We introduce a new quantity for describing nonclassicality of an arbitrary optical two-mode Gaussian state which remains invariant under any global photon-number preserving unitary transformation of the covariance matrix of the state. The invariant naturally splits into an entanglement monotone and local-nonclassicality quantifiers applied to the reduced states. This shows how entanglement can be converted into local squeezing and vice versa. Twin beams and their transformations at a beam splitter are analyzed as an example providing squeezed light. An extension of this approach to pure three-mode Gaussian states is given.

Faculty of Physics Adam Mickiewicz University PL 61 614 Poznan Poland

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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