Summary form only given. With the availability of ultrafast optical measurements, renewed interest was devoted to the properties of the secondary emission from quantum well excitons. It has been shown recently by many experiments, and in particular by means of spectral interferometry, that the early-time secondary emission at low temperature and exciton density is dominated by resonant Rayleigh scattering (RRS) induced by the localized exciton states subject to disorder. On the basis of speckle averaging up-conversion measurements, it has been claimed that RRS is the dominant signal even up to 20 ps. Such long-lived RRS signal cannot be explained by models that make use of classical exciton center-of-mass propagation.