Information, complexity, and quantum entanglement on doubly excited states of helium atom

Abstract

The electronic density ρ(r) in atoms, molecules and solids is, in general, a distribution that can be observed experimentally, containing spatial information projected from the total wave function. These density distributions can be though as probability distributions subject to the scrutiny of the analytical methods of information theory, namely, entropy measures, quantifiers for the complexity, or entanglement measures. Although the classification of resonant doubly excited states of He-like atoms in terms of labels of approximate quantum numbers have not been exempt from controversies, a well known proposal follows after the works by Herrick and Sinanoglu and Lin, with a labeling based on K, T, and A numbers for the Rydberg series of increasing n2 and for a given ionization He+ (N = n1). In this work we intend to justify this kind of classification from the topological analysis of the one-particle ρ(r) and two-particle \r{ho}(r1,r2) density distributions of the localized part of the resonances (computed with a Feshbach projection formalism and configuration interaction wave functions in terms of B-splines bases), using global quantifiers (Shannon) as well as local ones (Fisher information). We also studied measures for the entanglement using the von Neumann and linear entropies computed from the reduced one-particle density matrix within our correlated configuration interaction approach. We find in this study that global measures like the Shannon entropy hardly distinguishes among resonances in the whole Rydberg series. On the contrary, measures like the Fisher information, von Neumann and linear entropies are able to qualitatively discriminate the resonances, grouping them according to their (K, T)A labels.