  Polaritons are quasi-particles originating from the coupling of light with
matter that demonstrated quantum phenomena at the many-particle mesoscopic
level, such as BEC and superfluidity. A highly sought and long-time missing
feature of polaritons is a genuine quantum manifestation of their dynamics at
the single-particle level. Although they are conceptually perceived as
entangled states and theoretical proposals abound for an explicit manifestation
of their single-particle properties, so far their behaviour has remained fully
accountable for by classical and mean-field theories. In this Article, we
report the first experimental demonstration of a genuinely-quantum
manifestation of microcavity polaritons, by swapping, in a two-photon entangled
state generated by parametric down-conversion, a photon for a polariton.
Furthermore, we show how single polaritons are affected by polariton-polariton
interactions in a propaedeutic demonstration of their qualities for quantum
information applications.
