Scientists create never-before-seen form of matter: What we have done is create a special type of medium in which photons interact with each other so strongly that they begin to act as though they have mass, and they bind together to form molecules...
An effect called a Rydberg blockade, Lukin said, which states that when an atom is excited, nearby atoms cannot be excited to the same degree. In practice, the effect means that as two photons enter the atomic cloud, the first excites an atom, but must move forward before the second photon can excite nearby atoms.
The result, he said, is that the two photons push and pull each other through the cloud as their energy is handed off from one atom to the next.
"It's a photonic interaction that's mediated by the atomic interaction..."