Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves. Introducing spatial structure to an optical beam, even for a single photon, reduces the group velocity of the light by a readily measurable amount.
So it appears there is no actual waveguide involved. I guess that means somehow they construct photons in a "waveguide-like configuration" but in free space?
Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves. Introducing spatial structure to an optical beam, even for a single photon, reduces the group velocity of the light by a readily measurable amount.
So it appears there is no actual waveguide involved. I guess that means somehow they construct photons in a "waveguide-like configuration" but in free space?