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Sub-μK temperatures by adiabatic cooling in a 3D optical lattice

Poul Jessen, Anders Kastberg, William D. Phillips, S. L. Rolston, R. J. C. Spreeuw

Published May 8, 1994• 0 citations

Abstract

In a one-dimensional polarization-gradient cooling scheme, with two counter-propagating laser beams having mutually orthogonal linear polarization, a linear array of potential wells for cold atoms are formed. These optical potential wells are created by the spatially varying light shift of the atomic ground state. Transitions between quantized vibrational states in such wells have been observed by stimulated1 and spontaneous2 Raman spectroscopy. Recently, this was extended to three dimensions using both four beams3 and six beams.4 In this work, we intersect four travelling-wave laser beams, all polarized in the same plane, as indicated in Fig. 1. This produces a threedimensional body-centered cubic lattice of potential wells for cesium atoms. The laser wavelength is tuned 25 linewidths below the 62S1/2(F = 4) → 62P3/2(F = 5) transition at 852 nm; and the resultant lattice constants (distances between wells of the same character) are λ / 2 perpendicular to the plane of polarization, and λ × 2 in the plane of polarization.

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In a one-dimensional polarization-gradient cooling scheme, with two counter-propagating laser beams having mutually orthogonal linear polarization, a linear array of potential wells for cold atoms are formed. These optical potential wells are created by the spatially varying light shift of the atomic ground state. Transitions between quantized vibrational states in such wells have been observed by stimulated1 and spontaneous2 Raman spectroscopy. Recently, this was extended to three dimensions using both four beams3 and six beams.4 In this work, we intersect four travelling-wave laser beams, all polarized in the same plane, as indicated in Fig. 1. This produces a threedimensional body-centered cubic lattice of potential wells for cesium atoms. The laser wavelength is tuned 25 linewidths below the 62S1/2(F = 4) → 62P3/2(F = 5) transition at 852 nm; and the resultant lattice constants (distances between wells of the same character) are λ / 2 perpendicular to the plane of polarization, and λ × 2 in the plane of polarization.

Quantization of atomic motion is observed in two- and three-dimensional cesium optical molasses by stimulated Raman spectroscopy. A new geometry is used with the minimum number of laser beams, so that the topography of the optical potential is not sensitive to phase drifts of the molasses beams. The deepest potential wells are located on a hexagonal lattice in two dimensions and on a body-centered-cubic lattice in three dimensions. They correspond to purely circularly polarized light, allowing a Lamb-Dicke narrowing of the transitions between vibrational levels to occur.

We present an experimental investigation of a three-dimensional (3D) optical lattice where the light polarization is linear at the bottom of the potential wells instead of being circular as in previous studies. In the present situation, optical pumping induces frequent transitions between differently light-shifted Zeeman sublevels (jumping regime). However, narrow vibrational lines are still observed as a result from a motional narrowing-like phenomenon taking place in this lattice. The atomic kinetic temperature is investigated and shown to be lower than in a standard 3D lin ⊥ lin optical lattice.

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.

In two recent experiments Verkerk et al. and our group have observed quantization of the center-of-mass motion of laser cooled atoms in sub-wavelength sized potential wells formed by the light field of a 1-dimensional optical molasses. Our 1-dimensional optical molasses consists of a pair of counterpropagating laser beams of orthogonal, linear polarization, creating a series of potential wells separated by {lambda}/4. Polarization gradient laser cooling reduces the thermal energy of the atoms until they are trapped in well resolved vibrational levels near the bottom of the wells. Transitions between these vibrational levels are seen in a high resolution spectrum of fluorescence, where they give rise to a Rayleigh peak centered at the laser frequency, and well resolved Raman sidebands shifted by a frequency {plus_minus}{omega}, typically on the order of 100 kHz. We find quantitative agreement between the measured level separation h{omega} and the results of a calculation of the bandstructure in the periodic optical potential. Based on the observed ratio of the strength of the blue to the red sideband we determine the temperature of the atoms, and find agreement with theoretical predictions. Based on the level spacing and temperature we find the atoms to be localized to {approx}/15, withmore » up to 50-60% of the population of the trapped atoms in the vibrational ground state. We observe intriguing differences between fluorescence radiated by localized atoms, and the fluorescence radiated by non-localized atoms in a 1-dimensional optical molasses composed of a pair of counterpropagating laser beams of opposite circular polarization. The prospect of observing quantum motion and spatial order in 2 and 3 dimensions, and the consequences for the spectrum of fluorescence, will be discussed.« less

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.