Light Stopped for a Minute

At 186,000 miles per second, the speed of light is unparalleled, so slowing it down, let alone stopping it, is a formidable challenge. But physicists in Germany report using a glasslike crystal to stop light for about one minute, which could have important implications for light-based data processing.

A team at the Institute of Applied Physics at the Technical University Darmstadt, led by Thomas Halfmann, achieved the record by combining various known methods. In addition to stopping light, they were also able to store images that were transferred by the light pulse into a crystal for the duration of a minute — a million times longer than previously possible.

Professor Thomas Halfmann. Images courtesy of Katrin Binner.

Over the past decade, researchers have reported short stop times for simple light pulses in extremely cold gases and using special crystals. In their method, the Darmstadt researchers used a glasslike crystal containing a low concentration of praseodymium ions in a setup that also included two laser beams.

The first beam, or control beam, changes the optical properties of the crystal to decelerate the second beam. When the second beam comes into contact with the crystal and the first light beam, it decelerates. When the physicists switched off the first beam at the same moment that the second beam was within the crystal, the decelerated beam came to a stop.

The team was able to store a simple image of three striped lines within the crystal. The information was read out by turning the control laser beam on again.

How well the system works depends strongly on the parameters of the driving optical fields, magnetic fields and the high-frequency pulses, they said. They used computer algorithms to optimize these factors so that the spin waves formed by freezing the beam could survive inside the crystal for as long as possible.

Researchers at the Technical University of Darmstadt stopped light by combining known optics-related methods.

The team now intends to explore techniques that can store light significantly longer — perhaps for a week — and to achieve a higher bandwidth and data transfer rate for efficient information storage via stopped light.

"Stopped Light and Image Storage by Electromagnetically Induced Transparency up to the Regime of One Minute" by Georg Heinze, Christian Hubrich and Thomas Halfmann appears in Physical Review Letters (doi: 10.1103/PhysRevLett.111.033601).

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