Basic primer on spontaneous parametric down-conversion: it's an optical phenomenon where a photon of a given frequency (energy) is split into two entangled photons whose frequencies add up to the original photon. The resulting frequencies differ so you can separate the photons spatially by refracting them. Has long been a workhorse for experiments requiring a pair of entangled particles (Bell tests and such). You can buy SPDC crystals online for about $500 these days[0].
I guess they managed to SPDC a photon into three photons instead of two.
I guess they managed to SPDC a photon into three photons instead of two.
[0] https://www.newlightphotonics.com/SPDC-Components/405nm-Pump...