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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

All-optical production of a doubly degenerate Fermi-Fermi mixture

F. M. Spiegelhalder, A. Trenkwalder, D. Naik, E. Wille, G. Hendl, Florian Schreck, Rudolf Grimm

Published Jun 1, 2009• 0 citations
Publisher

Abstract

Ultracold Fermi-Fermi mixtures offer intriguing possibilities to study fermionic pairing phenomena and superfluidity in new regimes beyond the currently employed spin mixtures of a single species. We produce a doubly-degenerate mixture of6Li and40K atoms by evaporative cooling in an optical dipole trap. The trapping light is provided by a 100-W fiber laser operating near a wavelength of 1070nm.

DOI
10.1109/cleoe-eqec.2009.5192440

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Ultracold Fermi-Fermi mixtures offer intriguing possibilities to study fermionic pairing phenomena and superfluidity in new regimes beyond the currently employed spin mixtures of a single species. We produce a doubly-degenerate mixture of6Li and40K atoms by evaporative cooling in an optical dipole trap. The trapping light is provided by a 100-W fiber laser operating near a wavelength of 1070nm.
The study of low density, ultracold atomic Fermi gases is a promising avenue to understand fermion superfluidity from first principles. One technique currently used to bring Fermi gases in the degenerate regime is sympathetic cooling through a reservoir made of an ultracold Bose gas. We discuss a proposal for trapping and cooling of two-species Fermi-Bose mixtures into optical dipole traps made from combinations of laser beams having two different wavelengths. In these bichromatic traps it is possible, by a proper choice of the relative laser powers, to selectively trap the two species in such a way that fermions experience a stronger confinement than bosons. As a consequence, a deep Fermi degeneracy can be reached having at the same time a softer degenerate regime for the Bose gas. This leads to an increase in the sympathetic cooling efficiency and allows for higher precision thermometry of the Fermi - Bose mixture.
We report efficient production of quantum gases of 6 Li using a sub-Doppler cooling scheme based on the D 1 transition. After loading in a standard magneto-optical trap, an atomic sample of 10 9 atoms is cooled at a temperature of 40 μ K by a bichromatic D 1 gray-molasses. More than 2 × 10 7 atoms are then transferred into a high-intensity optical dipole trap, where a two-spin state mixture is evaporatively cooled down to quantum degeneracy. We observe that D 1 cooling remains effective in the deep trapping potential, allowing an effective increase of the atomic phase-space density before starting the evaporation. In a total experimental cycle of 11 s, we produce weakly-interacting degenerate Fermi gases of 7 × 10 5 atoms at T/T F < 0.1 and molecular Bose-Einstein condensates of up 5 × 10 5 molecules. We further describe a simple and compact optical system both for high-resolution imaging and for imprinting a thin optical barrier on the atomic cloud; this represents a first step towards the study of quantum tunneling in strongly interacting superfluid Fermi gases.
We present our experimental setup to produce ultracold strongly correlated fermionic superfluids made of a two-component spin-mixture of 6Li atoms. Employing standard cooling techniques, we achieve quantum degeneracy in a single-beam optical dipole trap. Our setup is capable of generating spin-balanced samples at temperatures as low as T/TF = 0.1 containing up to 5 × 10^4 atomic pairs. We can access different superfluid regimes by tuning the interparticle interactions close to a broad magnetic Feshbach resonance. In particular, we are able to explore the crossover from the molecular Bose-Einstein condensate (BEC) to the Bardeen-Cooper-Schrieffer (BCS) superfluid regimes. In the near future, we plan to study different collective excitations in these superfluid samples.
We report the experimental production of degenerate Fermi gases of 6Li atoms in an optical dipole trap. The gray-molasses technique is carried out to decrease the atomic temperature to 57 μK, which facilitates the efficient loading of cold atoms into the optical dipole trap. The Fermi degeneracy is achieved by evaporative cooling of a two-spin mixture of 6Li atoms on the Feshbach resonance. The degenerate atom number per spin is 3.5 × 104, and the reduced temperature T/T F is as low as 0.1, where T F is the Fermi temperature of the non-interacting Fermi gas. We also observe the anisotropic expansion of the atom cloud in the strongly interacting regime.
We propose the use of a combined optical dipole trap to achieve Fermi degeneracy by sympathetic cooling with a different bosonic species. Two far-detuned pairs of laser beams focused on the atomic clouds are used to confine the two atomic species with different trapping strengths. We show that a deep Fermi degeneracy regime can be potentially achieved earlier than Bose-Einstein condensation, as discussed in the favorable situation of a {sup 6}L i-{sup 23}Na mixture. This opens up the possibility of experimentally investigating a mixture of superfluid Fermi and normal Bose gases.
We report on the realization of an ultracold (<25 μK) mixture of rubidium (87Rb) and metastable triplet helium (4He) in an optical dipole trap. Our scheme involves laser cooling in a dual-species magneto-optical trap, simultaneous MW- and RF-induced forced evaporative cooling in a quadrupole magnetic trap, and transfer to a single-beam optical dipole trap. We observe long trapping lifetimes for the doubly spin-stretched spin-state mixture and measure much shorter lifetimes for other spin-state combinations. We discuss prospects for realizing quantum degenerate mixtures of alkali-metal and metastable helium atoms.