A team of researchers has created the technology to make an actual lightsaber (for those who don't know what this is, it's a... well, just look it up on Wikipedia). Only they haven't. Not really. But that hasn't stopped Star Wars nerds from frothing at the mouth and setting the Internet ablaze with the 'news'.
The hype
What the team at the Harvard-MIT Centre for Ultracold Atoms has actually accomplished is the minor feat of creating a completely new form of matter, one which had previously only been theorised and whose existence runs contrary to decades of accepted wisdom about the nature of light.
According to one of the research group leaders, Harvard Professor of Physics Mikhail Lukin, 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. What he and the other team members, including second leader, MIT Professor of Physics, Vladan Vuletic, have managed to do is contradict the conventional wisdom by coaxing photons into binding together to form molecules.
“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 inapt analogy to compare this to lightsabers.” No doubt it’s this statement which was (in)advertently responsible for the hype which ensued.
The science
The researchers began by pumping 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, 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,” Lukin said. “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.”
When Lukin and colleagues fired two photons into the cloud, they were surprised to see them exit together, as a single molecule. The mechanism responsible for this is 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 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.
The relevance
Practical applications for the discovery include quantum computing, where it could be used to create quantum switches or photonic logic gates, or even classical computing where it could aid the development of optical routers that convert light signals into electrical signals. Lukin also suggested that the system might one day be used to create complex three-dimensional structures – such as crystals – wholly out of light.
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