Science Express:University of Glasgow:
Scientists slow down the speed of light travelling through air
Issued: Fri, 23 Jan 2015 00:01:00 GMT
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 particles of light, known as photons, to be slowed as they travel through free space, unimpeded by interactions with any materials.
In a new paper published in Science Express today (Friday 23 January), researchers from the University of Glasgow and Heriot-Watt University describe how they have managed to slow photons in free space for the first time. They have demonstrated that applying a mask to an optical beam to give photons a spatial structure can reduce their speed. ...
The work was carried out by a team from the University of Glasgow’s Optics Group, led by Professor Miles Padgett, working with theoretical physicists led by Stephen Barnett, and in partnership with Daniele Faccio from Heriot-Watt University.
Spatially structured photons that travel in free space slower than the speed of light
Daniel Giovannini, Jacquiline Romero et al
Science DOI: 10.1126/science.aaa3035
Margaritahttp://arxiv.org/abs/1411.3987
Photons that travel in free space slower than the speed of light
Daniel Giovannini, Jacquiline Romero, Vaclav Potocek, Gergely Ferenczi, Fiona Speirits, Stephen M. Barnett, Daniele Faccio, Miles J. Padgett
(Submitted on 14 Nov 2014)
That the speed of light in free space is constant is a cornerstone of modern physics. However, light beams have finite transverse size, which leads to a modification of their wavevectors resulting in a change to their phase and group velocities. We study the group velocity of single photons by measuring a change in their arrival time that results from changing the beam's transverse spatial structure. Using time-correlated photon pairs we show a reduction of the group velocity of photons in both a Bessel beam and photons in a focused Gaussian beam. In both cases, the delay is several microns over a propagation distance of the order of 1 m. Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves. Introducing spatial structure to an optical beam, even for a single photon, reduces the group velocity of the light by a readily measurable amount.