Formation of matter-wave polaritons in an optical lattice

The polariton—a quasiparticle formed by the strong coupling of a photon to a matter excitation—is a fundamental ingredient of emergent photonic quantum systems ranging from semiconductor nanophotonics to circuit quantum electrodynamics. Exploiting the interaction between polaritons has led to the realization of superfluids of light as well as of strongly correlated phases in the microwave domain, with similar efforts underway for microcavity excitons–polaritons. Here we develop an ultracold-atom analogue of an exciton–polariton system in which interacting polaritonic phases can be studied with full tunability and in the absence of dissipation. In our optical lattice system, the exciton is replaced by an atomic excitation, whereas an atomic matter wave is substituted for the photon under a strong dynamical coupling between the two constituents that hybridizes the two dispersion relations. We spectroscopically access the band structure of the matter-wave polariton by coupling the upper and lower polariton branches, as well as explore polaritonic transport in the superfluid and Mott-insulating regimes, finding quantitative agreement with our theoretical expectations. Our work sheds light on fundamental polariton properties and related many-body phenomena, and opens up novel possibilities for studies of polaritonic quantum matter.