Experimental observation of the Bogoliubov transformation for a Bose-Einstein condensed gas

Summary form only given. Phonons with wavevector q//spl planck/-were optically imprinted into a Bose-Einstein condensate. Their momentum distribution was analyze using Bragg spectroscopy with a high momentum transfer. The wavefunction of these elementary excitations was shown to be a superposition of +q and -q free particle momentum states, in agreement with the Bogoliubov quasiparticle picture.