  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.
