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Physical Review A

Atomic spin polarization by Raman cooling

H. J. Lee, Steven Chu

Published Apr 1, 1998• 8 citations
Publisher

Abstract

We have simultaneously spin polarized and subrecoil cooled sodium atoms trapped in a blue detuned optical dipole trap. Temperatures of 1.5 \ensuremath{\mu}K and less have been achieved for atoms which have been polarized into any of three $F=1$ ground-state magnetic sublevels of sodium. The final phase-space density is comparable to the best achieved by an all-optical cooling technique.

DOI
10.1103/physreva.57.2905

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We have simultaneously spin polarized and subrecoil cooled sodium atoms trapped in a blue detuned optical dipole trap. Temperatures of 1.5 \ensuremath{\mu}K and less have been achieved for atoms which have been polarized into any of three F=1F=1F=1 ground-state magnetic sublevels of sodium. The final phase-space density is comparable to the best achieved by an all-optical cooling technique.
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.
We develop a simple and practical scheme to apply sideband cooling to a cloud of rubidium atoms. A sample containing 4 × 10 70 87 Rb is trapped in a far red detuned optical lattice. Through optimizing the relevant parameters, i.e., laser detuning, magnetic field, polarization, and duration time, a temperature around 1.5 μK and phase space density close to 1/500 are achieved. Compared with polarization gradient cooling, the temperature decreases by around one order of magnitude. This technique could be used in high precision measurement such as atomic clocks and atom interferometer. It could also serve as a precooling means before evaporation cooling in a dipole trap, and may be a promising method of achieving quantum degeneracy with purely optical means.
Conditions for trapping sodium atoms in far detuned dipole traps are explored with the goal of maximizing phase space density and coherence times. With red detuned traps, we have been able to achieve densities as high as 2 ×1012 atoms/cm3 at temperatures on the order of (11ħk)2/2M. We also report Raman cooling in a red de‐tuned trap to a one dimensional “temperature” of 0.7(ħk)2/2M. With a blue detuned trap, we have been able to observe Ramsey interference fringes between the ground states of sodium for over 4 seconds. Finally, we report an atom interferometer based on adiabatic transfer of ground state populations.
Laser cooled sodium atoms are trapped in an optical dipole force trap formed by the intersection of two 1.06 \ensuremath{\mu}m laser beams. Densities as high as $4\ifmmode\times\else\texttimes\fi{}{10}^{12}$ atoms/cm3{\mathrm{m}}^{3}m3 at a temperature of $\ensuremath{\sim}140$ \ensuremath{\mu}K have been obtained in a $\ensuremath{\sim}900$ \ensuremath{\mu}K deep trap. By reducing the trap depth over a 2 s interval, we have evaporatively cooled the atoms to a final temperature of $\ensuremath{\sim}4$ \ensuremath{\mu}K at a density of $6\ifmmode\times\else\texttimes\fi{}{10}^{11}$ atoms/cm3{\mathrm{m}}^{3}m3. This corresponds to a factor of 28 increase in atomic phase-space density.
We trap 107{10}^{7}107 cesium atoms in a far red detuned 1D optical lattice. With degenerate Raman sideband cooling we achieve a vibrational ground state population of 80% for the steep trapping direction. Collisional coupling enables us to cool the spin-polarized gas in 3D without loss of atoms to a peak phase space density of 1/1801/1801/180 at a mean temperature of $2.8\ensuremath{\mu}\mathrm{K}$ and a density of $1.4\ifmmode\times\else\texttimes\fi{}{10}^{13}{\mathrm{cm}}^{\ensuremath{-}3}$.