Scientists Slow Down the Speed of Light Travelling through Air

Scientists have long known that the speed of light can be slowed slightly as it travels through materials such as water or glass. However, it has generally been thought impossible for photons to be slowed as they travel through free space, unimpeded by interactions with any materials. Researchers from the University of Glasgow and Heriot-Watt University have managed to slow photons in free space for the first time.
The team’s experiment was configured like a time trial race, with two photons released simultaneously across identical distances towards a defined finish line. The researchers found that one photon reached the finish line as predicted, but the structured photon which had been reshaped by a mask arrived later. Over a distance of one metre, the team measured a slowing of up to twenty wavelengths, many times greater than the measurement precision.
The work demonstrates that, after passing the light beam through a mask, photons move more slowly through space. Crucially, this is very different to the slowing effect of passing light through a medium, where the light is only slowed during the time it is passing through the material – it returns to the speed of light after it comes out the other side. The effect of passing the light through the mask is to limit the top speed at which the photons can travel.
The work was carried out by a team from the University of Glasgow’s Optics Group, led by Miles Padgett, working with theoretical physicists led by Stephen Barnett, and in partnership with Daniele Faccio from Heriot-Watt University. The team measured similar effects in both Bessel and Gaussian beams.
Co-lead author Jacquiline Romero said: “Although we measure the effect for a single photon, it applies to bright light beams too. The effect is biggest when the lenses used to create the beam are large and when the distance over which the light is focused is small, meaning the effect only applies at short range.”
Professor Padgett added: “It might seem surprising that light can be made to travel more slowly like this, but the effect has a solid theoretical foundation and we’re confident that our observations are correct. The results give us a new way to think about the properties of light and we’re keen to continue exploring the potential of this discovery in future applications. We expect that the effect will be applicable to any wave theory, so a similar slowing could well be created in sound waves, for example.” (Source: Optik & Photonik 1 / 2015)

Referenz: D. Giovannini et al.: Spatially structured photons that travel in free space slower than the speed of light, Science, online 22 January 2015; DOI: 10.1126/science.aaa3035

Links:  University of Glasgow

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