September 26, 2013
Creating Matter That Behaves Like Luke Skywalker’s Light Saber
redOrbit Staff & Wire Reports - Your Universe Online
Scientists from Harvard University and the Massachusetts Institute of Technology (MIT) have joined forces to create a never-before-seen form of matter that is said to behave like the legendary light sabers of Star Wars fame.Harvard physics professor Mikhail Lukin, MIT physics professor Vladan Vuletic and their colleagues were able to coax photons into binding together to form what they describe as “photonic molecules.”
In a paper that appeared in Wednesday’s edition of the journal Nature, the authors explain how their findings run contrary to widely-accepted knowledge about the way in which light behaves.
Scientists have long believed that photons are massless particles which do not interact with each other, the researchers said, noting that if you shine two laser beams at one another, they would simply pass through each other. Photonic molecules behave differently, however, added Lukin.
“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,” he said. “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,” the Harvard professor 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.”
So how did Lukin, Vuletic and their colleagues manage to get the ordinarily massless photons to bind to one another? They started by pumping rubidium atoms into a vacuum chamber, and then used lasers to cool the cloud of atoms to only a few degrees above absolute zero.
Using extremely weak laser pulses, they fired single photons into the cloud of atoms. As the photons entered that cloud, the energy produced excited atoms along the way, causing the photon to slow dramatically. The energy is passed from atom to atom along the way, and eventually exits the cloud with the photon.
“When the photon exits the medium, its identity is preserved,” explained Lukin. “It's the same effect we see with refraction of light in a water glass. The light enters the water, it hands off part of its energy to the medium, and inside it exists as light and matter coupled together, but when it exits, it's still light.”
“The process that takes place is the same it's just a bit more extreme – the light is slowed considerably, and a lot more energy is given away than during refraction,” he added. Much to their surprise, when they fired two photos into the cloud, they exited together as a lone molecule due to an effect known as the Rydberg blockade.
The Rydberg blockade, Lukin explained, states that when an atom is excited, nearby atoms cannot be excited to the same degree. Essentially, this phenomenon means that when two photos enter an atomic cloud together, the first one excites an atom but must move forward before the other one can excite nearby items. As a result, the two photons push and pull with each other as their energy is passed from one atom to another, the researchers explained.
“It's a photonic interaction that's mediated by the atomic interaction. 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,” Lukin said, noting that the odd phenomenon has some practical applications, such as quantum computing.
“We do this for fun, and because we're pushing the frontiers of science,” the professor added. “But it feeds into the bigger picture of what we're doing because photons remain the best possible means to carry quantum information. The handicap, though, has been that photons don't interact with each other.”
In order to build a quantum computer, Lukin said that developers first need to come up with a way to preserve quantum information and process it using quantum logic operations. However, quantum logic requires interactions between individual quanta so that these types of systems can successfully process information.
“What we demonstrate with this process allows us to do that,” he explained. “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.”