Resonant Cascaded Down-Conversion
  We analyze an optical parametric oscillator (OPO) in which cascaded down-conversion occurs inside a cavity resonant for all modes but the initial pump. Due to the resonant cascade design, the OPO present two \chi^2 level oscillation thresholds that are therefore remarkably lower than for a \chi^3 OPO. This is promising for reaching the regime of an effective third-order nonlinearity well above both thresholds. Such a \chi^2 cascaded device also has potential applications in frequency conversion to far infra-red regimes. But, most importantly, it can generate novel multi-partite quantum correlations in the output radiation, which represent a step beyond squeezed or entangled light. The output can be highly non-Gaussian, and therefore not describable by any semi-classical model. 
Christian Weedbrook, Ben Perrett, Karen V. Kheruntsyan, Peter D.   Drummond, Raphael C. Pooser, Olivier Pfister