Passing photons through an optical mask can give them a transverse spatial structure that reduces their speed. Researchers from Heriot-Watt University and the University of Glasgow have reported delaying single photons by several microns over a distance of one meter in air. An unaltered beam was used for comparison. The experiments were conducted using both Gaussian and Bessel beams. An artist’s depiction of two photons racing. The photon on the left has been slowed by passing through an optical mask. Courtesy of Heriot-Watt University. Although the effect was demonstrated with single photons, it also applies to beams, according to Glasgow postdoctoral researcher Jacquiline Romero. “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,” she said. The phenomenon is different from the slowing effect of light passing through media such as glass or water, the researchers said. After exiting these media, light will resume its typical speed. But light that has been slowed by an optical mask has its top speed permanently downgraded. “The exciting discovery here is that this speed is the true speed of light only for plane waves, that is waves that are perfectly flat,” said Heriot-Watt professor Dr. Daniele Faccio. “In everyday situations however, we interact with light that is not a plane wave but has some kind of structure on it. The presence of this structure (think of the light beam emitted from a laser pointer) forces the light to actually move slower.” The team compares a beam of light containing many photons to a group of cyclists. Although the group travels along the road as a unit, the speeds of individual cyclists vary. The group formation can make it difficult to define a single velocity for all cyclists, and the same applies to light. “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,” said Glasgow professor Dr. Miles Padgett. “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.” The research was published in Science (doi: 10.1126/science.aaa3035). For more information, visit www.hw.ac.uk and www.gla.ac.uk.