Far from equilibrium field theory for strongly coupled light and matter: dynamics of frustrated multi-mode cavity QED

Light-matter interfaces have now entered a new stage marked by the ability to engineer quantum correlated states under driven-dissipative conditions. To propel this new generation of experiments, we are confronted with the need to model non-unitary many-body dynamics in strongly coupled regimes, by transcending traditional approaches in quantum optics. In this work, we contribute to this program by adapting a functional integral technique, conventionally employed in high-energy physics, in order to obtain non-equilibrium dynamics for interacting light-matter systems. Our approach is grounded in constructing 'two-particle irreducible' (2PI) effective actions, which provide a non-perturbative and conserving framework for describing quantum evolution at a polynomial cost in time. We apply our method to complement the analysis of spin glass formation in the context of frustrated multi-mode cavity quantum electrodynamics, initiated in our accompanying work [H. Hosseinabadi, D. Chang, J. Marino, arXiv:2311.05682]. Finally, we outline the capability of the technique to describe other near-term platforms in many-body quantum optics, and its potential to make predictions for this new class of experiments.