  A patterned Si nanobeam is formed which supports co-localized acoustic and
optical resonances that are coupled via radiation pressure. Starting from a
bath temperature of T=20K, the 3.68GHz nanomechanical mode is cooled into its
quantum mechanical ground state utilizing optical radiation pressure. The
mechanical mode displacement fluctuations, imprinted on the transmitted cooling
laser beam, indicate that a final phonon mode occupancy of 0.85 +-0.04 is
obtained.
