Using a sheet of laser light from an argon ion laser, we have been able to excite Bose condensates locally, creating sound-waves which travel along the length of the condensate. These localized perturbations allow us to measure the speed of sound and to further explore excitations and damping in a dilute Bose condensate.

 

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Generation of sound-waves in a Bose condensate. Using a laser light sheet to repel atoms from the center of the condensate, localized sound-wave packets can be produced.

In case (a), a condensate is formed with the light sheet off. Then at time t=0, the laser light sheet is turned on, pushing atoms away from the center of the trap and creating local increases in density. These density "blips" then travel as two localized sound waves away from the center of the condensate.

In case (b), a condensate is formed while the light sheet is on. At time t=0, the light sheet is turned off, leaving a density dip in the center of the condensate. This dip then splits in two as it travels away from the center of the condensate.

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Sound Propagation in a Bose-Einstein Condensate. This picture displays a series of images taken of a single condensate, the earliest image being at the left, and the latest image taken being at the right. The time interval between images is 1.3 ms.

The left-most image was taken just after the light sheet was turned on. The introduction of the light sheet caused the atoms in the center of the condensate to be pushed aside, creating the two dense bulges of atoms near the center of the condensate. These density perturbations then propagated away from the center of the condensate as sound waves.

The lower image was taken at lower condensate density where the speed of sound was smaller.

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