Figure 1
From: Exploration of multiphoton entangled states by using weak nonlinearities

The schematic diagram of creating multiphoton entangled states with linear optics and weak nonlinearities.
are input ports and each port is supplied with an arbitrary single-photon state, while
are the corresponding outputs, respectively. θ and 2θ represent phase shifts in the coherent probe beam
induced by Kerr interaction between photons.
is a phase shift gate. Each
represents a phase shift on the signal photons based on classical feed-forward information.