Scientists from Harvard and MIT have jointly demonstrated that, in specific conditions, photons can be made to interact with each other and form molecules. Such groupings of photons, dubbed “Photonic molecules”, constitute an entirely new form of matter, which until recently was purely theoretical. Combining the properties of light and those of solids, in terms of physics this new form of matter is not unlike a certain material that millions of Star Wars fans are already well familiar with – lightsaber material.
"It's not an in-apt analogy to compare this to light sabers," said Harvard Professor of Physics Mikhail Lukin, who heads the research group at the Harvard-MIT Center for Ultracold Atoms together with his MIT counterpart, Professor of Physics Vladan Vuletic. "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." explains Professor Lukin.
Photons are massless and do not interact with each other. While the first part of this statement remains true, the discovery challenges the second part by demonstrating that in a specially prepared medium, photons can act as though they interacted with each other, by doing so indirectly via atoms.
What conditions are then required? The scientists first pumped rubidium atoms into a vacuum chamber and cooled them down with lasers to temperatures barely above absolute zero. They then shot single photons into the cloud of atoms.
What happens to photons in this medium is similar to what happens to light refracted in a glass full of water. As photons enter the cloud, they loose part of their energy to the medium. Photons start exciting the atoms in their path and, in effect, slow down dramatically.
However, this exchange of energy between photons and atoms is governed by an effect called the Rydberg blockade, whereby in a certain volume no two atoms can be excited to the same degree. This means that, once a photon has excited an atom, it has to move forward before another photon can do the same.
Effectively, photons start pushing and pulling each other through the medium, behaving much like molecules. On leaving the medium, photons are back to their original energy state (the energy is transferred back from the atoms to the photons) but they emerge as molecules rather than as single photons.
While it is safe to assume we are not about to witness lightsaber mass production any time soon, the discovery may have important real-life implications. One field that may benefit from it is quantum computing, with the demonstrated behavior potentially bringing scientists a step closer to building quantum logic gates. Professor Lukin also points out that, with this new photonic bound state at our disposal, creating complex 3D structures out of light is suddenly completely imaginable.