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Physical Review Letters

Degenerate Raman Sideband Cooling of Trapped Cesium Atoms at Very High Atomic Densities

Vladan Vuletić, Cheng Chin, Andrew J. Kerman, Steven Chu

Published Dec 28, 1998• 213 citations
Publisher

Abstract

We trap ${10}^{7}$ 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 $1/180$ at a mean temperature of $2.8\ensuremath{\mu}\mathrm{K}$ and a density of $1.4\ifmmode\times\else\texttimes\fi{}{10}^{13}{\mathrm{cm}}^{\ensuremath{-}3}$.

DOI
10.1103/physrevlett.81.5768

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We study one-dimensional (1D) sideband cooling of cesium atoms strongly confined in a far-detuned optical lattice. The Lamb-Dicke regime is achieved in the lattice direction whereas the transverse confinement is much weaker. The employed sideband cooling method, first studied by Vuleti\ifmmode \acute{c}\else \'{c}\fi{} et al., uses Raman transitions between Zeeman levels and produces a spin-polarized sample. We present a detailed study of this cooling method and investigate the role of elastic collisions in the system. We accumulate 83(5)% of the atoms in the vibrational ground state of the strongly confined motion, and elastic collisions cool the transverse motion to a temperature of $2.8\ensuremath{\mu}\mathrm{K}=0.7\ensuremath{\Elzxh}{\ensuremath{\omega}}_{\mathrm{osc}}{/k}_{B},$ where ${\ensuremath{\omega}}_{\mathrm{osc}}$ is the oscillation frequency in the strongly confined direction. The sample then approaches the regime of a quasi-2D cold gas. We analyze the limits of this cooling method and propose a dynamical change of the trapping potential as a mean of cooling the atomic sample to still lower temperatures. Measurements of the rate of thermalization between the weakly and strongly confined degrees of freedom are compatible with the zero-energy scattering resonance observed previously in weak 3D traps. For the explored temperature range the measurements agree with recent calculations of quasi-2D collisions. Transparent analytical models reproduce the expected behavior for ${k}_{B}T\ensuremath{\gg}\ensuremath{\Elzxh}{\ensuremath{\omega}}_{\mathrm{osc}}$ and also for ${k}_{B}T\ensuremath{\ll}\ensuremath{\Elzxh}{\ensuremath{\omega}}_{\mathrm{osc}}$ where the 2D features are prominent.
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 nv = 0.75. In a polarization gradient lattice, the final temperature is T = 3.6 μK corresponding to nv 2.4.
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.