Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Quantum entanglement for two electrons in the excited states of
  helium-like systems   The quantum entanglement for the two electrons in the excited states of the
helium-like atom/ions is investigated using the two-electron wave functions
constructed by the B-spline basis. As a measure of the spatial
(electron-electron orbital) entanglement, the von Neumann entropy and linear
entropy of the reduced density matrix are calculated for the 1s2s 1,3S excited
states for systems with some selected Z values from Z=2 to Z=100. Results for
the helium atom are compared with other available calculations. We have also
investigated the entropies for these excited states when the nucleus charge is
reduced from Z=2 to Z=1. At such a critical charge, all the singly-excited
states of this system become unbound, and the linear entropies and the von
Neumann entropies for the excited states are approaching 1/2 and 1,
respectively, the limits for the entropies when one electron is bound to the
nucleus, and the other being free.

1
The categories of this paper are: cs
6
The categories of this paper are: astro-ph
30
The categories of this paper are: q-alg
9
The categories of this paper are: nlin
5
The categories of this paper are: gr-qc
8
The categories of this paper are: hep-ex
32
The categories of this paper are: atom-ph
15
The categories of this paper are: nucl-ex
33
The categories of this paper are: plasm-ph
10
The categories of this paper are: q-bio
4
The categories of this paper are: cond-mat
7
The categories of this paper are: hep-th
26
The categories of this paper are: bayes-an
21
The categories of this paper are: adap-org
31
The categories of this paper are: ao-sci
16
The categories of this paper are: stat