Coherent propagation and quantum beats of quadrupole polaritons in Cu_{2}O

Coherent propagation of quadrupole polaritons in ${\mathrm{Cu}}_2$O is demonstrated by time-resolved spectroscopy. This manifests itself in a strong distortion of the temporal shape of a picosecond optical pulse which is in resonance with the 1S exciton polariton. Oscillations with time-dependent period in the transmitted light intensity are quantitatively explained as resulting from quantum beats between the two branches of the exciton polariton. The analysis yields the homogeneous linewidth and oscillator strength.