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None. This is predicted by existing theories. The setup they have here is basically a waveguide[1][2]. You can show classically, using Maxwell's equations, tha

None. This is predicted by existing theories. The setup they have here is basically a waveguide[1][2]. You can show classically, using Maxwell's equations, that the group velocity of a wave of light when confined to a waveguide is less than light in free space. Thinking classically, the actual wave that is transmitted is a superposition of a bunch of plane waves bouncing off the walls. The complete wave moves slower than the individual waves that make it up [3]. What the authors have experimentally verified here is that single photons also travel at the group velocity. This is mostly expected, but difficult to measure (the time difference is 30 femtoseconds after travelling a meter). It's somewhat counterintuitive because the superposition is caused by the photon interacting with itself; but that's pretty par for the course with single photon experiments. I'm mildly surprised we haven't done this until now; probably one of those things where it's been generally accepted but no one's bothered to test directly. Cool experiment. [1] http://en.wikipedia.org/wiki/Waveguide http://en.wikipedia.org/wiki/Waveguide [2] Technically, it's a Bessel beam http://en.wikipedia.org/wiki/Bessel_beam http://en.wikipedia.org/wiki/Bessel_beam ; but if you understand why this should be in a waveguide, you understand what's going on. They make this analogy directly in the paper. [3] http://en.wikipedia.org/wiki/Group_velocity http://en.wikipedia.org/wiki/Group_velocity

tjradcliffe 12y ago
If there is a waveguide involved why does the headline talk about photons in free space? Photons in waveguides are not in free space. The title should be corrected to something truthful, like "Photons in waveguide-like configuration travel slower than light due to self-interference, as predicted". Anything else is deliberately, willfully, misleading click-bait (I am a physicist and clicked on the comments specifically to avoid clicking on a click-bait headline I knew was almost certainly a lie.)
- ScottBurson 12y ago
  The last two sentences of the abstract are: 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?
- cynicalkane 12y ago
  There isn't a physical waveguide. It's a Bessel beam.

tjradcliffe 12y ago

If there is a waveguide involved why does the headline talk about photons in free space? Photons in waveguides are not in free space. The title should be corrected to something truthful, like "Photons in waveguide-like configuration travel slower than light due to self-interference, as predicted". Anything else is deliberately, willfully, misleading click-bait (I am a physicist and clicked on the comments specifically to avoid clicking on a click-bait headline I knew was almost certainly a lie.)

ScottBurson 12y ago
The last two sentences of the abstract are: 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?
cynicalkane 12y ago
There isn't a physical waveguide. It's a Bessel beam.