A schematic diagram of the first ECP. The single photons in the spatial modes b₁ and b₂ can make the coherent state pick up the phase shifts of −θ/N and θ, respectively. BS means 50:50 beam splitter. After the BS, the output photons in the spatial modes d₁ and d₂ will be detected by the single-photon detectors D₁ and D₂, respectively.