Twin Photons in a Light Cavity express
Inside a tiny mirrored cavity, smaller than the thickness of a hair, light is trapped and bounces off the walls many times. If you place a material that can absorb and emit light inside, the light particles (photons) start to 'dress' in excitations of the material — this is how polariton modes are born. Usually, a bright coherent light, similar to a powerful laser beam, dominates in such a cavity. But physicists from France found a way to extract weak quantum fluctuations — single excitations that appear and disappear spontaneously — from this bright background.
When the average number of such quantum fluctuations is less than one, the researchers measured the arrival times of photons from two sides of the spectrum — red and blue. It turned out that photons arrive in pairs, always in the same order: first red, then blue. This is surprising because in ordinary thermal light, photons are not correlated in time. This ordering arises spontaneously because frequency-resolved detection 'cuts out' certain combinations from the internal quantum structure of elementary fluctuations.
The practical benefit of this discovery lies in the possibility of controlling single quantum states of light inside a solid-state device. Such states are needed for quantum computing and secure communication. Instead of fighting fluctuations as noise, they can be used as a resource: for example, as a source of entangled photon pairs with a predetermined temporal order.
In the experiment, the average number of quantum fluctuations was less than one — meaning that at any given moment, there was usually no 'extra' particle in the cavity, and pairs were born from vacuum fluctuations.