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Journal of the Physical Society of Japan

Experimental Study on Evaporative Cooling and Bose–Einstein Condensation of Sodium Atoms

Kou Matsui, Takahiko Shobu, Yasunori Tanami, H. Nakamatsu, Hideki Tanaka, Atsuo Morinaga

Published Aug 10, 2009• 2 citations
Publisher

Abstract

We have developed an apparatus for creating Bose–Einstein condensates of sodium atoms. Radio-frequency-induced forced evaporative cooling was studied experimentally and optimized to stably create sodium condensates containing a large number of atoms. The transition temperature, the spatial distribution of the condensate, and the aspect ratio of the Bose–Einstein condensate were analyzed quantitatively and compared with theoretical predictions. Several methods of ramping down the rf for evaporative cooling were tested.

DOI
10.1143/jpsj.78.084301

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We have developed an apparatus for creating Bose–Einstein condensates of sodium atoms. Radio-frequency-induced forced evaporative cooling was studied experimentally and optimized to stably create sodium condensates containing a large number of atoms. The transition temperature, the spatial distribution of the condensate, and the aspect ratio of the Bose–Einstein condensate were analyzed quantitatively and compared with theoretical predictions. Several methods of ramping down the rf for evaporative cooling were tested.
We report on a highly efficient evaporative cooling optimized experimentally. We successfully created sodium Bose-Einstein condensates with 6.4\ifmmode\times\else\texttimes\fi{}107{}^{7}7 atoms starting from 6.6\ifmmode\times\else\texttimes\fi{}109{}^{9}9 thermal atoms trapped in a magnetic trap by employing a fast linear sweep of radio frequency at the final stage of evaporative cooling so as to overcome the serious three-body losses. The experimental results such as the cooling trajectory and the condensate growth quantitatively agree with the numerical simulations of evaporative cooling on the basis of the kinetic theory of a Bose gas carefully taking into account our specific experimental conditions. We further discuss theoretically a possibility of producing large condensates, more than 108{}^{8}8 sodium atoms, by simply increasing the number of initial thermal trapped atoms and the corresponding optimization of evaporative cooling.
We describe experimental setups for producing large Bose-Einstein condensates of Na23 and Rb87. In both, a high-flux thermal atomic beam is decelerated by a Zeeman slower and is then captured and cooled in a magneto-optical trap. The atoms are then transferred into a cloverleaf-style Ioffe-Pritchard magnetic trap and cooled to quantum degeneracy with radio-frequency-induced forced evaporation. Typical condensates contain 20×106 atoms. We discuss the similarities and differences between the techniques used for producing large Rb87 and Na23 condensates in the context of nearly identical setups.
Bose-Einstein condensation of 87Rb{}^{87}\mathrm{Rb}87Rb has been observed in a vapor cell time-averaged orbiting potential trap, and the trapped condensates have been studied using in situ absorption and dark-ground imaging methods. Condensates of $3\ifmmode\times\else\texttimes\fi{}{10}^{4}$ atoms were observed after 26 s of evaporative cooling. The evaporative cooling sequence consisted of a combination of cooling with ramping magnetic-field strengths and radio-frequency-induced cooling. The measured sizes, numbers of atoms, and transition temperatures of the condensates are consistent with theoretical predictions that include the effects of atomic interactions.
The dynamics of evaporative cooling of magnetically trapped 87 Rb atoms is studied on the basis of the quantum kinetic theory of a Bose gas. The theoretical optimization of the cooling trajectory clarifies that the acceleration of evaporative cooling around the transition point of Bose-Einstein condensation is very effective against serious three-body recombination loss. The number of condensed atoms is expected to be largely enhanced by the optimization as compared with the typical value produced by the conventional evaporative cooling, where the frequency of the radio-frequency magnetic field is swept exponentially. Next, we precisely examine how the growth of a condensate depends on the experimental parameters of evaporative cooling in this inefficient exponential-sweep cooling. We find that, due to large nonlinear three-body loss, the number of condensed atoms finally becomes insensitive to the initial number of trapped atoms, the initial temperature, and the bias field of a magnetic trap. This result opens up the experimental possibility that the final number of condensed atoms can be well stabilized against the shot-to-shot fluctuations of experimental conditions in evaporative cooling.
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.