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.
Giovannini, D., Romero, J., Poto ek, V., Ferenczi, G., Speirits, F., Barnett, S. M., Faccio, D., & Padgett, M. J. (2015). Spatially structured photons that travel in free space slower than the speed of light. Science. https://doi.org/10.1126/science.aaa3035