

Photons that travel in free space slower than the speed of light - mwc
http://arxiv.org/abs/1411.3987

======
aortega
The paper actually has a nice one-sentence summary:

"The group velocity of light in free space is reduced by controlling the
transverse spatial structure of the light beam."

The key word here is "group velocity"

~~~
nilkn
As a non-physicist, though, that sentence is still mostly unintelligible to
me. Could anybody explain it in simple terms? I don't really know what group
velocity is supposed to mean, nor do I really know what the spatial structure
of a light beam is.

~~~
grondilu
A light beam has an extension in space, and does so even if we're talking
about the probability amplitude of a single photon. Therefore there are
several ways to define the notion of velocity. One of them is the _group
velocity_ , which intuitively is the speed of propagation for the "shape" of
the wave, as opposed to the _phase velocity_ , which is the speed propagation
of its "zeros". There is a quite extensive Wikipedia article on the subject,
check it out:
[https://en.wikipedia.org/wiki/Group_velocity](https://en.wikipedia.org/wiki/Group_velocity)

Basically this ArXiv article shows that when a light beam has an unusual
spatial shape, its group velocity can become measurably slower than light. The
group velocity is usually equal to the actual speed of photons, so if it's
still the case here, the authors have shown that in certain conditions light
can propagate in vacuum slightly slower than it usually does.

------
rpedela
What is the effect of this result, if any, on established theories?

~~~
archgoon
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
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
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?

