The basic idea of the evaporative cooling is simple. First, let's consider atoms trapped by the magnetic potential as shown in the panel (a) of the picture below. The most energetic (i.e. fastest, hottest) atoms, in the course of their movement within the trap, can go much higher up the potential walls. Now, we lower the walls of the trap potential. As a result, the most energetic atoms will likely fall out of the trap. The remaining atoms will collide with each other, exchanging momentum and reaching a new, lower temperature equilibrium; this is called rethermalization. After the atoms have been allowed to rethermalize, we lower the walls of the trap potential again, and allow the atoms to rethermalize again. This procedure is repeated until the critical temperature of a BEC is reached.
In practice, the magnetic trapping potential can be modified by a radio frequency (RF) signal. The RF signal introduces a coupling between the trapping state m=2 and the anti-trapping state m=-2. As a result of this coupling, the height of the top of the trapping potential becomes a function of the RF frequency. The higher the frequency, the higher the magnetic trap walls. The picture below shows the effect of the evaporative cooling. We send a 8 MHz RF signal to the atoms in the Z-trap. In 10 s, the RF frequency is ramped down linearly to different final frequencies as indicated by each panel.
In our first realization of a BEC, right after the Z-trap compression a 16 MHz sinusoid RF wave was delivered to the atoms by an antenna outside the chamber. The RF frequency was then linearly decreased to around 160kHz over the course of 16 s. We have since sped up the process considerably: the ramp takes 5 s, and our bottom frequency is around 850kHz. At the same time, the -y direction bias magnetic field OCis increased linearly from 40 G to 60 G, which makes the Z-trap tighter and tighter, increasing the rethermalization rate. The final Z-trap is 350 μm away from the mirror and has a gradient of 360 G/cm.
Now, we may have a BEC in the Z-trap! The question is how we are going to observe it?
|Step 3: Magnetic Trap||Step 5: Observation of a BEC|
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