entanglement indicators obtained from quantum tomograms'
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[*Keywords*]{}: Energy-level crossings, Quantum entanglement, Tomograms, Tomographic entanglement indicator

\[sec:intro\]Introduction
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The measurement of any observable in a quantum mechanical system yields a histogram of the state of the system in the basis of that observable. In particular, in the context of quantum optics, measurements of a judiciously chosen quorum of field observables yield a set of histograms (the optical tomogram) from which the density matrix is reconstructed. The latter is needed in standard procedures for estimating the extent of entanglement between the subsystems of bipartite or multipartite systems. A standard measure of the entanglement between the two subsystems of a bipartite system is the subsystem von Neumann entropy $\xi_{\textsc{svne}}=
-\Tr\,(\rho \,\log_{2} \,\rho)$, where $\rho$ is the density matrix of either one of the subsystems [@niel]. In the case of continuous-variable (CV) quantum systems, an infinite set of histograms is required, in principle, in order to obtain complete information about the density matrix. In practice, however, only a finite set of histograms (corresponding to measurement of a finite set of observables) can be obtained. As state reconstruction from tomograms [@QSRecon] typically involves error-prone statistical techniques such as maximum likelihood estimates, it is preferable 