  The entanglement properties of two-electron atomic systems have been the
subject of considerable research activity in recent years. These studies are
still somewhat fragmentary, focusing on numerical computations on particular
states of systems such as Helium, or on analytical studies of model-systems
such as the Moshinsky atom. Some general trends are beginning to emerge from
these studies: the amount of entanglement tends to increase with energy and, in
the case of excited states, entanglement does not necessarily tend to zero in
the limit of vanishing interaction between the two constituting particles. A
physical explanation of these properties, shared by the different two-electrons
models investigated so far, is still lacking. As a first step towards this goal
we perform here, via a perturbative approach, an analysis of entanglement in
two-electrons models that sheds new light on the physical origin of the
aforementioned features and on their universal character.
