediction of 595a0595{a}_{0}595a0​ for the triplet sss-wave scattering length for Rb in the $\ensuremath{\mid}F=2,{m}_{F}=2⟩$ and Cs in the $\ensuremath{\mid}F=4,{m}_{F}=4⟩$ magnetic states. We briefly speculate on the prospects for reaching the Bose-Einstein condensation of Cs inside a magnetic trap through sympathetic cooling.
In our experiment, we study and intend to manipulate the interaction between the two species viz. rubidium (Rb) and caesium (Cs) at ultracold temperatures. We propose to explore the world of many body physics and small particle physics by inserting a few Cs atoms into a Rb condensate. Our experimental set up consists of a double MOT system, with a magnetic trap in the QUIC configuration. The Rb atoms are cooled evaporatively using a species selective microwave frequency such that Cs remains unaffected due to a 2.4 Ghz difference in their hyperfine transitions, producing a Rb condensate with around 105 atoms.
We investigate sympathetic cooling fermions 40 K by evaporatively cooling bosonic 87 Rb atoms in a magnetic trap with microwave and radio frequency induced evaporations in detail. The mixture of bosonic and fermionic atoms is prepared in their polarized spin states | F = 9/2, m F = 9/2〉 for 40 K and | F = 2, m F = 2〉 for 87 Rb, which is trapped in Quadrupole—Ioffe—Configuration trap. Comparing microwave with radio frequency evaporatively cooling bosonic 87 Rb atoms with sympathetically cooling Fermi gas 40 K, we find that the presence of rubidium atoms in the |2, 1〉 Zeeman states, which are generated in the evaporative process, gives rise to a significant loss of 40 K due to inelastic collisions. Thus, the rubidium atoms populated in the |2, 1〉 Zeeman states should be removed in order to effectively perform sympathetically cooling 40 K with the evaporatively cooled 87 Rb atoms.
We model sympathetic cooling of ground-state CaF molecules by ultracold Li and Rb atoms. The molecules are moving in a microwave trap, while the atoms are trapped magnetically. We calculate the differential elastic cross sections for CaF-Li and CaF-Rb collisions, using model Lennard-Jones potentials adjusted to give typical values for the sss-wave scattering length. Together with trajectory calculations, these differential cross sections are used to simulate the cooling of the molecules, the heating of the atoms, and the loss of atoms from the trap. We show that a hard-sphere collision model based on an energy-dependent momentum transport cross section accurately predicts the molecule cooling rate but underestimates the rates of atom heating and loss. Our simulations suggest that Rb is a more effective coolant than Li for ground-state molecules, and that the cooling dynamics is less sensitive to the exact value of the sss-wave scattering length when Rb is used. Using realistic experimental parameters, we find that molecules can be sympathetically cooled to $100\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$ in about 10 s. By applying evaporative cooling to the atoms, the cooling rate can be increased and the final temperature of the molecules can be reduced to $1 \ensuremath{\mu}\mathrm{K}$ within 30 s.