Chinese scientists have cracked a long-standing puzzle in quantum optics, creating a tiny device that spits out pairs of light particles on demand with unprecedented purity and efficiency that could upend the quantum race.Single-photon devices are maturing rapidly, but the field is now pursuing efficient two-photon sources that can pave the way for sharper medical imaging, unbreakable encryption and next-generation sensors. In precision measurement, for instance, using two photons can double...