  We study a coherent superposition of field annihilation and creation operator
acting on continuous variable systems and propose its application for quantum
state engineering. Specifically, it is investigated how the superposed
operation transforms a classical state to a nonclassical one, together with
emerging nonclassical effects. We also propose an experimental scheme to
implement this elementary coherent operation and discuss its usefulness to
produce an arbitrary superposition of number states involving up to two
photons.
