GREAT UNKNOWNS:

Can you trap light in a box?

OF COURSE YOU CAN. In fact, you likely have a device in your home right now that not only traps light and stores it – but keeps it cold, too! That’s right, your humble refrigerator: Open the door and voila! 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 being told that the light in your fridge is not the same light all the time, but actually emanates from something called a light bulb 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 Massachusetts Institute of Technology physics professor Vladan Vuletić.

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 solid,’ says Vuletić. ‘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, the way music might be captured on magnetic tape.

In a vacuum, Harvard University physicist Lene Hau created a tiny cloud of sodium atoms so cold that their movements synchronised. 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 travel 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 revolutionise 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 cannot tackle. Like, say, how to save time in a bottle.

98