Seeing Light in a New Light

Sep. 25, 2013 — Harvard and MIT scientists are challenging the conventional wisdom about light, and they didn’t need to go to a galaxy far, far away to do it.

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.

At first sight that appears impossible, although photos do interact gravitationally with each other in a peculiar way.

> Researchers began by pumped rubidium atoms into a vacuum chamber, then used lasers to cool the cloud of atoms to just a few degrees above absolute zero. Using extremely weak laser pulses, they then fired single photons into the cloud of atoms.
As the photons enter the cloud of cold atoms, Lukin said, its energy excites atoms along its path, causing the photon to slow dramatically. As the photon moves through the cloud, that energy is handed off from atom to atom, and eventually exits the cloud with the photon. When the photon exits the medium, its identity is preserved. It’s the same effect we see with refraction of light in a water glass.

I begin to see the light. Atoms in rubidium vapour were used for the first ever successful experiments that slowed light to sub-relativistic speeds.

> When Lukin and colleagues fired two photons into the cloud, they were surprised to see them exit together. The reason they form the never-before-seen molecules? An effect called a Rydberg blockade 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 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.

Excellent. So far as I know this is a totally new effect. It has implications not just in pure science but also for fast computers based on photons rather than electrons.