Slow light that does not go back to it's original speed in vacuum 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? 
 A: 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.
