2014/11/14 by Daniel Giovannini, Jacquiline Romero, Václav Potoček +7 · 1 voice · 173 citations
Physics and Astronomy · #Beam (structure) #Bessel beam #Cold Atom Physics and Bose-Einstein Condensates #Free space #Gaussian beam #Group velocity #Light beam #Optics #Orbital Angular Momentum in Optics #Photon #Physics #Quantum optics and atomic interactions #Speed of light (cellular automaton) #Transverse plane #physics.optics #quant-ph
paper · pdf · doi:10.1126/science.aaa3035
published in Science 347(6224), 857-860 (American Association for the Advancement of Science) · 8 pages, 4 figures
arxiv created 2014/11/14 · openalex publication_date 2015/01/23 · arxiv updated 2017/07/26 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
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.