  In a new branch of quantum computing, information is encoded into coherent
states, the primary carriers of optical communication. To exploit it, quantum
bits of these coherent states are needed, but it is notoriously hard to make
superpositions of such continuous-variable states. We have realized the
complete engineering and characterization of a qubit of two optical
continuous-variable states. Using squeezed vacuum as a resource and a special
photon subtraction technique, we could with high precision prepare an arbitrary
superposition of squeezed vacuum and a squeezed single photon. This could lead
the way to demonstrations of coherent state quantum computing.
