  Multimode nonclassical states of light are an essential resource in quantum
computation with continuous variables, for example in cluster state
computation. They can be generated either by mixing different squeezed light
sources using linear optical operations, or directly in a multimode optical
device. In parallel, frequency combs are perfect tools for high precision
metrological applications and for quantum time transfer. Synchronously Pumped
Optical Parametric Oscillators (SPOPOs) have been theoretically shown to
produce multimode non-classical frequency combs. In this paper, we present the
first experimental generation and characterization of a femtosecond quantum
frequency comb generated by a SPOPO. In particular, we give the experimental
evidence of the multimode nature of the generated quantum state and, by
studying the spectral noise distribution of this state, we show that at least
three nonclassical independent modes are required to describe it.
