The operational principle of the photon scattering and entanglement processes. A single-photon wave packet is predominantly reflected by elastic scattering on a two-level emitter, while the two-photon wave packet can be inelastically scattered in the forward direction, thereby generating energytime entanglement. The entanglement is probed using two UMZIs (Fig. 2 ). Each UMZI is used to realize time projections onto the superposition state (\frac{\left\vert s\right\rangle +{{\mathrm{e}}}^{i\phi }\left\vert l\right\rangle }{\sqrt{2}}), as illustrated on a Bloch sphere, where (\left\vert s\right\rangle) ( (\left\vert l\right\rangle) ) corresponds to a photon taking the short (long) path, and ϕ is the phase setting of the UMZI. ϕ = 0 (blue vector) and ϕ = π (red vector) refer to the two settings for data in b .
