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I have watched today episode of scishow where Hank claims that photons can be permanently slowed down to speed that is less than speed of light.

https://www.youtube.com/watch?v=YkGC47qD7qs&t=1m26s

How is this possible?

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The total confusion comes from mixing classical concepts, light, with quantum mechanical ones, photons. The paper just demonstrated that light can move slower in vacuum if manipulated optically before.

A light beam is composed out of zillions of photons , and its properties are emergent, are built up, from the wavefunctions of individual photons. What the experiment showed is that a relationship of phases and angles built up through the optical masks is retained when in vacuum; surprising, but not theory shaking, as the article explains:

The researchers explain this result by noting that they were using group velocity to measure the light's speed—a measurement of the group's envelope speed. The mask, they explain, caused some of the photons in the group to move at a slight angle to the other's causing a slowdown for the group as a whole. Thus, their results are not going to upend one of the basic tenets of modern physics, it is more likely that future researchers will have to make sure lab or astronomical observations are not being impacted by shape changes that occur naturally.

In the arxiv copy of the experiment, studied as a light beam:

The analytical form of this predicted delay (Eq. 1) suggests a simple geometrical model,where the delay arises from the additional length of the diagonal ray, propagating at an angle with respect to the optical axis.

The individual photons that make up the beam retain this form, which has been measured as a difference in arrival at a detector of two entangled single photons.

It is not single photons that have been measured to slow down, rather two photons traveling different distances enforced by the optical masking relative to each other.

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  • $\begingroup$ This does not address the issue. From the abstract: "We study the group velocity of single photons by measuring a change in their arrival time..." They are discussing single photons. $\endgroup$ – garyp Jan 9 '16 at 14:51
  • $\begingroup$ @garyp can a single photon have a group velocity? look at the one sentence summary arxiv.org/ftp/arxiv/papers/1411/1411.3987.pdf . It is light that has a group velocity . $\endgroup$ – anna v Jan 9 '16 at 16:13
  • $\begingroup$ Yes, all that's needed is a nonlinear relationship between $\omega$ and $k$. With a beam, we normally take $k$ in the longitudinal direction, say the $z$ direction. As long as the beam has transverse structure, $k_z$ will be vary as the angle of the wavevector component for a fixed frequency. The authors point out that this is well-studied classically; they have done a quantum optics experiment to show that it works at the single photon level. I'll admit that I don't understand your answer, but your last sentence is not correct. And, by the way, the experiment is not trivial $\endgroup$ – garyp Jan 9 '16 at 18:22
  • $\begingroup$ @garyp I have edited. BTW single photons have a single frequency characterized by E=h*nu. Their wave function being complex has phases and that join up to make a classical EM wave. Have a look at the photon part motls.blogspot.com/2011/11/… $\endgroup$ – anna v Jan 9 '16 at 19:11
  • $\begingroup$ An excitation of a mode of an EM field ( a single "photon") has indeterminate phase. It makes no sense to add the wavefunctions of separate photons. That's meaningless. One can have only multiple excitations of a single mode. Motls's post is very long, but he seems to be presenting the standard development wherein the the quantum state most closely resembling a classical wave is the coherent ("Glauber") state. But this has nothing to do with the question. When I have more time I'll address your assertion about longer path length. $\endgroup$ – garyp Jan 9 '16 at 21:36

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