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.

Dipartimento di Scienza e Alta Tecnologia Università degli Studi dell'Insubria Via Valleggio 11 22100 Como Italy

Institute of Physics of CAS Joint Laboratory of Optics of Palacký University and Institute of Physics 17 listopadu 50a 771 46 Olomouc Czech Republic

Istituto di Fotonica e Nanotecnologie Consiglio Nazionale delle Ricerche Via Valleggio 11 22100 Como Italy

RCPTM Joint Laboratory of Optics of Palacký University and Institute of Physics of the Czech Academy of Sciences 17 listopadu 12 77146 Olomouc Czech Republic

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