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Physical Review Letters

3D Raman Sideband Cooling of Cesium Atoms at High Density

Dian-Jiun Han, Steffen Wolf, Steven J. Oliver, Colin McCormick, Marshall T. DePue, David S. Weiss

Published Jul 24, 2000• 108 citations
Publisher

Abstract

We laser cool $5\ifmmode\times\else\texttimes\fi{}{10}^{7}$ Cs atoms to a spin-polarized phase space density of 1/30, the highest ever obtained by laser cooling. It is achieved by compression and polarization gradient cooling in a 3D far-off-resonant optical lattice, followed by 3D Raman sideband cooling optimized at a density of $1.5\ifmmode\times\else\texttimes\fi{}{10}^{12}\mathrm{atoms}/{\mathrm{cm}}^{3}$, and adiabatic release. In the lattice, $23%$ of the sites are occupied, $95%$ of the atoms are in the lowest energy magnetic sublevel, and $37%$ are in the lowest 3D vibrational state.

DOI
10.1103/physrevlett.85.724

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We laser cool 5\ifmmode×\else\texttimes\fi1075\ifmmode\times\else\texttimes\fi{}{10}^{7}5\ifmmode×\else\texttimes\fi107 Cs atoms to a spin-polarized phase space density of 1/30, the highest ever obtained by laser cooling. It is achieved by compression and polarization gradient cooling in a 3D far-off-resonant optical lattice, followed by 3D Raman sideband cooling optimized at a density of 1.5\ifmmode×\else\texttimes\fi1012atoms/cm31.5\ifmmode\times\else\texttimes\fi{}{10}^{12}\mathrm{atoms}/{\mathrm{cm}}^{3}1.5\ifmmode×\else\texttimes\fi1012atoms/cm3, and adiabatic release. In the lattice, 2323%23 of the sites are occupied, 9595%95 of the atoms are in the lowest energy magnetic sublevel, and 3737%37 are in the lowest 3D vibrational state.
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 $1.4\ifmmode\times\else\texttimes\fi{}{10}^{13}{\mathrm{cm}}^{\ensuremath{-}3}$.
We demonstrate a simple, general purpose method to cool neutral atoms. A sample containing $3\ifmmode\times\else\texttimes\fi{}{10}^{8}$ cesium atoms prepared in a magneto-optical trap is cooled and simultaneously spin polarized in 10 ms at a density of $1.1\ifmmode\times\else\texttimes\fi{}{10}^{11}\mathrm{cm}{}^{\ensuremath{-}3}$ to a phase space density $n{\ensuremath{\lambda}}_{\mathrm{dB}}^{3}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/500$, which is almost 3 orders of magnitude higher than attainable in free space with optical molasses. The technique is based on 3D degenerate Raman sideband cooling in optical lattices and remains efficient even at densities where the mean lattice site occupation is close to unity.
This dissertation reports on quantum state preparation of cesium atoms in a two-dimensional optical lattice, by resolved-sideband Raman cooling. An optical lattice is a periodic potential produced by the light shift interaction between an atom and light field. Laser cooled atoms can become strongly localized about the bottom of potential wells in an optical lattice, where they occupy a discrete spectrum of bound vibrational energy levels. The distribution over vibrational levels of atoms in the lattice is characterized by the mean vibrational excitation, n . In an optical lattice, absorption and emission of photons from lattice beams causes n to increase in time. This source of heating is always present, but its rate can be greatly reduced in a lattice detuned far from the atomic resonance. Sideband cooling is an efficient means of transferring atoms from higher into lower-lying vibrational levels and, thus, it reduces n for the ensemble. If the sideband cooling rate is much greater than the heating rate, then n approaches zero and virtually all atoms are in the lowest vibrational level in their potential wells. Our sideband cooling scheme involves stimulated Raman transitions between bound states in the potential wells of a pair of magnetic sublevels, followed by optical pumping, for a net loss of one quantum of vibration per cooling cycle. The process accumulates 98% of atoms in the ground vibrational level of a potential well associated with a single Zeeman substate. Each atom in the lattice is then very close to a pure state. For two-dimensional lattice with sideband cooling we find nx≈ny≈0.008 &parl0;16&parr0; . Various issues related to state preparation and sideband cooling are also discussed in the context of a one dimensional lin ⊥ lin optical lattice. These include improvement of laser cooling in a near resonance lattice by application of weak magnetic fields, transfer of atoms from near into far off-resonance lattices, and heating rates in far off-resonance lattices.
Sideband laser cooling using stimulated Raman transitions is performed on trapped cesium atoms. The confinement is produced by a far–off-resonance dipole trap consisting of two crossed YAG beams which, by interference, create a one-dimensional optical lattice. In a pure intensity lattice, we measure a 1D temperature of T = 6 μK corresponding to a mean quantum vibrational number of about ⟨ n v ⟩ = 0.75. In a polarization gradient lattice, the final temperature is T = 3.6 μK corresponding to ⟨ n v ⟩ ≃ 2.4.
We report on a realization of sub-Doppler laser cooling of 39K^{39}\mathrm{K}39K atoms using degenerate three-dimensional Raman sideband cooling. We take advantage of the well-resolved excited hyperfine states on the D1{D}_{1}D1​ optical transition to produce spin-polarized samples with $1.4\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms at temperatures of 1.8 $\ensuremath{\mu}\mathrm{K}$. The phase-space densities are $\ensuremath{\ge}{10}^{\ensuremath{-}4}$, which significantly improves the initial conditions for a subsequent evaporative cooling step. The presented cooling technique using the D1{D}_{1}D1​ line can be adapted to other atomic species and is applicable to high-resolution imaging schemes in far off-resonant optical lattices.
We report a way to overcome heating due to the reabsorption of scattered light when relatively many atoms are laser cooled at high density. A sample of $8\ifmmode\times\else\texttimes\fi{}{10}^{7}\mathrm{Cs}$ atoms, with $6\ifmmode\times\else\texttimes\fi{}{10}^{11}{\mathrm{atoms}\mathrm{}\mathrm{cm}}^{\mathrm{\ensuremath{-}}3},$ is loaded into a deep far-detuned one-dimensional optical lattice, which acts as a large volume three-dimensional (3D) trap. Polarization gradient cooling with a separate set of cooling beams is found to work well in this trap, populating the trap's vibrational ground state with more than half the atoms. After cooling, the lattice is shut off adiabatically, leaving a still dense sample at 1.2 \ensuremath{\mu}K. We propose and demonstrate a generalization to a 3D lattice, which will allow many more atoms to be loaded into conservative light traps, and can greatly increase the initial phase-space density for loading magnetic traps.