A quantum electrodynamic calculation of the interaction of an excited-state atom with a ground-state atom is performed. For an excited reference state and a lower-lying virtual state, the contribution to the interaction energy naturally splits into a pole term and a Wick-rotated term. The pole term is shown to dominate in the long-range limit, altering the functional form of the interaction from the retarded 1/R7 Casimir-Polder form to a long-range tail - provided by the Wick-rotated term - proportional to cos[2(Em - En)R/(ħc)]/R2, where Em < En is the energy of a virtual state, lower than the reference-state energy En, and R is the interatomic separation. General expressions are obtained which can be applied to atomic reference states of arbitrary angular symmetry. Careful treatment of the pole terms in the Feynman prescription for the atomic polarizability is found to be crucial in obtaining correct results.




This research was supported by the National Science Foundation (Grant No. PHY-1403973).

Keywords and Phrases

Atoms; Electrodynamics; Ground state; Poles; Quantum electronics; Quantum theory; Van der Waals forces, Atomic polarizability; Functional forms; General expression; Ground-state atoms; Interaction energies; Interatomic separation; Quantum electrodynamics; Van Der Waals interactions, Excited states

International Standard Serial Number (ISSN)

2469-9926; 2469-9934

Document Type

Article - Journal

Document Version

Final Version

File Type





© 2017 American Physical Society (APS), All rights reserved.

Publication Date

01 Apr 2017

Included in

Physics Commons