Spatially structured photons that travel in free space slower than the speed of light

Science DOI: 10.1126/science.aaa3035


Posted but not written by: Louis Sheehan

Spatially structured photons that travel in free space slower than the speed of light

  1. Daniel Giovannini1,*,
  2. Jacquiline Romero1,*,
  3. Václav Potoček1,
  4. Gergely Ferenczi1,
  5. Fiona Speirits1,
  6. Stephen M. Barnett1,
  7. Daniele Faccio2,
  8. Miles J. Padgett1,

+Author Affiliations

  1. 1School of Physics and Astronomy, SUPA, University of Glasgow, Glasgow G12 8QQ, UK.

  2. 2School of Engineering and Physical Sciences, SUPA, Heriot-Watt University, Edinburgh EH14 4AS, UK.
  1. Corresponding author. E-mail:
  1. * These authors contributed equally to this work.

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 micrometers 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.

Twitter: Lou Sheehan@LouSheehan

[ One of my intentions with this blog is to simply collect articles of interest to me for purposes of future reference. I do my best to indicate who has actually composed the articles. NONE of the articles have been written by me. Further, this ‘blog’ will contain various drafts of unknown writings just to be saved in the event they are needed by me, etc.– Louis Sheehan ]

Feel free to ignore this blog! I am intending to use it as a repository of various writings: drafts, doodles, etc. If there ARE any articles here, they are posted but not written by: Lou Sheehan


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