For the first time, light has been slowed to the speed of sound using a technique that could improve optical storage and the processing of quantum information. Scientists at the University of Glasgow dragged light through various substances, such as water or solids, and reduced its wavelength speed – slowing it down to the speed of sound and sending it through a rotating crystal. According to most people, the speed of light is constant, but this is only the case in a vacuum, such as space, where it travels at 671 million mph. However, when it travels through substances such as water or solids, its speed is reduced, with different wavelengths traveling at different speeds. The green laser is shown as it leaves the ruby crystal. (Image: University of Glasgow) It has also been observed, but is not widely appreciated, that 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 and observed 100 years later. “The speed of light is a constant only in vacuum,” said Miles Padgett, a professor in the Optics Group in the School of Physics & Astronomy. “When light travels through glass, movement of the glass drags the light with it, too. Spinning a window as fast as you could is predicted to rotate the image of the world behind it ever so slightly. This rotation would be about a millionth of a degree and imperceptible to the human eye.” Dr. Sonja Franke-Arnold, Dr. Graham Gibson and professor Padgett, in collaboration with their colleague professor Robert Boyd at the universities of Ottawa and Rochester, took a different approach and set up an experiment: 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 3000 rpm. Once the light enters the ruby, its speed is slowed down to around the speed of sound (approximately 741 mph), and the spinning motion of the rod drags the light with it, resulting in the rotation of the image by almost five degrees – large enough to see with the naked eye. Franke-Arnold came up with the idea of using slow light in ruby to observe the photon drag. “We mainly wanted to demonstrate a fundamental optical principle, but this work has possible applications, too,” he said. “Images are information, and the ability to store their intensity and phase is an important step to the optical storage and processing of quantum information, potentially achieving what no classical computer can ever match. “The option to rotate an image by a set arbitrary angle presents a new way to code information, a possibility not accessed by any image coding protocol so far.” For more information, visit: www.gla.ac.uk