  We report preparation and characterization of coherent superposition states t
|0> + alpha |1> of electromagnetic field by conditional measurements on a
beamsplitter. The state is generated in one of the beam splitter output
channels if a coherent state and a single-photon Fock state |1> are present in
the two input ports and a single photon is registered in the other beam
splitter output. The single photon thus plays a role of a "catalyst": it is
explicitly present in both the input and the output channels of the interaction
yet facilitates generation of a nonclassical state of light.
