Quantum beats in picosecond time-resolved resonance Quorescence are investigated in Cu20 for the yellow 1S quadrupole exciton polariton. In order to obtain information on the coherence of the resonant intermediate polariton states, we have studied the electric-dipole allowed lightscattering processes involving one and two I'3 phonons. A quantitative analysis provides excitonpolariton coherence times of about 1 ns at 2 K, decreasing with higher temperatures and larger wave vectors due to phonon scattering. From the magnetic-field dependence, low-field g values are determined with high accuracy. The measurements permit a clear distinction between Raman and hot-luminescence-like contributions to the scattering intensity. The strong eKect of the spectral width of the excitation pulse on the time behavior of the scattering, as well as the temperature dependence of the coherence time, indicate that it is essential to take into account the polariton character.