In contrast, for the LP wavepacket, Fig. shows a more gradual expansion of the polariton wavepacket. For instance, at t = 45 fs, the LP wavepacket (red curves) spans a width ranging from about 18 µm to 22 µm only, which is similar to the width of the corresponding dark-state wavepacket at t = 45 fs. This is because the LP wavepacket advances at a rate comparable to the rate of LP to dark state transition, resulting in a synchronized expansion of both LP and dark exciton wavepacket. In addition, the interference contribution for MSD is destructive, which further reduces the transient MSD (see Fig. in Supporting Information).