Cavity quantum electrodynamics (QED) manipulates the coupling of light with matter, and allows several emitters to couple coherently with one light mode1. However, even in a many-body system, the light–matter coupling mechanism has so far been restricted to one-body processes. Leveraging cavity QED for the quantum simulation of complex, many-body systems has thus far relied on multi-photon processes, scaling down the light–matter interaction to the low energy and slow time scales of the many-body problem2,3,4,5. Here we report cavity QED experiments using molecular transitions in a strongly interacting Fermi gas, directly coupling cavity photons to pairs of atoms. The interplay of strong light–matter and strong interparticle interactions leads to well-resolved pair polaritons—hybrid excitations coherently mixing photons, atom pairs and molecules. The dependence of the pair-polariton spectrum on interatomic interactions is universal, independent of the transition used, demonstrating a direct mapping between pair correlations in the ground state and the optical spectrum. This represents a magnification of many-body effects by two orders of magnitude in energy. In the dispersive regime, it enables fast, minimally destructive measurements of pair correlations, and opens the way to their measurement at the quantum limit and their coherent manipulation using dynamical, quantized optical fields.
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All data files are available from the corresponding author upon request. Accompanying data, including those for figures, are available from the Zenodo repository (https://doi.org/10.5281/zenodo.489675710.5281/zenodo.4896757).
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We thank T. Donner for discussions and careful reading of the manuscript; R. Hulet, P. Julienne and J. Hutson for discussions; C. Vale, H. Hu and J. Drut for providing the contact data; and T. Zwettler for experimental assistance. We acknowledge funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 714309), the Swiss National Science Foundation (grant no. 184654), the Sandoz Family Foundation-Monique de Meuron program for Academic Promotion and EPFL.
The authors declare no competing interests.
Peer review information Nature thanks Hui Zhai, Florian Schreck and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
a, Spectrum of PA4 at 832 G averaged over three realizations. b, Spectrum reconstructed by equation (2) using the fit results. The solid and dashed lines indicate the fitted positions of the PA resonance and the dispersively shifted cavity resonance. The colour scale is identical to that in the main text.
Positions of PA1–PA4 (green open circles, purple filled diamonds, orange open squares and light blue crosses, respectively) as a function of magnetic fields. The value at 730 G is subtracted for clarity. Linear fits presented by the solid lines yield 0.31, 0.67, 0.89 and −0.83 MHz G−1 for the four PA resonances, respectively.
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Konishi, H., Roux, K., Helson, V. et al. Universal pair polaritons in a strongly interacting Fermi gas. Nature 596, 509–513 (2021). https://doi.org/10.1038/s41586-021-03731-9