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Ordinary light is a stream of photons flying every which way: sometimes more, sometimes fewer, and in that chaos, measurement precision gets lost. Physicists have learned to "tame" this stream by creating squeezed light — where the noise in one characteristic drops below the vacuum level. It's like tightening a string: it still vibrates, but it doesn't flop around.

In the new work, squeezed light is born right inside a glass cell filled with rubidium vapor heated to 120 degrees. A laser beam passes through the vapor and, interacting with atoms, generates pairs of photons — a signal photon and an idler photon. Their intensities are so tightly correlated that the difference between them fluctuates 6.3 times less than in ordinary light. The device, the size of a palm, runs on 300 milliwatts, like a laser pointer.

Such a source is needed for quantum sensors: for example, a magnetometer based on rubidium atoms could sense fields thousands of times weaker than Earth's magnetic field. And the narrow emission bandwidth, just a few megahertz, allows direct interfacing with quantum memory, where photons are stored in a cloud of cold atoms. This is a step toward a quantum internet, where information is transmitted via light protected by the laws of physics.