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
- Daniel Giovannini1,*,
- Jacquiline Romero1,*,
- Václav Potoček1,
- Gergely Ferenczi1,
- Fiona Speirits1,
- Stephen M. Barnett1,
- Daniele Faccio2,
- Miles J. Padgett1,†
1School of Physics and Astronomy, SUPA, University of Glasgow, Glasgow G12 8QQ, UK.
2School of Engineering and Physical Sciences, SUPA, Heriot-Watt University, Edinburgh EH14 4AS, UK.
↵* 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.
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