A robust verification of the quantum nature of light

We present a conditional experiment involving a parametric amplifier and an avalanche photodetector to generate highly nonclassical states of the radi ation field. The nonclassicality is robust against amplifier gain, detector efficiency and dark counts. At the output all the generalized Wigner funct ions have negative values, and this is exploited in order to reveal the non classicality through quantum homodyne tomography. (C) 2001 Elsevier Science B.V. All rights reserved.