The new process of quantum injection into an optical parametric amplifier operating in entangled configuration is adopted to “amplify” into a large dimensionality spin- $\frac{1}{2}$ Hilbert space the quantum entanglement and superposition properties of the photon couples generated by parametric down-conversion. The structure of the Wigner function and of the field's correlation functions shows a decoherence-free multiphoton Schroedinger-cat behavior of the emitted field which is largely detectable against the squeezed-vacuum noise.

• Received 5 March 1998

DOI:https://doi.org/10.1103/PhysRevLett.81.2842