Aug 072013


Efficient information storage by stopped light.

Physicists in Darmstadt have been able to stop something that has the greatest possible speed and that never really stops: light. The physicists, headed by Thomas Halfmann, stopped light for about one minute. They were also able to save images that were transferred by the light pulse into the crystal for a minute – a million times longer than previously possible.

Already a decade ago, physicists stopped light for a very short moment: In previous years, this extended towards stop times of a few seconds for simple light pulses in extremely cold gases and special crystals.But now the researchers at the Institute of Applied Physics of the Technische Universität Darmstadt extended the possible duration and applications for freezing the motion of light considerably.The researchers achieved the record by cleverly combining various known methods of their field. The result will have practical significance in future data processing systems that operate using light.
A glass-like crystal to stop the light

To stop the light, the physicists used a glass-like crystal that contains a low concentration of ions – electrically charged atoms – of the element praseodymium. The experimental setup also includes two laser beams. One is part of the deceleration unit, while the other is to be stopped.

The first light beam, called the “control beam”, changes the optical properties of the crystal: the ions then change the speed of light to a high degree. The second beam, the one to be stopped, now comes into contact with this new medium of crystal and laser light and is slowed down within it. When the physicists switch off the control beam at the same moment that the other beam is within the crystal, the decelerated beam comes to a stop.

More precisely, the light turns into a kind of wave trapped in the crystal lattice. This can be explained in greatly simplified form as follows. The praseodymium ions are orbited by electrons. These behave similarly to a chain of magnets: if you put one into motion, the movement – mediated by magnetic forces – propagates in the chain like a wave.

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