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arXiv:1406.4788 · Physical Review A

Efficient all-optical production of large Li6 quantum gases using D1 gray-molasses cooling

A. Burchianti, Giacomo Valtolina, Jorge Amin Seman, E. Pace, Marco De Pas, M. Inguscio, Matteo Zaccanti, G. Roati

Published Oct 8, 2014• 101 citations
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Abstract

We use a gray molasses operating on the ${D}_{1}$ atomic transition to produce degenerate quantum gases of $^{6}\mathrm{Li}$ with a large number of atoms. This sub-Doppler cooling phase allows us to lower the initial temperature of $10{}^{9}$ atoms from 500 to 40 $\ensuremath{\mu}\mathrm{K}$ in 2 ms. We observe that ${D}_{1}$ 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 $\ifmmode\times\else\texttimes\fi{}$ $10{}^{5}$ molecules and weakly interacting degenerate Fermi gases of $7\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms at $T/{T}_{F}<0.1$ with a typical experimental duty cycle of 11 s.

DOI
10.1103/physreva.90.043408

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We use a gray molasses operating on the D1{D}_{1}D1​ atomic transition to produce degenerate quantum gases of 6Li^{6}\mathrm{Li}6Li with a large number of atoms. This sub-Doppler cooling phase allows us to lower the initial temperature of 10910{}^{9}109 atoms from 500 to 40 \ensuremathμK\ensuremath{\mu}\mathrm{K}\ensuremathμK in 2 ms. We observe that D1{D}_{1}D1​ 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 \ifmmode×\else\texttimes\fi\ifmmode\times\else\texttimes\fi{}\ifmmode×\else\texttimes\fi 10510{}^{5}105 molecules and weakly interacting degenerate Fermi gases of 7\ifmmode×\else\texttimes\fi1057\ifmmode\times\else\texttimes\fi{}{10}^{5}7\ifmmode×\else\texttimes\fi105 atoms at T/TF<0.1T/{T}_{F}<0.1T/TF​<0.1 with a typical experimental duty cycle of 11 s.
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.
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 demonstrate the production of large 7Li^{7}\mathrm{Li}7Li Bose-Einstein condensates in an optical dipole trap using D1{D}_{1}D1​ gray molasses. The sub-Doppler cooling technique reduces the temperature of $4\ifmmode\times\else\texttimes\fi{}{10}^{9}$ atoms to $25\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$ in 3 ms. After microwave evaporation cooling in a magnetic quadrupole trap, we transfer the atoms to a crossed optical dipole trap, where we employ a magnetic Feshbach resonance on the $|F=1,{m}_{F}=1\ensuremath{\rangle}$ state. Fast evaporation cooling is achieved by tilting the optical potential using a magnetic field gradient on the top of the Feshbach field. Our setup produces pure condensates with $2.7\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms in the optical potential for every 11 s. The trap tilt evaporation allows rapid thermal quench, and spontaneous vortices are observed in the condensates as a result of the Kibble-Zurek mechanism.
We report on the simultaneous sub-Doppler laser cooling of 6Li^{6}\mathrm{Li}6Li and 7Li^{7}\mathrm{Li}7Li isotopes using gray molasses operating on their respective D1{D}_{1}D1​ atomic transitions. For 7Li^{7}\mathrm{Li}7Li we show that the sub-Doppler cooling can be achieved with two distinct $\mathrm{\ensuremath{\Lambda}}$-type transitions, where the upper level can be either of the two 22P1/2{2}^{2}\phantom{\rule{0.16em}{0ex}}{P}_{1/2}22P1/2​ hyperfine states. We obtain temperatures of $\ensuremath{\sim}85\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{K}$, with atom numbers of $\ensuremath{\sim}{10}^{8}$, and phase-space densities in the range of ${10}^{\ensuremath{-}6}\ensuremath{-}{10}^{\ensuremath{-}5}$ for both isotopes. These conditions provide a good starting point for loading the mixture into an optical dipole trap and performing evaporative cooling to quantum degeneracy. Our work provides a valuable simplification for the preparation of ultracold $^{6}\mathrm{Li}\ensuremath{-}^{7}\mathrm{Li}$ mixtures, which were proven to be a successful system for the study of impurity physics and Bose-Fermi superfluids.
We present an efficient scheme to implement a gray optical molasses for sub-Doppler cooling of 6Li^{6}\mathrm{Li}6Li atoms with minimum experimental overhead. To integrate the D1{D}_{1}D1​ light for the gray molasses cooling into the same optical setup that is used for the D2{D}_{2}D2​ light for a standard magneto-optical trap (MOT), we rapidly switch the injection seeding of a slave laser between the D2{D}_{2}D2​ and the D1{D}_{1}D1​ light sources. Switching times as short as 30 $\ensuremath{\mu}\mathrm{s}$ can be achieved, inferred from monitor optical beat signals. The resulting low-intensity molasses cools a sample of $N=9\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms to about 60 $\ensuremath{\mu}\mathrm{K}$. A maximum phase-space density of $\ensuremath{\rho}=1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ is observed. On the same setup, the performance of the GM is compared to that of narrow-line cooling in an ultraviolet (UV) MOT, following the procedure in Sebastian et al. [J. Sebastian, Ch. Gross, K. Li, H. C. J. Gan, W. Li, and K. Dieckmann, Phys. Rev. A 90, 033417 (2014)]. Further, we compare the production of a degenerate Fermi gas using both methods. Loading an optical dipole trap from the gray molasses yields a quantum degenerate sample with $3.3\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms, while loading from the denser UV MOT yields $2.4\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms. Where the highest atom numbers are not a priority this implementation of the gray molasses technique yields sufficiently large samples for a comparatively low technical effort.
We use D1{D}_{1}D1​ gray molasses to achieve Bose-Einstein condensation of a large number of 41K^{41}\mathrm{K}41K atoms in an optical dipole trap. By combining a specific configuration of a compressed magneto-optical trap with D1{D}_{1}D1​ gray molasses, we obtain a cold sample of $2.4\ifmmode\times\else\texttimes\fi{}{10}^{9}$ atoms with a temperature as low as 42 $\ensuremath{\mu}\mathrm{K}$. After magnetically transferring the atoms into the final glass cell, we perform a two-stage evaporative cooling. A condensate with up to $1.2\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms in the lowest Zeeman state $|F=1,{m}_{F}=1\ensuremath{\rangle}$ is achieved in the optical dipole trap. Furthermore, we observe two narrow Feshbach resonances in the lowest hyperfine channel, which are in good agreement with theoretical predictions.