  Generating nonclassical states of mechanical systems is a challenge relevant
for testing the foundations of quantum mechanics and developing quantum
technologies. Significant effort has been made to search for such states in the
stationary behaviour of cavity optomechanical systems. We focus instead on the
transient dynamics. We find that in the strong coupling regime the presence of
an optical drive causes an initial mechanical coherent state to evolve to a
nonclassical state, with strongly negative Wigner function. An analytical
treatment for weak drive reveals that these states are quantum superpositions
of coherent states. Numerical simulation shows that the presence of Wigner
negativity is robust against weak dissipation.
