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arXiv:1808.06836 · Physical Review A

Comparison of an efficient implementation of gray molasses to narrow-line cooling for the all-optical production of a lithium quantum gas

Christine Satter, Shina Tan, Kai Dieckmann

Published Aug 24, 2018• 10 citations
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Abstract

We present an efficient scheme to implement a gray optical molasses for sub-Doppler cooling of $^{6}\mathrm{Li}$ atoms with minimum experimental overhead. To integrate the ${D}_{1}$ light for the gray molasses cooling into the same optical setup that is used for the ${D}_{2}$ light for a standard magneto-optical trap (MOT), we rapidly switch the injection seeding of a slave laser between the ${D}_{2}$ and the ${D}_{1}$ 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.

DOI
10.1103/physreva.98.023422

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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 \ensuremathμs\ensuremath{\mu}\mathrm{s}\ensuremathμs can be achieved, inferred from monitor optical beat signals. The resulting low-intensity molasses cools a sample of N=9\ifmmode×\else\texttimes\fi108N=9\ifmmode\times\else\texttimes\fi{}{10}^{8}N=9\ifmmode×\else\texttimes\fi108 atoms to about 60 \ensuremathμK\ensuremath{\mu}\mathrm{K}\ensuremathμK. A maximum phase-space density of \ensuremathρ=1.2\ifmmode×\else\texttimes\fi10\ensuremath−5\ensuremath{\rho}=1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\ensuremathρ=1.2\ifmmode×\else\texttimes\fi10\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×\else\texttimes\fi1053.3\ifmmode\times\else\texttimes\fi{}{10}^{5}3.3\ifmmode×\else\texttimes\fi105 atoms, while loading from the denser UV MOT yields 2.4\ifmmode×\else\texttimes\fi1062.4\ifmmode\times\else\texttimes\fi{}{10}^{6}2.4\ifmmode×\else\texttimes\fi106 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 have used the narrow $2{S}_{1/2}\ensuremath{\rightarrow}3{P}_{3/2}$ transition in the ultraviolet (uv) to laser cool and magneto-optically trap (MOT) 6{}^{6}6Li atoms. Laser cooling of lithium is usually performed on the $2{S}_{1/2}\ensuremath{\rightarrow}2{P}_{3/2}$ (D2) transition, and temperatures of $\ensuremath{\sim}$300 $\ensuremath{\mu}$K are typically achieved. The linewidth of the uv transition is seven times narrower than the D2 line, resulting in lower laser cooling temperatures. We demonstrate that a MOT operating on the uv transition reaches temperatures as low as 59 $\ensuremath{\mu}$K. Furthermore, we find that the light shift of the uv transition in an optical dipole trap at 1070 nm is small and blueshifted, facilitating efficient loading from the uv MOT. Evaporative cooling of a two spin-state mixture of 6{}^{6}6Li in the optical trap produces a quantum degenerate Fermi gas with $3\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms in a total cycle time of only 11 s.
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Sub-Doppler laser cooling of40K with Raman gray molasses on the D2{D}_{2}D2​ line
Gray molasses is a powerful tool for sub-Doppler laser cooling of atoms to low temperatures. For alkaline atoms, this technique is commonly implemented with cooling lasers which are blue-detuned from either the D 1 or D 2 line. Here we show that efficient gray molasses can be implemented on the D 2 line of 40 K with red -detuned lasers. We obtained temperatures of , which enables direct loading of atoms from a magneto-optical trap into an optical dipole trap. We support our findings by a one-dimensional model and three-dimensional numerical simulations of the optical Bloch equations which qualitatively reproduce the experimentally observed cooling effects.
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 $\ensuremath{\mu}\mathrm{K}$ 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 $\ifmmode\times\else\texttimes\fi{}$ 10510{}^{5}105 molecules and weakly interacting degenerate Fermi gases of $7\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms at T/TF<0.1T/{T}_{F}<0.1T/TF​<0.1 with a typical experimental duty cycle of 11 s.
We report on the realization of sub-Doppler laser cooling of sodium atoms in gray molasses using the D1{D}_{1}D1​ optical transition $(3s\phantom{\rule{0.16em}{0ex}}^{2}S_{1/2}\ensuremath{\rightarrow}3p\phantom{\rule{0.16em}{0ex}}^{2}P_{1/2})$ at 589.8 nm. The technique is applied to samples containing $3\ifmmode\times\else\texttimes\fi{}{10}^{9}$ atoms, previously cooled to $350\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$ in a magneto-optical trap, and it leads to temperatures as low as $9\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$ and phase-space densities in the range of ${10}^{\ensuremath{-}4}$. The capture efficiency of the gray molasses is larger than 2/3, and we observe no density-dependent heating for densities up to ${10}^{11}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$.
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.