We report preparation and characterization of coherent superposition states $t|0〉+\alpha |1〉$ of the electromagnetic field by conditional measurements on a beam splitter. This state is generated in one of the beam splitter output channels if a coherent state $|\alpha 〉$ 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.

• Received 27 February 2002

DOI:https://doi.org/10.1103/PhysRevLett.88.250401