Capturing Light in a Very Cold Bottle

My “cold” theme continues, even though the cold front that hit the western states has passed and we are ringing in the New Year with a city full of slush. Oh winter, thou art a bittersweet lover. How ’bout some physics to warm your soul?

Physicists at Harvard University (it’s a little school near Boston) led by Lene Hau, have improved a technique for capturing beams of light, then releasing them a second and a half later. Blocking light is one thing, but stopping it and then releasing it so that it continues the way it began, is quite another. They have, in some sense, captured light in a bottle. Eventually this could change means of communication or the storage of quantum information.

Here’s how they did it: the scientists shot a beam of light at a group of very cold atoms. And we are are talking chilly. These atoms are so cold that they form what are called Bose-Einstein Condensates [BEC’s], where the atoms all assume a coherent quantum state. I guess it’s like when a group of people go out in the freezing cold and after a while their individual identities go away because they all just become a collection of teeth-chattering frozen replicas of each other. Or something like that.

Anyway – the scientists first applied a control laser and a magnetic field to the BEC. They then struck it with a pulse of light, which, due to the cold particle group, slowed down to about 15 miles per hour, down from its regular 671 million mph. The researchers found that the light actually crumples up like an accordion and pack itself into the bunch of atoms. The slowed down light imprints itself on the BEC’s and when the control laser is turned off, the BEC’s retain that information, which includes the frequency of the light, as well as all of it’s quantum information. For a moment, they had captured light.

In the image, courtesy of Ann Goodsell, light enters the BEC and slows down, then is revived and emitted in the other direction, weaker than before.

Previous experiments of this kind could only hope to revive the light beam within a few milliseconds, otherwise its memory would vanish from the particles as they bumped around in a group. But the Harvard scientists decided to herd the storage particles away from the group so they don’t lose the information through collisions. When they turn on the secondary laser and the magnetic field, a beam of light nearly identical to the original springs forth. Out of the bottle or back from the dead, depending on how you want to look at it.

But like coffee that’s been in the freezer for too long, the reborn light beam isn’t quite as strong as the original. Still, it retains many important properties and could still carry information about the original beam of light to a new source. The group hopes that better magnetic fields can improve the new beam’s strength.

This technique is better than previous efforts both in terms of time (the light can be revived 1.5 seconds after freezing, compared to the previous record of about 0.6 seconds) and a lower loss of energy from the original beam to the revived one.

From an article in (a publication of IOP) quoting Hau:

…the technique could be adapted to process the information contained in the pulse. For example, the drop could be split into two before revival – which would create two entangled pulses of light. Another option, according to Hau, is that only one of the drops is revived – creating a pulse of light that is entangled with the remaining atoms. Another possibility is the creation of “squeezed” light pulses, in which the number of photons in the pulse is set by the number of atoms in the BEC.

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