Experimental demonstration of sub-shot-noise intensity correlations in an intense twin beam

Twin-beam states of light produced by spontaneous parametric downconversion are endowed with sub-shot-noise photon-number correlations at any intensity regime. We have demonstrated sub-shot-noise photon-number correlations in a ps-pulsed intense (more than 1000 photons) twin beam by measuring the variance of the difference in the number of photons detected in the two outputs of the crystal. The variance is 3.25 dB below the shot-noise level. Measurements show that detection of sub-shot-noise correlation requires a careful selection of twin coherence areas and that these areas depend on the pump intensity.