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