Figure 6.

Interference in the interaction with delocalized modes. For the two-wavepacket beam configuration of Figure 5c and a sample that has lateral translational invariance, a single electron of split wave function emits in-plane polaritons and transition radiation with an intensity that is insensitive to the longitudinal and lateral wavepacket separations a and b. This is in contrast to the emission intensity observed when each wavepacket is populated by one electron (two-electron solid curve) or when considering two electrons with each of them equally shared among the two wavepackets (two-electron dashed curve). We adopt the same beam parameters as in Figure 5 (see also Methods).