Exact evaluation of entropic quantities in a solvable two-particle model

It has long been known that the von Neumann entropy SN and the Jozsa–Robb–Wootters subentropy QJRW [R. Jozsa, et al., Phys. Rev. A 49 (1994) 668] are, respectively, upper and lower bounds on the accessible information one can obtain about the identity of a pure state by performing a quantum measurement on a system whose pure state is initially unknown. We determine these bounds exactly in terms of the occupation numbers of normalized natural orbitals of an externally confined interacting two-particle model system. The occupation numbers are obtained via a sign-correct direct decomposition of the underlying exact Schrödinger wave function in terms of an infinite sum of products of Löwdin's natural orbitals, avoiding thus the solution of the eigenvalue problem with the corresponding reduced one-particle matrix.