Scientists drag light by 5° by slowing to the speed of sound

By slowing a beam of light down to the speed of sound, UK researchers have dragged photons by an unprecedented 5°, proving a longstanding theory of physics and opening up potential applications in quantum data storage.

The speed of light is constant in a vacuum, such as space, where it travels at 671 million miles per hour.

When it travels through different substances, such as water or solids, its speed is reduced, with different wavelengths (colours) travelling at different speeds.

In addition, light can be dragged when it travels through a moving substance such as glass, air or water — a phenomenon first predicted by Augustin-Jean Fresnel in 1818.

Indeed, spinning a pane of glass as fast as possible is predicted to rotate the image of the world behind it slightly, but this rotation would be about a millionth of a degree and imperceptible to the human eye.

In an effort to understand the phenomenon in more detail, a team from Glasgow University tried to increase the rotational dragging effect, as Dr Sonja Franke-Arnold explained to The Engineer.

‘We were looking at this dragging effect for some time but couldn’t resolve rotation by more than a micro-degree — so small you can’t even measure it properly.

‘Then I listened to a talk on slow light and this whole idea of using it in order to increase the dragging effect came to me, and I approached Robert Boyd [of Rochester University], who is a world-leading expert in the field.’

The group’s experimental set-up involves shining a primitive image made up of the elliptical profile of a green laser through a ruby rod spinning on its axis at up to 3,000rpm.

Once the light enters the ruby, its speed slows down to around the speed of sound (approximately 741mph) and the spinning motion of the rod drags the light with it, resulting in the image being rotated by almost 5° — large enough to see with the naked eye.

‘If you ever wanted to do proper quantum communications then photons are fantastic storage devices for quantum information, but they travel so fast you can’t really access that and having a medium that can store this light, or at least slow it down for long enough so you can decide what to do next — that would be useful,’ Franke-Arnold said.