An ordinary laser emits photons one by one, like a faucet dripping water. But what if you need several particles of light born at the same instant? Physicists have made a cloud of cold rubidium atoms emit quadruples of photons — as if the faucet suddenly released four drops at once, each falling into its own test tube.
The cloud is cooled to near absolute zero so the atoms barely move. Two laser beams are shone through it, and inside the atoms the light combines in a special way: from two pump photons, four new ones are born. All four appear within twenty nanoseconds — a million times shorter than a blink of an eye. The instruments recorded 2.5 million such quadruples every second.
The key difference from previous experiments is that no powerful pulsed laser is needed. Here, a regular continuous beam works, like the steady light of a lamp rather than a flash. The photons emerge at frequencies that rubidium atoms "recognize" and with a narrow bandwidth — roughly like a single note in music. Such particles can be easily written into atomic memory and transmitted over a quantum network.
The rubidium cloud now acts as a tiny factory of entangled quadruples. Each quadruple is a ready-made resource for quantum computing and secure communication, where light and atoms speak the same language.
Rubidium-87 is the very element used in the world's most accurate atomic clocks. Now it also gives birth to photons in fours.