Until now, light molecules did not exist. They're a new form of matter, which basically everyone who has ever lived thought was impossible. Even Professor Lukin himself is pretty sure that he's created something straight out of sci-fi:

"It's not an in-apt analogy to compare this to light sabers," Lukin added. "When these photons interact with each other, they're pushing against and deflect each other. The physics of what's happening in these molecules is similar to what we see in the movies."

To create their lightsaber molecules, or "photonic molecules" as they're being called, Lukin and Vuletic had to create an environment where weightless photos would begin to act like they have mass. Once this was accomplished, it was relatively simple to coax the affected photons to bond with one another, creating molecules.