  We investigate the multiphoton states generated by high-gain optical
parametric amplification of a single injected photon, polarization encoded as a
''qubit''. The experiment configuration exploits the optimal phase-covariant
cloning in the high gain regime. The interference fringe pattern showing the
non local transfer of coherence between the injected qubit and the mesoscopic
amplified output field involving up to 4000 photons has been investigated. A
probabilistic new method to extract full information about the multiparticle
output wavefunction has been implemented.
