Most quantum optics experiments are done by simply attenuating laser pulses until they have on average much less than one photon per pulse. This way the probabi

Most quantum optics experiments are done by simply attenuating laser pulses until they have on average much less than one photon per pulse. This way the probability of having more than one photon is very low (at the cost of reduced count rates since most pulses don't contain any photons). These pulses can then be used to create entangled photon pairs using SPDC. There are also several ways of creating pulses that contain exactly one photon, but this is way more complicated.