Light can been slowed down to just over 200 kilometres per hour using only simple desktop equipment at room temperature, US researchers have shown.



In a vacuum, light travels at 300,000 kilometres per second. It is slowed, for example, by a third when it passes through glass. But slowing light to 200 kph – a factor of 5.4 million – normally requires large and complex laboratory equipment and cryogenic cooling.



The researchers fired two laser beams tuned to slightly different frequencies at a ruby crystal. The frequency difference produced a rhythmic vibration in atoms in the crystal, which in turn altered the refractive index of the material.



The effect, called coherent population oscillation, is well known but has never been put to this use before. The refractive index determines the speed of light and can be tuned by changing the frequency of the lasers.

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