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

We are taught that the speed of light in free space is one of the universal physical constants: c. Giovannini et al. now show that there are certain conditions under which such certainty can be broken (see the Perspective by Sambles). Adding spatial structure to an optical beam of single photons reduced the speed of light. The magnitude of the decrease depended on the complexity of the structure imprinted onto the photons. Science , this issue p. 857 ; see also p. 828 Introducing spatial structure to an optical beam reduces the speed of light. [Also see Perspective by Sambles ] 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.