Anatomy of entanglement and nonclassicality criteria

Abstract: We examine the internal structure of two-mode entanglement criteria for quadrature and number-phase squeezed states. (i) We first address entanglement criteria obtained via partial transpose of Schrodinger-Robertson inequality, where presence of an extra term makes them stronger. We show that this extra term performs an optimization within two intra-mode rotations, for the variables mixing in the entanglement criterion. We demonstrate this situation both for quadrature and number-phase squeezed states. (ii) We realize that Simon's criterion performs this optimization automatically. (iii) Hence, we derive a Simon-like criterion for number-phase squeezed states which performs this optimization in the number-phase plane. The method can be generalized also for other states, e.g., amplitude-squeezed states. In addition to examining the intra-mode rotations, (iv) we also present an entanglement scheme in terms of product of the noises of the two modes, i.e., noise-area. We question, analytically and numerically, whether the well-known entanglement criteria are actually search mechanisms for a noise-area below unity. In this regard, we also numerically show the following. We consider an entangled state. We minimize the noise-area for the product form of the Duan-Giedke-Cirac-Zoller criterion (Mancini {\it et. al.}) with respect to the intra-mode rotations. We observe that this noise-area (actually) is the input nonclassicality that a beam-splitter necessitates in order to generate exactly the amount of entanglement our particular state possesses. (vi) Additionally, we define an alternative entanglement measure for Gaussian states which can also be adapted to multimode entanglement. We further raise some intriguing questions, e.g., on the existence of an easier definition of an entanglement depth for number-phase squeezed states.