[abdullahkhalids]

There is a process called spontaneous parametric downconversion (SPDC), where if you shine laser of frequency f at a crystal, it will with small probability p emit two photons of frequency f/2 (energy conservation in play here).

If done correctly, the outgoing laser light and the two photons all travel in different directions and so can be separated and further directed using mirrors or optical fiber cables.

Because the process is non-deterministic, what we usually do is direct one of the photon beams towards a "heralding" [1] detector, while the other is directed towards the optical setup where we need a single photon [2]. If at a given moment a photon pair is produced, then the heralding detector will click; which tells us that is also a photon currently in our optical setup.

Finally, there is a ~p^2 probability that two photon-pairs will be produced at the same time by this process (and p^3 etc). To eliminate this possibility, in this experiment their heralding detector can detect how many photons landed on it any given moment. So if they see 2 or more photons in their heralding detector, then they discard this run, because now there are multiple photons heading towards the human eye.

[1] Herald as in the guy who announced that the King was approaching.

[2] In this case, towards the human eye.

[abdullahkhalids]

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

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