The light enters one cloud of atoms and is revived in another

The quantum sleight of hand exploits the properties of super-cooled matter known as a Bose-Einstein condensate.

The emerging pulse was slightly weaker than the high-speed beam that entered the experimental setup, but was identical in all other respects.

The work, published in the journal Nature, could one day lead to advances in computing and optical communication.

"Instead of light shining through optical fibres into boxes full of wires and semiconductor chips, intact data, messages, and images will be read directly from the light," said Professor Lene Vestergaard Hau of Harvard University and one of the authors of the Nature paper.

Exotic freezer

The Harvard team rose to prominence in the late 1990s when it slowed light from its constant 299,792km/s (186,282mps) to a leisurely 61km/h (38mph).

It applied the brakes by shining light into a cloud of sodium atoms trapped in a vacuum and cooled to just above absolute zero (-273C), the theoretical state of zero heat.

The two atom clouds were separated and had never seen each other before

Lene Hau

At this temperature the atoms coalesce to form a Bose-Einstein condensate, an exotic quantum entity first predicted by Albert Einstein and created in the lab in 1995.

A second laser tuned the tiny atomic cloud to slow the pulse of light.

In 2001, working with a team from the Harvard Smithsonian Center for Astrophysics, the same group brought light to a halt, by slowly turning off the second control laser.

Switching the laser back on set the light free.

The new experiment builds on this work.

Light switch

Instead of just one cloud of sodium atoms, the new setup used two, a fraction of a millimetre apart.

"The two atom clouds were separated and had never seen each other before," said Professor Hau.

The team had previously "frozen" a beam of light in a gas cloud

A pulse of light was shone on the first cloud, impressing a "cast" of the pulse into a clump of spinning sodium atoms, nudged in the direction of the second condensate.

This slowly moving clump was composed entirely of sodium atoms, effectively turning light into matter.

Once the "messenger" group had merged with the second cloud, a second laser was shone through the condensate to revive the original pulse of light.

From a standing start, the reconstructed beam sped back up to the normal speed of light. Analysis showed that it possessed exactly the same shape and wavelength of the original beam, although it was slightly weaker.

Writing in an accompanying article in Nature, Professor Michael Fleischhauer of the University of Kaiserslautern in Germany described the experiment as "striking and intriguing."

He said that science was entering a period of "unprecedented experimental control" of light and matter.

"That could bring very real technological benefits," he wrote.

Applications could include optical storage devices and quantum computers, far quicker and more powerful than today's PCs.