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Out of the Lab: Storing Light

The ability to store light in an optical memory buffer could have huge implications for photonic routing

January 22, 2001

3 Min Read
Out of the Lab: Storing Light

It’s often said that genuine optical routers won’t arrive until there’s been a fundamental breakthrough in physics -- until someone has figured out a way of buffering light so that packets can be stored and forwarded the way they are in today’s electronic routers.

Well, it looks as though we've got that fundamental breakthrough. In fact, two independent groups of scientists claim to have managed to slow down and "stop" light -- and both groups are aiming on publishing their findings this month.

The first paper, due to be published on Thursday, Jan. 25, in the journal Nature, describes how scientists from the Rowland Institute of Science in Cambridge, Mass., have arrested a pulse of light inside a gas of cold sodium atoms.

The second paper, due to be published next Monday, Jan. 29, in the journal Physical Review Letters (PRL) gives results of a similar experiment conducted by scientists at the Harvard-Smithsonian Center for Astrophysics.

The fact that two independent outfits have come up with similar results is significant. While the process of peer review in scientific journals weeds out most of the crackpots, there have been a few instances -- like cold fusion -- where work has proved impossible to reproduce, and thus of no practical use. This appears unlikely to happen with this research.

The key difference between the two experiments was the material used. Scientists at the Rowland's Institute employed a gas of cold sodium atoms, while the researchers at the Harvard-Smithsonian worked with a glass cell containing rubidium, which they heat up to create rubidium vapour.

In both cases, the atoms in the gas normally absorb light -- in other words, the gas is opaque. It can be made transparent (non-absorbing) to a particular wavelength by illuminating it with a so-called coupling laser. If the coupling laser gets turned off as a pulse is passing through the gas cloud, the pulse "stops." Turn the coupling laser back on, and the pulse continues on its journey. If information is encoded on the pulse, it can be recovered later, just like a letter delayed in the mail.

The idea of stopping light is one that would curl Einstein's hair. The trick is that the light in the pulse isn’t actually stopped at all. Instead, the information in the pulse has been transferred to the surrounding gas atoms, while the energy it contains passes into the coupling beam. The information stays trapped in the atoms until they get a kick of energy from the coupling laser being turned back on. Then -- kapow! -- the pulse springs back to life.

Of course, there's a lot of work to be done before it will be possible to engineer this technology into something practical. "This technology could be 10 years from application, it could be 50," says David Phillips, lead author on the PRL paper.

He adds, "There are still a bunch of physics issues to understand before you start engineering it into a box." Experiments that take place at very low temperatures -- like the work at Harvard-Smithsonian -- are useful in this respect because they are a lot easier to understand from a physics perspective."

Pretty soon, it should be possible to store light in solid materials, as well as in gases. "For real applications, solid state is where you'll think about doing things," notes Phillips. In fact, he believes that researchers are already submitting proposals to do these experiments -- storing light -- inside doped optical fiber.

— Pauline Rigby, senior editor, Light Reading http://www.lightreading.com

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