Entanglement and nonclassicality in four-mode Gaussian states generated   via parametric down-conversion and frequency up-conversion
  Multipartite entanglement and nonclassicality of four-mode Gaussian states generated in two simultaneous nonlinear processes involving parametric down-conversion and frequency up-conversion are analyzed assuming the vacuum as the initial state. Suitable conditions for the generation of highly entangled states are found. Transfer of the entanglement from the down-converted modes into the up-converted ones is also suggested. The analysis of the whole set of states reveals that sub-shot-noise intensity correlations between the equally-populated down-converted modes, as well as the equally-populated up-converted modes, uniquely identify entangled states. They represent a powerful entanglement identifier also in other cases with arbitrarily populated modes. 
Ievgen I. Arkhipov, Jan Pe\v{r}ina Jr., Ond\v{r}ej Haderka, Alessia   Allevi, Maria Bondani
quant-ph