  We demonstrate, theoretically and experimentally, that statistical mixtures
of the vacuum state |0> and the single-photon Fock state |1> are nonclassical
according to the Vogel criterion (W. Vogel, Phys. Rev. Lett. 84, 1849 (2000)),
regardless of the vacuum fraction. The ensembles are synthesized via
conditional measurements on biphotons generated by means of parametric
downconversion, and their quadrature statistics are measured using balanced
homodyne detection. A comparative review of various quantum state
nonclassicality criteria is presented.
