Two Atoms Send Photons Marching in Triplets express
Light is a stream of photons. In a special waveguide, resembling a narrow corridor, a 'gatekeeper' atom usually lets them through one at a time. This phenomenon is the domain of single-particle spectroscopy.
When two 'gatekeepers' are placed side by side, they force photons to gather in trios. In the experiment, two artificial atoms — essentially tiny superconducting circuits — connected by a common waveguide, produced exactly this effect. Photons, which normally ignore each other, suddenly start traveling in groups of three. They still race along at the speed of light, but their behavior becomes radically different. This result aligns with the Standard Model of particle physics and paves the way for multitasking quantum optics.
Pioneers of quantum optics dreamed of this for decades — Leonard Mandel and Roy Glauber.
To see individual photons, you need total darkness and ultrasensitive detectors. In an ordinary lightbulb, there are so many that counting them is pointless.
In science fiction, quantum communication promises instant data transfer. Photon triples aren't capable of that yet, but they help us better understand the quantum world.