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Could You Design a Box That Traps Light Inside of It?
Of course you can. In fact, you likely have a device in your home right now that not only traps light and stores it—it keeps it cold, too! That’s right, your humble refrigerator: Open the door and violà, light. It’s always there, ready to illuminate your carefully curated array of fine comestibles. We’re not sure how those photons get in there in the first place. They probably inject them at the factory, like Freon or something . . .
Hold on. We’re now told that the light in your fridge is not the same light all the time, but actually emanates from something called a “lightbulb” that turns on when you open the door. What will they think of next?
The good news, our fact checkers inform us, is that the actual answer is also yes—though to truly understand how scientists can trap light, and even move it from place to place, would require a degree in physics, which, sadly, we don’t have. We’ll give this our best shot, though.
Scientists have devised several ways to trap light and save it. The “easy” way is to get two perfect mirrors and face them precisely at each other. Then you can “bounce” a beam of light back and forth between them as many as 500,000 times. “With the best mirrors, if you arrange them at some distance you can store light for a fraction of a millisecond or so,” says MIT physics professor Vladan Vuletic.
But suppose you want to keep your pet light beam for ten seconds or a minute—an eternity in the light-storage game. “The best way is to actually store light in gas or in a solid,” says Vuletic. “The idea is you don’t keep the light in the form of photons. You store it in a reversible way in atoms, in such a way that all the information of the light’s properties is conserved.” To oversimplify, it’s a little like “recording” light, like the way music might be captured on magnetic tape.
In a vacuum, Harvard University physicist Lene Hau created a tiny cloud of sodium ions so cold that their movements synchronized. She then shot a light beam into the cloud at the same time she fired a laser of a different frequency into the cloud from the side. This laser confused the electrons of the sodium atoms and kept them from absorbing the primary light beam. As a result, the light moved very slowly through the cloud, changing the electrons as it passed, and leaving an “imprint.”
Hau showed that if you block the laser at the correct moment, the light beam stops, though its particulars are still there, encoded in the imprint of the electrons. If you turn the laser back on, the beam will resume its travels and shoot out the other side. Thus, light can, in essence, be frozen. Hau was even able to transfer the imprint of the light beam into a different cloud and reconstitute it up to a minute later. “You’re really turning light into a matter copy, moving the matter copy around, and then turning that matter copy back to light,” Hau says. “That’s when you can start to say you can put light on the shelf in matter form.”
Crazy stuff. The remaining question, we suppose, is Why? What’s the point of capturing light? The big promise is that it could revolutionize computing. As we gain the ability to manipulate light and photons, the ability to store and transfer energy within a photon of light becomes conceivable. That would vastly increase computing power, and allow us to solve problems that today’s most powerful computers can’t tackle. Like how to save time in a bottle, say.
