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arXiv:1502.00280 · Chinese Physics Letters

All-Optical Production of Quantum Degeneracy and Molecular Bose-Einstein Condensation of 6 Li

Shu-Jin Deng, Peng-Peng Diao, Qian-Li Yu, Hai-Bin Wu

Published May 1, 2015• 13 citations
arXivHTMLPDF

Abstract

We achieve a highly degenerate and strongly interacting Fermi gas in a mixture of the two lowest hyperfine states of 6 Li by direct evaporative cooling in a high power crossed optical dipole trap. The trap is loaded from a large atom number magneto-optical trap realized by a laser system of 2.5-W intracavity-frequency-doubled light output at 671 nm. With this system, we also demonstrate the production of a molecular Bose-Einstein condensate (mBEC) of 6 Li 2 , and observe the anisotropic expansion of Fermi gases in the so-called BEC-Bardeen-Cooper-Schrieffer crossover regime.

DOI
10.1088/0256-307x/32/5/053401

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Physical Review Letters•Oct 1, 2014•0 citations
We achieve a highly degenerate and strongly interacting Fermi gas in a mixture of the two lowest hyperfine states of 6 Li by direct evaporative cooling in a high power crossed optical dipole trap. The trap is loaded from a large atom number magneto-optical trap realized by a laser system of 2.5-W intracavity-frequency-doubled light output at 671 nm. With this system, we also demonstrate the production of a molecular Bose-Einstein condensate (mBEC) of 6 Li 2 , and observe the anisotropic expansion of Fermi gases in the so-called BEC-Bardeen-Cooper-Schrieffer crossover regime.
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.
Abstract We report a new apparatus for the study of two-species quantum degenerate mixture of 41 K and 6 Li atoms. We develop and combine several advanced cooling techniques to achieve both a large atom number and high phase space density of the two-species atom clouds. Furthermore, we build a high efficiency two-species magnetic transport system to transfer atom clouds from the 3D magneto-optical-trap chamber to a full glass science chamber of an extreme high vacuum environment and good optical access. We perform a forced radio-frequency evaporative cooling for 41 K atoms while the 6 Li atoms are sympathetically cooled in an optically plugged magnetic trap. Finally, we achieve the simultaneous quantum degeneracy for the 41 K and 6 Li atoms. The Bose–Einstein condensate of 41 K has 1.4 × 10 5 atoms with a condensate fraction of about 62%, while the degenerate Fermi gas of 6 Li has a total atom number of 5.4 × 10 5 at 0.25 Fermi temperature.
We report on an efficient production scheme for a large quantum degenerate sample of fermionic lithium. The approach is based on our previous work on narrow-line $2{S}_{1/2}\ensuremath{\rightarrow}3{P}_{3/2}$ laser cooling resulting in a high phase-space density of up to $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. This allows utilizing a large-volume crossed optical dipole trap with a total power of 45W45\phantom{\rule{0.16em}{0ex}}\mathrm{W}45W, leading to high loading efficiency and $8\ifmmode\times\else\texttimes\fi{}{10}^{6}$ trapped atoms. The same optical trapping configuration is used for rapid adiabatic transport over a distance of 25cm25\phantom{\rule{0.16em}{0ex}}\mathrm{cm}25cm in 0.9s0.9\phantom{\rule{0.16em}{0ex}}\mathrm{s}0.9s, and subsequent evaporative cooling. With optimized evaporation we achieve a degenerate Fermi gas with $1.7\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms at a temperature of 60nK60\phantom{\rule{0.28em}{0ex}}\mathrm{nK}60nK, corresponding to T/TF=0.16(2)T/{T}_{\text{F}}=0.16\left(2\right)T/TF​=0.16(2). Furthermore, the performance is demonstrated by evaporation near a broad Feshbach resonance creating a molecular Bose-Einstein condensate of $3\ifmmode\times\else\texttimes\fi{}{10}^{5}$ lithium dimers.
We achieve degeneracy in a mixture of the two lowest hyperfine states of 6Li by direct evaporation in a CO2 laser trap, yielding the first all optically produced degenerate Fermi gas. More than 10(5) atoms are confined at temperatures below 4 microK at full trap depth, where the Fermi temperature for each state is 8 microK. This degenerate two-component mixture is ideal for exploring mechanisms of superconductivity ranging from Cooper pairing to Bose-Einstein condensation of strongly bound pairs.
We use a gray molasses operating on the D[subscript 1] atomic transition to produce degenerate quantum gases of [superscript 6]Li with a large number of atoms. This sub-Doppler cooling phase allows us to lower the initial temperature of 10[superscript 9] atoms from 500 to 40 μK in 2 ms. We observe that D[subscript 1] cooling remains effective into a high-intensity infrared dipole trap where two-state mixtures are evaporated to reach the degenerate regime. We produce molecular Bose-Einstein condensates of up to 5 × 10[superscript 5] molecules and weakly interacting degenerate Fermi gases of 7×10[superscript 5] atoms at T/T[subscript F] < 0.1 with a typical experimental duty cycle of 11 s.