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Narrow line laser cooling of lithium: A new tool for all-optical production of a degenerate Fermi gas

Pedro Duarte

Published Jan 1, 2011• 2 citations
OpenAlex

Abstract

We have used the narrow 2 S 1/2 [arrow right] 3 P 3/2 transition in the ultraviolet (UV) to laser cool and magneto-optically trap (MOT) 6 Li atoms. Laser cooling of lithium atoms is usually performed on the 2 S 1/2 [arrow right] 2 P 3/2 (D2) transition, where temperatures of twice the Doppler limit, or ∼300 μ K for lithium, are achieved. The linewidth of the UV transition is seven times narrower than the D2 line, resulting in a lower Doppler limit. We show that a MOT operating on the UV transition reaches temperatures as low as 59 μ K. We load 6 million atoms from this UV MOT into a 1070 nm optical dipole trap (ODT). We show that the light shift of the UV transition in the ODT is small and blue-shifted, facilitating efficient loading. Evaporative cooling of a two spin-state mixture of 6 Li in the ODT produces a quantum degenerate Fermi gas with 3 million atoms in only 11 seconds.

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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) ${}^{6}$Li 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 ${}^{6}$Li 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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All-optical production and transport of a large Li6 quantum gas in a crossed optical dipole trap

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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 $45\phantom{\rule{0.16em}{0ex}}\mathrm{W}$, 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 $25\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$ in $0.9\phantom{\rule{0.16em}{0ex}}\mathrm{s}$, 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 $60\phantom{\rule{0.28em}{0ex}}\mathrm{nK}$, corresponding to $T/{T}_{\text{F}}=0.16\left(2\right)$. 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.

arXiv:1605.03298•Physical Review A•May 26, 2016•25 citations
We have used the narrow 2 S 1/2 [arrow right] 3 P 3/2 transition in the ultraviolet (UV) to laser cool and magneto-optically trap (MOT) 6 Li atoms. Laser cooling of lithium atoms is usually performed on the 2 S 1/2 [arrow right] 2 P 3/2 (D2) transition, where temperatures of twice the Doppler limit, or ∼300 μ K for lithium, are achieved. The linewidth of the UV transition is seven times narrower than the D2 line, resulting in a lower Doppler limit. We show that a MOT operating on the UV transition reaches temperatures as low as 59 μ K. We load 6 million atoms from this UV MOT into a 1070 nm optical dipole trap (ODT). We show that the light shift of the UV transition in the ODT is small and blue-shifted, facilitating efficient loading. Evaporative cooling of a two spin-state mixture of 6 Li in the ODT produces a quantum degenerate Fermi gas with 3 million atoms in only 11 seconds.
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.
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 report an experimental study of peak and phase-space density of a two-stage magneto-optical trap (MOT) of 6Li^{6}\mathrm{Li}6Li atoms, which exploits the narrower $2{S}_{1/2}\ensuremath{\rightarrow}3{P}_{3/2}$ ultraviolet (UV) transition at 323 nm following trapping and cooling on the more common D2D2D2 transition at 671 nm. The UV MOT is loaded from a red MOT and is compressed to give a high phase-space density up to $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. Temperatures as low as $33\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{K}$ are achieved on the UV transition. We study the density limiting factors and in particular find a value for the light-assisted collisional loss coefficient of $1.3\ifmmode\pm\else\textpm\fi{}0.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ cm3{\mathrm{cm}}^{3}cm3/s for low repumping intensity.
Laser cooling to micro-Kelvin temperatures requires a laser with active frequency stabilization. The linewidth of an atomic transition sets a lower bound on the Doppler cooling temperature. The 2s-2p transition in lithium-6 has a Doppler temperature of ~140 μK In contrast, the 2s-3p transition has a narrower linewidth and thus provides a lower temperature limit of ~20 μK. We present a method for stabilizing a laser to an atomic line in a vapor cell using modulation transfer spectroscopy and a home-built lock-in amplifier. Our results demonstrate successful locking of a 323 nm laser to the 2s-3p transition. The stabilized laser provides a second stage of magneto-optical trapping that results in a factor of ∼5 increase in the phase space density before evaporating to degeneracy in an optical dipole trap.
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 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.