Experimental realization of macroscopic coherence by phase-covariant cloning of a single photon

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 nonlocal transfer of coherence between the injected qubit and the mesoscopic amplified output field involving up to 4000 photons has been investigated. A probabilistic method to extract full information about the multiparticle output wave function has been implemented.