There was a fascinating article, by Eugenie Samuel, at It explains an experiment in which they stopped light dead in its tracks, then released it, letting it speed off, as usual.

Stop Light: The REAL Spin Doctors

Scientists have stopped light in its tracks in two landmark experiments. In doing so they have overcome a fundamental obstacle to the development of quantum computers.

Light normally travels at 300,000 km per second but both groups of researchers slowed a laser beam to a complete standstill by passing it through a specially prepared cell of gas atoms. Later the researchers restarted the light beam and sent it speeding off again.

Ron Walsworth from the Harvard Smithsonian Centre for Astrophysics led one of the groups and says the demonstration shows how information could be transported in a quantum computer. "The light could take information from node to node as required," he told New Scientist.

"Everybody thought it was pretty wild," says Seth Lloyd, a quantum computing engineer from MIT who attended the Physics Optics and Electronics conference in Utah where the Smithsonian group presented their idea. "We thought it would be what's needed in quantum computing."

Steady state

In conventional quantum computing, researchers aim to store quantum states in individual atoms. However the states are very delicate and liable to be destroyed by background noise.

In contrast, in the light stopping experiment, the information is contained in the electromagnetic fields of the light beam and is transferred to the state of the gas atoms. "We have over 1012 atoms which makes the state very robust," says David Phillips, who worked on the experimental set-up at Harvard-Smithsonian.

Both Phillip's group and Lene Hau's at Harvard University and the Rowland Institute in Cambridge, Massachusetts, prepared their gas atoms in the same way. They used a control laser beam tuned at just the right frequency to put the atoms into a so-called dark state. In this state the atoms cannot absorb light as usual.

When a second pulse beam is passed into the cell, rather than being absorbed it interacts with the atoms by flipping their spins. Doing this slows the beam by an amount that depends on the intensity of the control beam.

So to slow the beam to a standstill, the researchers fade out the control beam completely. But they have to do it smoothly or the dark state is destroyed. "Showing that could be done was the clever theory," says Phillips.

This theoretical breakthrough was achieved in 2000 by Misha Lukin and Suzanne Yellin at Harvard-Smithsonian and Michael Fleischhauer at the University of Kaiserslautern, Germany.

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