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Science Measurements


Ballistic expansion of a cold atom cloud

In CAL's Ground Testbed, a vapor of rubidium (Rb) atoms is laser cooled, magnetically trapped, transported into an atom chip trap, and then evaporatively cooled down to nanoKelvin temperatures. These ultra cold atoms are then released and observed to fall due to gravity (in terrestrial experiments such as this one). During its fall this thermal atom cloud expands due to its finite temperature. We image the atoms with a pair of laser beam flashes; the first flash captures a "shadow" of the atom cloud, while the second flash records a reference image. We can then process the images to get the density distribution of the cloud (in these images RED is the most dense while BLACK is zero density). The expansion rate provides a measurement of the temperature of the ultra cold Rb. The snapshots in this clip indicate the time of flight in milliseconds of the dropped cloud in the upper left. The radio frequency (RF) is noted in the upper right corner indicating the final value of the RF knife that we applied during forced evaporative cooling.

Image credit: Dr. David C. Aveline, CAL Ground Testbed, Jet Propulsion Laboratory





Evaporative cooling with an RF Knife

In CAL's Ground Testbed, a vapor of rubidium (Rb) atoms is evaporatively cooled in an atom chip trap utilizing a radio frequency (RF) "knife" that continuously slices away the hottest atoms. With the hottest atoms removed, the cloud can re-thermalize at a cooler temperature much like a hot cup of tea gets cooler over time as the hottest molecules evaporate. This RF knifeā€™s frequency is smoothly ramped down from ~30 MHz down to ~1 MHz. The lower the cut with the RF frequency, the colder and more dense the atom cloud becomes, and if done efficiently it may cross the critical temperature to achieve a Bose-Einstein Condensate (BEC). The snapshots in this clip show the decreasing size (increasing density) and decreasing temperature of the Rb cloud when lower final RF values are applied.

Image credit: Dr. David C. Aveline, CAL Ground Testbed, Jet Propulsion Laboratory