  Cavity quantum electrodynamics (QED) manipulates the coupling of light with
matter, and allows for several emitters to couple coherently with one light
mode. However, even in a many-body system, the light-matter coupling mechanism
was so far 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 problem. Here we report on 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 inter-particle 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 towards their measurements at the quantum limit and their coherent
manipulation using dynamical, quantized optical fields.
