We use the time-of-flight (TOF) method to distinguish between a thermal cloud and a BEC. The idea of TOF is to turn the trap off and wait a certain amount of time before taking a shadow image of the atoms. During this time, the atoms will drop due to gravity and expand due to their non-zero temperature. A normal thermal atom cloud will display a Gaussian distribution after TOF. However, when atoms reach the condensation state, their temperature (as observered from their velocity distribution) has a sudden drop. From the shadow image after a long enough TOF, we should observe two distributions of atoms right at the BEC transition. The image below shows our observation of the BEC.

We have a BEC!!!

Step 4: Evaporative Cooling

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