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That the speed of light in free space is constant is a cornerstone of modernphysics. However, light beams have finite transverse size, which leads to amodification of their wavevectors resulting in a change to their phase andgroup velocities. We study the group velocity of single photons by measuring achange in their arrival time that results from changing the beam's transversespatial structure. Using time-correlated photon pairs we show a reduction ofthe group velocity of photons in both a Bessel beam and photons in a focusedGaussian beam. In both cases, the delay is several microns over a propagationdistance of the order of 1 m. Our work highlights that, even in free space, theinvariance of the speed of light only applies to plane waves. Introducingspatial structure to an optical beam, even for a single photon, reduces thegroup velocity of the light by a readily measurable amount.

Photons that travel in free space slower than the speed of light