Fundamental limitation of ultrastrong coupling between light and atoms

TL;DR

This paper explores the fundamental limits of ultrastrong light-atom coupling, revealing that superradiant criticality is closely linked to molecular and crystalline phase boundaries.

Contribution

It introduces a tractable polarization field model within a generalized gauge framework to analyze light-matter interaction limitations.

Findings

Superradiant criticality is near covalent molecule formation.

Electrostatic dipole-dipole interactions between atoms are canceled.

Framework helps determine fundamental limits of light-matter coupling.

Abstract

In a recent work of ours [Phys. Rev. Lett. 112, 073601 (2014)], we generalized the Power-Zineau-Woolley gauge to describe the electrodynamics of atoms in an arbitrary confined geometry. Here we complement the theory by proposing a tractable form of the polarization field to represent atomic material with well-defined intra-atomic potential. The direct electrostatic dipole-dipole interaction between the atoms is canceled. This theory yields a suitable framework to determine limitations on the light-matter coupling in quantum optical models with discernible atoms. We find that the superradiant criticality is at the border of covalent molecule formation and crystallization.