Seeing Light in a New Light

Working with colleagues at the Harvard-MIT Center for Ultracold Atoms, a group led by Harvard Professor of Physics Mikhail Lukin and MIT Professor of Physics Vladan Vuletic have managed to coax photons into binding together to form molecules — a state of matter that, until recently, had been purely theoretical. The work is described in a September 25 paper in Nature.
The discovery, Lukin said, runs contrary to decades of accepted wisdom about the nature of light. Photons have long been described as massless particles which don’t interact with each other — shine two laser beams at each other, he said, and they simply pass through one another.
"Photonic molecules," however, behave less like traditional lasers and more like something you might find in science fiction — the light saber.
"Most of the properties of light we know about originate from the fact that photons are massless, and that they do not interact with each other," Lukin said. "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. This type of photonic bound state has been discussed theoretically for quite a while, but until now it hadn’t been observed.
"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 get the normally-massless photons to bind to each other, Lukin and colleagues, including Harvard post-doctoral fellow Ofer Fisterberg, former Harvard doctoral student Alexey Gorshkov and MIT graduate students Thibault Peyronel and Qiu Liang couldn’t rely on something like the Force — they instead turned to a set of more extreme conditions.