[abdullahkhalids]

I drew a schematic using https://asciiflow.com/

                                        +---------------------+
                                        | Heralding detector  |
                                        |                     |
                                        +---------------------+
                                       /
                                      /
                       +----------+  /  Photon 1
                       |          | /
    Laser beam ------> | Crystal  | ------------> Outgoing laser beam
                       |          | \
                       +----------+  \ Photon 2
                                      \
                                       \
                                        To human eye