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arXiv:1709.06467 · Scientific Reports

Λ-enhanced grey molasses on the D2 transition of Rubidium-87 atoms

Sara Rosi, A. Burchianti, Stefano Conclave, D. Naik, G. Roati, Chiara Fort, Francesco Minardi

Published Jan 16, 2018• 62 citations
arXivHTMLPDF

Abstract

Abstract Laser cooling based on dark states, i.e. states decoupled from light, has proven to be effective to increase the phase-space density of cold trapped atoms. Dark-states cooling requires open atomic transitions, in contrast to the ordinary laser cooling used for example in magneto-optical traps (MOTs), which operate on closed atomic transitions. For alkali atoms, dark-states cooling is therefore commonly operated on the D1 transition nS1/2 → nP1/2. We show that, for 87Rb, thanks to the large hyperfine structure separations the use of this transition is not strictly necessary and that “quasi-dark state” cooling is efficient also on the D2 line, 5S1/2 → 5P3/2. We report temperatures as low as (4.0 ± 0.3) μK and an increase of almost an order of magnitude in the phase space density with respect to ordinary laser sub-Doppler cooling.

DOI
10.1038/s41598-018-19814-z

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Abstract Laser cooling based on dark states, i.e. states decoupled from light, has proven to be effective to increase the phase-space density of cold trapped atoms. Dark-states cooling requires open atomic transitions, in contrast to the ordinary laser cooling used for example in magneto-optical traps (MOTs), which operate on closed atomic transitions. For alkali atoms, dark-states cooling is therefore commonly operated on the D1 transition nS1/2 → nP1/2. We show that, for 87Rb, thanks to the large hyperfine structure separations the use of this transition is not strictly necessary and that “quasi-dark state” cooling is efficient also on the D2 line, 5S1/2 → 5P3/2. We report temperatures as low as (4.0 ± 0.3) μK and an increase of almost an order of magnitude in the phase space density with respect to ordinary laser sub-Doppler cooling.
We present a systematic study of the $\mathrm{\ensuremath{\Lambda}}$-enhanced gray molasses cooling ($\mathrm{\ensuremath{\Lambda}}$-GMC) of cesium atoms on the D2{D}_{2}D2​ transition. Due to the large splitting in the excited hyperfine transitions of the cesium D2{D}_{2}D2​ line, it is relatively simple to implement the $\mathrm{\ensuremath{\Lambda}}$-GMC with a suitable frequency control of the trapping and repumping lasers of the magneto-optical trap (MOT). We achieve a temperature of $1.7\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{K}$ with $3.2\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms. The phase space density of atoms is $1.43\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. Compared to the condition with a bare MOT or a MOT with gray-molasses cooling without $\mathrm{\ensuremath{\Lambda}}$ enhancement, there is an increase in the phase space density by a factor of more than 103{10}^{3}103 and 10, respectively.
Alkali-metal atoms are the foremost candidates for studies of degenerate quantum gases employing sub-Doppler cooling techniques operating either on the D1 or D2 transition. However, the case of 39K atoms has a narrow excited-state structure of the D2 transition compromising difficulty to achieve below the Doppler limit temperature (TD = hΓ/2kB = 140μK) by employing conventional cooling techniques such as optical molasses or polarization gradient cooling on this transition. In this work, a sub-Doppler cooling known as Gray molasses operating on the ¦42S½} → ¦42P½} 770nm transition D1 is applied to cool a cloud of 39K atoms to sub-Doppler temperatures, which is an important step to reach the quantum degeneracy of the quantum gases. With this approach we reach temperatures below the Doppler limit around 14.3μK by the formation of dark states using the cooling and repumping lasers detuned to the blue side at the Raman resonance (two-photon) in Λ configuration. Then, we ramp down the intensities of each laser beam, in order to cool the atoms to an ultra-low temperature of 8.62μK, getting closer to the photon recoil limit of (Tr = h2k2L/2mkB = 0.4μK). In our results, we present a study of the behaviour of the temperature and the number of atoms by the influence of the relative detuning, and also a particular analysis of the temperature by the influence of the global detuning. In the following, we present the lowest temperature as a function of the final intensities D1 molasses. Finally, we compare the performance of our Gray molasses cloud with other works by using a phase space density analysis.
We report on the efficient gray molasses cooling of sodium atoms using the D 2 optical transition at 589.1 nm. Thanks to the hyperfine split about 6 Γ between | F ′ = 2 ⟩ and | F ′ = 3 ⟩ in the excited state 3 2 P 3 / 2 , this atomic transition is effective for the gray molasses cooling mechanism. Using this cooling technique, the atomic sample in F = 2 ground manifold is cooled from 700 μ K to 56 μ K in 3.5 ms. We observe that the loading efficiency into magnetic trap is increased due to the lower temperature and high phase space density of atomic cloud after gray molasses. This technique offers a promising route for the fast cooling of the sodium atoms in the F = 2 state .
We present new techniques in cooling 39 K atoms using laser light close to the D1 transition.First, a new compressed-MOT configuration is taking advantage of gray-molasses-type cooling induced by blue-detuned D1 light.It yields an optimized density of atoms.Then, we use pure D1 gray molasses to further cool the atoms to an ultra-low temperature of 6 μK.The resulting phase-space density is 2 × 10 -4 and will ease future experiments with ultracold potassium.As an example, we use it to directly load up to 3 × 10 7 atoms in a far detuned optical trap, a result that opens the way to the all-optical production of potassium degenerate gases.
We discuss our experimental results on forced evaporative cooling of cold rubidium 87 Rb atoms to quantum degeneracy in an Optical Dipole Trap. The atoms are first trapped and cooled in a magneto-optical trap (MOT) loaded from a continuous beam of cold atoms [1]. More than 10 10 atoms are trapped in the MOT and then about 10 8 atoms are transferred to a Quasi-Electrostatic Trap (QUEST) formed by tightly focused CO2 laser (λ = 10.6μ m ) beams intersecting at their foci in an orthogonal configuration in the horizontal plane. Before loading the atoms into the dipole trap, the phase-space density of the atomic ensemble was increased making use of sub-doppler cooling at large detuning and the temporal dark MOT technique. In a MOT the phase-space density of the atomic ensemble is six orders of magnitude less than what is required to achieve quantum degeneracy. After transferring atoms into the dipole trap efficiently, phase-space density increases by a factor of 10 3 . Further increase in phase-space density to quantum degeneracy is achieved by forced evaporative cooling of atoms in the dipole trap. The evaporative cooling process involves a gradual reduction of the trap depth by ramping down the trapping laser intensity over a second. The temperature of the cold atomic cloud was measured by time-of-flight (TOF) technique. The spatial distribution of the atoms is measured using absorption imaging. We report results of evaporative cooling in a single beam and in a crossed double-beam dipole traps. Due to the large initial phase space density, and large initial number of atoms trapped, the quantum phase transition occurs after about 600 ms of evaporative cooling in our optimized crossed dipole trap.
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 ${10}^{11}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$.