Formation of a Bose-Einstein condensate. Two-dimensional probe absorption images, after 6 msec time of flight, show the sharpness of the phase transition. The evaporative cooling was induced by an externally applied rf field. As the final rf frequency (labeled on the plots) was lowered, lower temperatures and higher phase space densities were reached. The cloud at the start of the animation had a temperature of about 5 microkelvin. Above the phase transition (frequency >700 kHz) the clouds expanded spherically, as expected for a normal thermal distribution. As the frequency was reduced, the spherical cloud shrank in size, due to the lower temperatures reached. Below the transition point (frequency <700 kHz, 2 microkelvin) an elliptical core appeared, which is the signature of the condensate. As the frequency was lowered the spherical part became invisible, corresponding to a pure condensate. Finally, when the threshold for evaporation reached the bottom of the trap (around 300 kHz), the condensate itself was lost by evaporation. Note that the color scale here has been chosen to represent optical density (OD) instead of absorption (A), as used in many other images. The two are related by OD=-ln(1-A). The rf frequency displayed in the animation changes when a new original frame is displayed and stays constant when interpolated frames are shown. The size of the frame is 1.1 by 1.6 mm.

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