Phenomena that produces photons in pairs as it passes through a certain type of crystal.

You can then detect one of the photons, and when you do you know that the other one is there as well and ready to be used. two photon interference experiment comes to mind, which is the basis of photonic quantum computer, where you need two photons to be produced at the exact same time to produce quantum entanglement.

One Photon In, TWO Photons Out by JQInews (2010)

Source.

Features Alan Migdall of the National Institute of Standards and Technology. Produced by the Joint Quantum Institute (JQI).

Mentions that this phenomena is useful to determine the efficiency of a single photon detector, as you have the second photon of the pair as a control.

Also briefly describes how the input energy and momentum must balance out the output energy and momentum of the two photons coming out (determined by the output frequency and angle).

Shows the crystal close up of the crystal branded "Cleveland Crystals Inc.". Mentions that only one in a billion photon gets scattered.

Also shows a photomultiplier tube.

Then shows their actual optical table setup, with two tunnels of adjustable angle to get photons with different properties.

Video 2.

How do you produce a single photon? by Physics World (2015)

Source.

Very short whiteboard video by Peter Mosley from the University of Bath, but it's worth it for newbs. Basically describes spontaneous parametric down-conversion.

One interesting thing he mentions is that you could get single photons by making your sunglasses thicker and thicker to reduce how many photons pass, but one big downside problem is that then you don't know when the photon is going to come through, that becomes essentially random, and then you can't use this technique if you need two photons at the same time, which is often the case, see also: two photon interference experiment.