We can use the time-of-flight (TOF) technique to find the velocity distribution of the atoms in the vertical dimension, and thus determine the temperature.
To use this method, the lattice is configured vertically.
The atoms are very cold and thus are very slow, so the effect of gravity can be clearly observed.
Once all the light in the chamber is extinguished, the atoms begin to freefall due to gravity.
After 20 cm of free fall, they pass through a cylindrically collimated sheet of light resonant on the |F=2> → |F'=3> transition.
The resulting flourecence is collected on a large area photodiode.
The FWHM of the Gaussian signal is related to the temperature of the atoms at the time of their release - the greater the spread in velocities (the larger the FWHM), the greater the temperature.
Further, from the average time of arrival we can determine their average starting velocity.
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