High-gain parametric down-conversion (PDC) is a source of bright squeezed vacuum, which is a macroscopic nonclassical state of light and a promising candidate for quantum information applications. Here we study its properties, such as the intensity spectral width and the spectral width of pairwise correlations. In agreement with the theory, we observe an increase in the spectral width by 27% compared with the low-gain PDC. Frequency cross- and auto-correlations are registered by measuring the reduction of noise in the difference of PDC intensities at various pairs of wavelengths. The noise reduction plots also demonstrate super-bunching typical for collinear frequency-degenerate PDC.

The rate of an n-photon effect generally scales as the nth order autocorrelation function of the incident light, which is high for light with strong photon-number fluctuations. Therefore, "noisy" light sources are much more efficient for multiphoton effects than coherent sources with the same mean power, pulse duration, and repetition rate. Here we generate optical harmonics of the order of 2-4 from a bright squeezed vacuum, a state of light consisting of only quantum noise with no coherent component. We observe up to 2 orders of magnitude enhancement in the generation of optical harmonics due to ultrafast photon-number fluctuations. This feature is especially important for the nonlinear optics of fragile structures, where the use of a noisy pump can considerably increase the effect without overcoming the damage threshold.

In this Letter, we experimentally study the statistical properties of a bright squeezed vacuum state containing up to 10(13) photons per mode (10 μJ per pulse), produced via high-gain parametric down conversion (PDC). The effects of bunching and superbunching of photons were observed for a single-mode PDC radiation by second-order intensity correlation function measurements with analog detectors.