## Copies of Light

From: "andrew cooke" <andrew@...>

Date: Thu, 8 Feb 2007 21:06:18 -0300 (CLST)

There's a summary at the BBC -
http://news.bbc.co.uk/2/hi/science/nature/6343311.stm - but the Nature
http://www.nature.com/news/2007/070205/full/070205-8.html

There's also an article at the Harvard site -
http://www.news.harvard.edu/gazette/2007/02.08/99-hau.html

Since I think Nature deletes or paywalls articles after a certain time,
here's a couple of selections from their text:

"First, they slow a light pulse  travelling, naturally, at the speed of
light  to a crawl by beaming it into an ultracold cloud of about two
million sodium atoms. Then they destroy the light beam entirely, but
imprint a memory of it in the sodium.

They shunt some of these atoms into a second cloud, and tickle them with
another laser beam. This triggers their 'memory' of the original pulse,
which emerges, much weaker but otherwise unchanged, from the second
cloud."

[...]

"The experiment relies on the way that, according to quantum mechanics,
atoms may behave as waves as well as particles. This enables atoms to do
some counterintuitive things, such as passing through two openings at
once.

Usually, each atom displays its wavelike behaviour independently of all
its neighbours, like a crowd of soccer fans each waving their arms about
at random. But if a group of atoms is cooled to very low temperatures  a
mere fraction of a degree above absolute zero  then they may all come
into step, like the fans conducting a Mexican wave.

In this state, called a Bose-Einstein condensate, information encoded in a
light pulse can be transferred to the atom waves. Because all the atoms
move coherently, the information doesn't dissolve into random noise and
get lost."

[...]

"At the same time, the light's momentum is transferred to the atoms, so
they move out of the Bose-Einstein condensate cloud to a second, similar
cloud that the researchers hold suspended in a magnetic trapping field
less than a millimetre away.

By turning the control beam on this second cloud, the added atoms are
encouraged to spread their 'message' throughout the whole cloud. All the
atoms come into step with each other again, giving this second cloud a
memory of the original laser pulse. They then re-radiate this pulse,
albeit with only about one-fiftieth as much of the original light energy.
The pulse crawls through the second cloud and speeds up once it leaves."

Andrew