Nonlinear quantum beats of propagating polaritons

We observe nonequidistant oscillations in the correlation trace of the nonlinear signal in a four-wave mixing experiment when exciting the upper polariton branch between the ${\mathit{A}}_{\mathit{n}=1\mathrm{}}$ and the ${\mathit{A}}_{\mathit{n}=2\mathrm{}}$ excitons in CdSe. The quantum beats are described qualitatively and quantitatively taking into account propagation interference effects on the third-order nonlinear polarization, and the homogeneous dampings of the exciton polaritons are determined.