  Quantum lithography proposes to adopt entangled quantum states in order to
increase resolution in interferometry. In the present paper we experimentally
demonstrate that the output of a high-gain optical parametric amplifier can be
intense yet exhibits quantum features, namely, sub-Rayleigh fringes, as
proposed by Agarwal et al. (Phys. Rev. Lett. 86, 1389 (2001)). We investigate
multiphoton states generated by a high-gain optical parametric amplifier
operating with a quantum vacuum input for a gain values up to 2.5. The
visibility has then been increased by means of three-photon absorption. The
present article opens interesting perspectives for the implementation of such
an advanced interferometrical setup.
