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Using dispersive (off -resonance) imaging techniques, we have successfully observed Bose-Einstein condensation inside of a magnetic trap. Dispersive imaging allows us to image very dense Bose condensates which would be completely saturated in an on-resonance absorption image. It also has the advantage that the amount of heat deposited into the atoms by the probing light is about 300 times lower than would be the case for an equivalent absorption image. This huge decrease in heat transfer means that we can image a Bose condensate without destroying it, giving us the ability to do repeated non-destructive measurements on a pure quantum mechanical wave function.
First direct imaging of a condensate using the dark-ground technique.
We have employed two different types of dispersive imaging, darkground and phase contrast. Phase contrast imaging, the method we currently use, is a well established method which we borrowed from the field of microscopy. In a phase contrast image, light which has been scattered by the atoms is made to interfere with unscattered light in a manner similar to homodyne detection.
Non-destructive imaging of a Bose-Einstein condensate. These images demonstrate the non-destructive nature of dispersive imaging. All of these images were taken of the same atom cloud using the phase contrast method. The leftmost image was taken first, and subsequent images were taken every ms. The color scale indicates the phase shift (in radians) that the light aquired passing through the atoms. As you can see, throughout the entire sequence of images the amount of heating is negligible and the Bose condensate is still intact. This means that we can use phase contrast imaging to watch the evolution of a Bose condensate in real time. This gives us the unique ability to directly observe the evolution of matter waves, which will help us to bridge the gap between the quantum and classical worlds.
Direct observation of the formation of a Bose-Einstein condensate using dispersive light scattering (phase contrast images). The intensity of the scattered light is a measure of the density of atoms (integrated along the line-of-sight). The left picture shows the cloud slightly above the BEC transition temperature. When the temperature was lowered, a dense core formed in the center of the trap - the Bose condensate. Further cooling increased the condensate fraction to close to 100% (right).
