Heralded Single-Photon Source — SPDC Pair Splitter

A pump laser fires repeated pulses into a nonlinear crystal. Almost every pulse passes straight through untouched — spontaneous parametric down-conversion is rare. Occasionally the crystal converts one pump photon into a correlated signal/idler pair, sent off in two different directions at the same instant. The idler arm's detector clicking is the herald: proof a signal photon exists in the other arm right now. Push the pump power slider up to raise the pair-generation rate — and watch the multi-pair leak counter grow and heralding purity fall, as pulses start producing two or more pairs at once instead of a clean single pair.