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Physical Review Letters

Evaporative Cooling of Sodium Atoms

Kendall B. Davis, M.‐O. Mewes, Michael A. Joffe, Michael R. Andrews, Wolfgang Ketterle

Published Jun 26, 1995• 260 citations
Publisher

Abstract

We have observed evaporative cooling of magnetically trapped sodium atoms. A novel technique, rf induced evaporation, was used to reduce the temperature by a factor of 12 and increase the phase space density by more than 2 orders of magnitude. The elastic collision cross section of cold sodium atoms in the $F\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$, ${m}_{F}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}1$ hyperfine state was determined to be $6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ c${\mathrm{m}}^{2}$, which implies a positive value of the scattering length.

DOI
10.1103/physrevlett.74.5202

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We have observed evaporative cooling of magnetically trapped sodium atoms. A novel technique, rf induced evaporation, was used to reduce the temperature by a factor of 12 and increase the phase space density by more than 2 orders of magnitude. The elastic collision cross section of cold sodium atoms in the F=1F\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1F=1, mF=\ensuremath−1{m}_{F}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}1mF​=\ensuremath−1 hyperfine state was determined to be 6\ifmmode×\else\texttimes\fi10\ensuremath−126\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}6\ifmmode×\else\texttimes\fi10\ensuremath−12 cm2{\mathrm{m}}^{2}m2, which implies a positive value of the scattering length.
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 have observed Bose-Einstein condensation of sodium atoms. The atoms were trapped in a novel trap that employed both magnetic and optical forces. Evaporative cooling increased the phase-space density by 6 orders of magnitude within seven seconds. Condensates contained up to 5\ifmmode\times\else\texttimes\fi{}105{10}^{5}105 atoms at densities exceeding 1014{10}^{14}1014 ${\mathrm{cm}}^{\ensuremath{-}3}$. The striking signature of Bose condensation was the sudden appearance of a bimodal velocity distribution below the critical temperature of \ensuremath{\sim}2\ensuremath{\mu}K. The distribution consisted of an isotropic thermal distribution and an elliptical core attributed to the expansion of a dense condensate.
We observe the spontaneous evaporation of atoms confined in a bubble-like radio frequency (rf)-dressed trap (Zobay and Garraway 2001 Phys. Rev. Lett. 86 1195; 2004 Phys. Rev. A 69 023605). The atoms are confined in a quadrupole magnetic trap and are dressed by a linearly polarized rf field. The evaporation is related to the presence of holes in the trap, at the positions where the rf coupling vanishes, due to its vectorial character. The final temperature results from a competition between residual heating and evaporation efficiency, which is controlled via the height of the holes with respect to the bottom of the trap. The experimental data are modelled by a Monte Carlo simulation predicting a small increase in phase-space density limited by the heating rate. This increase was within the phase-space density determination uncertainty of the experiment.
We have evaporatively cooled caesium atoms in a magnetic trap to temperatures as low as 8 nK and produced a final phase space density within a factor of four of that required for the onset of Bose–Einstein condensation. At the end of the forced radio-frequency evaporation, 1500 atoms in the F = 3, m F = −3 state remain in the magnetic trap. We observe a decrease in the one-dimensional evaporative cooling efficiency at very low temperatures as the trapped sample enters the collisionally thick (hydrodynamic) regime. To alleviate this problem we propose a modified trapping scheme where three-dimensional evaporation is possible. In addition, we report measurements of the two-body inelastic collision rates for caesium atoms as a function of magnetic field. We confirm the positions, with reduced uncertainties, of three previously identified resonances at magnetic fields of 108.87(6), 118.46(3) and 133.52(3) G.
We describe the realization of a magnetically guided beam of cold rubidium atoms, with a flux of $7\ifmmode\times\else\texttimes\fi{}{10}^{9}\text{ }\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}/\mathrm{s}$, a temperature of $400\text{ }\ensuremath{\mu}\mathrm{K}$, and a mean velocity of 1 m/s1\text{ }\mathrm{m}/\mathrm{s}1 m/s. The rate of elastic collisions within the beam is sufficient to ensure thermalization. We show that the evaporation induced by a radio-frequency wave leads to appreciable cooling and an increase in the phase space density. We discuss the perspectives to reach the quantum degenerate regime using evaporative cooling.