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

Phonons with wavevector $q\mathrm{/}\mathrm{\hslash }$ were optically imprinted into a Bose-Einstein condensate. Their momentum distribution was analyzed using Bragg spectroscopy with a high momentum transfer. The wavefunction of the phonons was shown to be a superposition of +q and -q free particle momentum states, in agreement with the Bogoliubov quasiparticle picture.