  We show that a continuous range of nonclassical states of light can be
generated using conditional measurements on the idler mode of an optical
parametric amplifier. The output state is prepared by introducing a coherent
state in the signal mode of the amplifier with a single photon in the idler
mode, followed by a conditional measurement of a single photon in the output
idler mode. By varying the gain of the amplifier, this approach can produce a
coherent state, a photon-added state, a displaced number state, or a continuous
range of other nonclassical states with intermediate properties. We note that
this approach can generate a photon-added state even though the post-selected
amplifier does not add any photons to the signal or idler modes. The ability to
generate a continuous range of nonclassical states may have practical
applications in quantum information processing.
