Light from a bulb or even a regular laser resembles a crowd at a subway exit: people move haphazardly, sometimes dense, sometimes sparse. Physicists call this behavior shot noise — random fluctuations in the number of particles arriving per second. But in a new experiment, researchers have created a light stream that behaves differently, like a squad of soldiers marching in lockstep.
The setup looks almost like a pot of hot steam. A glass cell contains rubidium atoms heated to about 100 degrees Celsius. A laser beam with a wavelength of 795 nanometers (just beyond the red edge of the rainbow, already invisible infrared) is directed into it. The rubidium atoms absorb two photons from the beam and almost instantly emit two new ones, but of different colors — one slightly 'redder,' the other slightly 'bluer' than the original. This gives birth to twin pairs: two photons tightly bound to each other.
The key trick is that these pairs are not just correlated but 'squeezed' in noise. If you measure the difference in photon number between the two beams, it fluctuates 5.5 times less than in ordinary light. Even after the beams are sent into optical fiber, like trying to march a squad through a narrow corridor without breaking step, the squeezing remains at a factor of 3. The temporal pattern of coincidences is also unusual: instead of a sharp peak, there's a flat-top rectangle. This indicates that pairs are generated not in a single process but in several, tuned like musical strings to different frets.
Such a source of paired light can already be integrated into quantum sensors — devices that detect magnetic fields or tiny displacements thousands of times more precisely than classical ones. It will also be useful for quantum communication, where each photon pair is like a sealed envelope: any eavesdropping is immediately noticeable. The experiment brings us back to that same pot of rubidium vapor, but now we know: inside it, not just a hot fog is born, but a perfectly synchronized light orchestra.
Rubidium is a soft metal that melts in your hand at 39 degrees Celsius, but in the experiment it is heated more to produce a dense vapor, like fog.
In Arthur Koestler's novel 'The Blind Darkness,' quantum effects are used for instantaneous communication — today's squeezed photons bring that idea a little closer to the lab bench.