Light from an ordinary lamp is chaos: photons scatter in all directions with no order. But if you make many atoms emit light simultaneously and in sync, you get a short, powerful flash, like applause that turns individual claps into a single burst. Such coordinated emission is called superradiance. It's hard to achieve: atoms must be very close to each other to sense their neighbors.
In the new work, physicists used a tiny glass cell just one millimeter long, filled with hot cesium vapor. The atoms in the vapor dart around chaotically, but at high density, the distance between them becomes smaller than the wavelength of the emitted light. Then the atoms start acting as a single ensemble. The scientists made them emit not single photons but pairs of entangled photons: this is needed for quantum communication and computing.
The result showed up in the temporal shape of the signal: the burst of photon pairs compressed from 0.60 to 0.17 nanoseconds. Such a sharp shortening is direct proof that the atoms emitted not individually but collectively. At the same time, the rate of detected pairs exceeded a million per second, and random coincidences were rare — only one in 280 events.
This work shows that superradiance can be achieved in a simple and cheap device, a glass cell with vapor, rather than in a complex setup with ultracold atoms. Hot cesium vapor, familiar from atomic clocks, turns out to be a convenient stage for a quantum choir. Perhaps soon such cells will become sources of quantum light in ordinary laboratories.
The cell is one millimeter long, roughly the thickness of ten human hairs laid side by side.
In the series "The Expanse," quantum communication works through entangled photon pairs, such a source could be aboard the Rocinante.