In their normal form, photons have no mass, move at the speed of light, and do not interact with each other in any way. However, a few years ago, the phenomenon of "photonic molecules" was discovered - structures that seem to consist of a pair of photons. At the Massachusetts Institute of Technology, they conducted an experiment on the generation of such molecules under ideal conditions and obtained "photon triplets", which can be conventionally called a new form of matter and light at the same time.
The rubidium cloud was cooled to almost absolute zero so that the atoms of the substance became almost motionless, after which a very weak laser beam was directed there. It emits only a few photons per flash, which is easy to track. And it turned out that at the exit from the cloud, the photons were grouped in 2 or 3, acquired a mass, albeit negligible, and because of this slowed down by a factor of 100, 000.
The parameter of the phase, the coefficient of interaction of photons, in triplets turned out to be three times greater than in photon pairs - they really did not accidentally "stick together", but acted as a single structure. Scientists have suggested that when passing through a cloud of atoms, photons were subjected to some kind of influence and turned into "polaritons". This is a term from quantum theory, a particle with signs of light and matter at the same time, so it can interact with others like itself.
Polaritons retained their properties outside the rubidium cloud, but their detailed behavior is a matter of subsequent experiments. In the meantime, scientists are trying to incorporate this discovery into the existing framework for understanding the world. If there is "space-time", then can there be "matter-light"? And if polaritons can be described according to the rules of quantum calculus, is there a chance to use them in quantum computers and similar applied systems of the future?