  The interaction energy between two atoms is crucially dependent on the
quantum state of the two-atom system. In this paper, it is demonstrated that a
steady resonance interaction energy between two atoms exists when the atoms are
in a certain type of coherent superposition of single-excitation states. The
interaction is tree-level classical in the sense of the Feynman diagrams. A
quantity called quantum classicality is defined in the present paper, whose
nonzero-ness ensures the existence of this interaction. The dependence of the
interatomic interaction on the quantum nature of the state of the two-atom
system may potentially be tested with Rydberg atoms.
