Collective resonances in $\chi^{(3)}$; a QED study

TL;DR

This paper presents a quantum electrodynamics (QED) analysis of collective resonances in the third-order susceptibility of two interacting atoms, revealing features observable in transmission spectroscopy that are missed by simpler models.

Contribution

It provides a full diagrammatic QED calculation of collective $ ext{chi}^{(3)}$ resonances, highlighting effects beyond effective interatomic coupling models.

Findings

QED corrections produce observable collective two-photon absorption resonances.

Full QED treatment reveals features absent in effective coupling models.

Predicted signals can be observed in transmission spectroscopy with shaped broadband pulses.

Abstract

We calculate the third order susceptibility $\chi^{(3)}$ of a pair of 2 level atoms which interact via the exchange of photons. QED corrections to second order in coupling to vacuum field modes yield collective two photon absorption resonances which can be observed in transmission spectroscopy of shaped broadband pulses. While some collective effects can be obtained by introducing an effective interatomic coupling using a quantum master equation, the predicted signals contain clear features that are missed by that level of theory and require a full diagrammatic QED treatment.