Beyond the Tavis-Cummings model: revisiting cavity QED with atomic ensembles

TL;DR

This paper revisits the Tavis-Cummings model in cavity QED with atomic ensembles, providing conditions for its validity, deriving a more general Hamiltonian, and demonstrating significant deviations in dense ensembles through experimental data.

Contribution

The authors derive a generalized Hamiltonian for dense atomic ensembles in cavity QED, extending beyond the traditional Tavis-Cummings model, and validate it with experimental results.

Findings

The Tavis-Cummings model is only valid for low optical depth ensembles.

A more general Hamiltonian accounts for cascaded photon-atom interactions.

Experimental data shows deviations from Tavis-Cummings predictions in dense ensembles.

Abstract

The interaction of an ensemble of $N$ two-level atoms with a single mode electromagnetic field is described by the Tavis-Cummings model. There, the collectively enhanced light-matter coupling strength is given by $g_N = \sqrt{N} \bar{g}_1$, where $\bar{g}_1$ is the ensemble-averaged single-atom coupling strength. Formerly, this model has been employed to describe and to analyze numerous cavity-based experiments. Here, we show that this is only justified if the effective scattering rate into non-cavity modes is negligible compared to the cavity's free-spectral range. In terms of experimental parameters, this requires that the optical depth of the ensemble is low, a condition that is violated in several state-of-the-art experiments. We give quantitative conditions for the validity of the Tavis-Cummings model and derive a more general Hamiltonian description that takes into account the cascaded interaction of the photons with all consecutive atoms. We show that the predictions of our model can differ quantitatively and even qualitatively from those obtained with the Tavis-Cummings model. Finally, we present experimental data, for which the deviation from the predictions of the Tavis-Cummings model is apparent. Our findings are relevant for all experiments in which optically dense ensembles of quantum emitters are coupled to an optical resonator.