  We report on the observation of broadband (40 THz) bright twin beams through
high-gain parametric down-conversion in an aperiodically poled lithium niobate
crystal. The output photon number is shown to scale exponentially with the pump
power and not with the pump amplitude, as in homogeneous crystals.
Photon-number correlations and the number of frequency/temporal modes are
assessed by spectral covariance measurements. By using sum-frequency generation
on the surface of a non-phasematched crystal, we measure a cross-correlation
peak with the temporal width 90 fs.
