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arXiv:1109.5237

Three Dimensional Raman Cooling using Velocity Selective Rapid Adiabatic Passage

Axel Kuhn, Hélène Perrin, W. Hänsel, Christophe Salomon

Published Sep 24, 2011• 2 citations
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Abstract

We present a new and efficient implementation of Raman cooling of trapped atoms. It uses Raman pulses with an appropriate frequency chirp to realize a velocity selective excitation through a rapid adiabatic passage. This method allows to address in a single pulse a large number of non zero atomic velocity classes and it produces a nearly unity transfer efficiency. We demonstrate this cooling method using cesium atoms in a far-detuned crossed dipole trap. Three-dimensional cooling of $1 \times 10^{5}$ atoms down to $2 μ$K is performed in 100 ms. In this preliminary experiment the final atomic density is $1.3\times 10^{12}$ at/cm$^3$ (within a factor of 2) and the phase-space density increase over the uncooled sample is 20. Numerical simulations indicate that temperatures below the single photon recoil temperature should be achievable with this method.

DOI
10.48550/arxiv.1109.5237

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We have Raman cooled sodium atoms below the photon recoil temperature in a novel type of blue-detuned optical dipole force trap. In this trap $4.5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms have been cooled to an effective three dimensional temperature of 1.0 \ensuremath{\mu}K at a final density of $4\ifmmode\times\else\texttimes\fi{}{10}^{11}{\mathrm{cm}}^{\ensuremath{-}3}$. No atoms were lost during the cooling process. The phase space density increased by a factor of 320 over the uncooled sample. This is the highest phase space density achieved by an all-optical cooling method.
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We use velocity-selective Raman pulses generated by two diode lasers to cool cesium atoms in 1 dimension to an r.m.s. of 1.2 mm/s. This is about 1/3 of the single-photon recoil velocity v rec = ℏk/M , where ℏk is the photon momentum and M the atom's mass. The corresponding effective temperature is a factor of 9 below the single-photon recoil temperature given by k B T rec /2 = 1/2 Mv rec 2 . Because of the high cesium mass, this temperature is only (23 ± 5) nanokelvin, the lowest 1D kinetic temperature reported to date.
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We present a cooling method that should be generally applicable to atoms with narrow optical transitions. This technique uses velocity-selective pulses to drive atoms towards a zero-velocity dark state and then quenches the excited state to increase the cooling rate. We demonstrate this technique of quenched narrow-line cooling by reducing the 1D temperature of a sample of neutral 40Ca{}^{40}\mathrm{Ca}40Ca atoms. We cool selected velocities with the 1S0(4s2)3P1(4s4p){}^{1}{S}_{0}{(4s}^{2}{)}^{3}{P}_{1}(4s4p)1S0​(4s2)3P1​(4s4p) 657-nm intercombination line and quench with the 3P1(4s4p)1S0(4s5s){}^{3}{P}_{1}{(4s4p)}^{1}{S}_{0}(4s5s)3P1​(4s4p)1S0​(4s5s) intercombination line at 553 nm, which increases the cooling rate eightfold. Limited only by available quenching laser power, we have transferred 18% of the atoms from our initial 2-mK velocity distribution and achieved temperatures as low as 4 \ensuremath{\mu}K, corresponding to a vrms{v}_{\mathrm{rms}}vrms​ of 2.8 cm/s or 2 recoils at 657 nm. This cooling technique, which is closely related to Raman cooling, can be extended to three dimensions.