  We demonstrate coherent control of population transfer between vibrational
states in an optical lattice by using interference between a one-phonon
transition at $2\omega$ and a two-phonon transition at $\omega$. The $\omega$
and $2\omega$ transitions are driven by phase- and amplitude-modulation of the
lattice laser beams, respectively. By varying the relative phase between these
two pathways, we control the branching ratio of transitions to the first
excited state and to the higher states. Our best result shows an improvement of
the branching ratio by a factor of 3.5$\pm$0.7. Such quantum control techniques
may find broad application in suppressing leakage errors in a variety of
quantum information architectures.
