Two laser beams of different colors strike a crystal, and inside it, triplets of light particles are occasionally born, like three coins popping out of the same vending machine slot. For the first time, this process has been made efficient enough: 11.6 triplets per second.
The beams were green and invisible infrared. Inside the crystal, their energies combined and transformed into three new particles that flew off in different directions. Detectors caught coincidences, moments when all three appeared simultaneously, to avoid confusing them with ordinary scattered light.
The authors explained the result with a simple model: random fluctuations of emptiness (quantum vacuum fluctuations) slightly "rock" the light in the crystal, and these nudges build up into triplets. The calculation matched the experiment.
For now, the triplets are too few for practical use, but the very fact is like the first successful engine start: all that remains is to ramp up the power.
For comparison: an ordinary light bulb emits billions of billions of photons per second, while here only about twelve triplets are born in the same time.