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.