2014/11/14 by Daniel Giovannini, Jacquiline Romero, Václav Potoček +7 · 1 voice · 7 citations
Physics and Astronomy · #Orbital Angular Momentum in Optics #Quantum optics and atomic interactions #Cold Atom Physics and Bose-Einstein Condensates
paper · pdf · doi:10.1126/science.aaa3035
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 wave vectors 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 in the group velocity of photons in both a Bessel beam and photons in a focused Gaussian beam. In both cases, the delay is several micrometers over a propagation distance of ~1 meter. Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves.