The low-momentum excitations of a Bose-Einstein condensate (BEC), using a novel matter-wave interference technique, was studied. The dynamics of phonons in freely expanding condensates was analyzed using both the Gross-Pitaevskii equation (GPE) and a dynamically rescaled Bogoliubov theory. It was found that axial low-momentum phonons were adiabatically converted by the expansion into free atoms with the same axial momentum. A strong matter-wave fringe patterns was observed in time-of-flight expansion images. It was shown that the fringe visibility was a sensitive spectroscopic probe of in-trap phonons, explained by use of Bogoliubov excitation projection method applied to the rescaled order parameter of the expanding condensate.
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This work was supported in part by the Israel Ministry of Science, the Israel Science Foundation, and the DIP Foundation.