We study low momentum excitations of a Bose-Einstein condensate using a novel
matter-wave interference technique. In time-of-flight expansion images we
observe strong matter-wave fringe patterns. The fringe contrast is a sensitive
spectroscopic probe of in-trap phonons and is explained by use of a Bogoliubov
excitation projection method applied to the rescaled order parameter of the
expanding condensate. Gross-Pitaevskii simulations agree with the experimental
data and confirm the validity of the theoretical interpretation. We show that
the high sensitivity of this detection scheme gives access to the quantized
quasiparticle regime.Comment: 5 pages, 5 figures, author list update