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Physical Review A

High-brightness atom source for atomic fountains

Philipp Treutlein, Keng Yeow Chung, Steven Chu

Published Apr 9, 2001• 82 citations
Publisher

Abstract

We launch Cs atoms using a moving three-dimensional (3D) optical lattice. Atoms are initially spin polarized and cooled to the ground state of the optical potential using 3D Raman sideband cooling and then accelerated in the lattice to velocities of up to $3 \mathrm{m}/\mathrm{s}.$ Subsequent adiabatic lowering of the potential releases the atoms from the lattice. Three-dimensional kinetic temperatures as low as $200 \mathrm{nK}$ were achieved. The observed temperature of the launch is independent of the acceleration and final velocity of the atoms. In an alternative approach, we first accelerate the atoms to velocities of up to 5 m/s using moving molasses and then cool them with the lattice in the comoving frame of the atoms to temperatures as low as 150 nK.

DOI
10.1103/physreva.63.051401

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We launch Cs atoms using a moving three-dimensional (3D) optical lattice. Atoms are initially spin polarized and cooled to the ground state of the optical potential using 3D Raman sideband cooling and then accelerated in the lattice to velocities of up to 3m/s.3 \mathrm{m}/\mathrm{s}.3m/s. Subsequent adiabatic lowering of the potential releases the atoms from the lattice. Three-dimensional kinetic temperatures as low as 200nK200 \mathrm{nK}200nK were achieved. The observed temperature of the launch is independent of the acceleration and final velocity of the atoms. In an alternative approach, we first accelerate the atoms to velocities of up to 5 m/s using moving molasses and then cool them with the lattice in the comoving frame of the atoms to temperatures as low as 150 nK.
We localize Cs atoms in wavelength-sized potential wells of an optical lattice, and cool them to a three-dimensional temperature of 700 nK by adiabatic expansion. In the optical lattice we precool the atoms to $\ensuremath{\approx}1\ensuremath{\mu}\mathrm{K}$. We then reduce the trapping potential in a few hundred $\ensuremath{\mu}$s, causing the atomic center-of-mass distribution to expand and the temperature to decrease by an amount which agrees with a simple 3D band theory. These are the lowest 3D kinetic temperatures ever measured.
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, 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.
By the moving molasses technique we have extracted laser-cooled cesium atoms in a continuous way directly from an optical molasses. The mean launching velocity is precisely tunable from 1 to 12 m/s. The atomic beam has a flux of $1.3\ifmmode\times\else\texttimes\fi{}{10}^{8} \mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}/\mathrm{s}$ at 7 m/s and a longitudinal temperature of $70 \ensuremath{\mu}\mathrm{K},$ which represents the highest flux and lowest velocity spread obtained so far in a continuous beam of cold atoms. These features makes it well suited for atomic fountains. The atomic flux can be slightly increased in a two-dimensional magneto-optical trap operation (+40(+40%).(+40 A simple model accounts for the observed dependence of the flux with the magnetic-field gradient.
We have measured the temperature of cesium atoms released from optical molasses. For a wide range of laser intensity and detuning from resonance, the temperature depends only on the intensity to detuning ratio. The lowest temperature achieved is 2.5±0.6 μK, which corresponds to an rms velocity of 12.5 mm/s or 3.6 times the single‐photon recoil velocity. This is, to our knowledge, the coldest kinetic temperature ever measured for three dimensional (3D) cooling. We then discuss the possibility of using such a cold sample of atoms for realizing a high performance atomic clock in a fountain geometry. In particular, a method for launching the atoms upwards, while maintaining the very cold temperature, is demonstrated.
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.
We demonstrate launching of laser-cooled Yb atoms in a continuous atomic beam. The continuous cold beam has significant advantages over the more-common pulsed fountain, which was also demonstrated by us recently. The cold beam is formed in the following steps---(i) Atoms from a thermal beam are first Zeeman slowed to a small final velocity, (ii) the slowed atoms are captured in a two-dimensional magneto-optic trap (2D-MOT), and (iii) atoms are launched {\em continuously} in the vertical direction using two sets of moving-molasses beams, inclined at ±15∘\pm 15^\circ±15∘ to the vertical. The cooling transition used is the strongly-allowed 1S0→1P1{^1S}_0 \rightarrow {^1P}_11S0​→1P1​ transition at 399 nm. We capture about 7×1067 \times 10^67×106 atoms in the 2D-MOT, and then launch them with a vertical velocity of 13 m/s at a longitudinal temperature of 125(6) mK.