25 September 2013
Photonic molecules behave like light saber, say scientists
by Will Parker
Photons have long been described as massless particles which don't interact with each other, but Harvard and MIT scientists have reported the creation of "photonic molecules" that behave as though they have mass. The researchers, from the Harvard-MIT Center for Ultracold Atoms, say that the photonic molecules represent a never-before-seen form of matter that was, until now, purely theoretical. Lead researchers Mikhail Lukin (Harvard) and Vladan Vuletic (MIT) have detailed their work in the latest issue of Nature.
"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 explained. "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. It's not an in-apt analogy to compare this to light sabers. 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, the researchers 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. What surprised them was that two single photons fired into the cloud would exit together, as a single photonic molecule.
The researchers say the light molecule is created by an effect called a Rydberg blockade, 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, Lukin explained, 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," according to Lukin. "That makes these two photons behave like a molecule, and when they exit the medium they're much more likely to do so together than as single photons."
According to the researchers, the effect does have some practical applications right now. "It feeds into the bigger picture of what we're doing because photons remain the best possible means to carry quantum information," Lukin said. "The handicap, though, has been that photons don't interact with each other."
To build a quantum computer, he explained, researchers need to build a system that can preserve quantum information, and process it using quantum logic operations. The challenge, however, is that quantum logic requires interactions between individual quanta so that quantum systems can be switched to perform information processing.
"What we demonstrate with this process allows us to do that," Lukin said. "Before we make a useful, practical quantum switch or photonic logic gate we have to improve the performance, so it's still at the proof-of-concept level, but this is an important step. The physical principles we've established here are important."
Lukin also suggested that the system might one day even be used to create complex three-dimensional structures wholly out of light. "What it will be useful for we don't know yet, but it's a new state of matter, so we are hopeful that new applications may emerge as we continue to investigate these photonic molecules' properties," he said.