  We demonstrate the generation of multi-photon quantum states of light by
cavity-enhanced parametric down-conversion in the high-repetition-rate pulsed
regime. An external enhancement cavity resonant with the spectral comb of modes
of a mode-locked pump laser provides a coherent build-up of the pump intensity
and greatly enhances the parametric gain without sacrificing its high
repetition rate and comb structure. We probe the parametric gain enhancement by
the conditional generation and tomographic analysis of two-photon Fock states.
Besides its potential impact to efficiently generate highly-nonclassical or
entangled multi-photon states in many existing experimental setups, this scheme
opens new and exciting perspectives towards the combination of quantum and comb
technologies for enhanced measurements and advanced quantum computation
protocols.
