So, the light trapped in the crystal is stationary, and not just "bouncing around" inside, where we can observe it?
I guess the question could be phrased - is the light really stopped, or just confined to a REALLY small area (but the light still vibrates, wobbles, spins, or whatever else light does when trapped in a dark place)?
That's where the above analogy doesn't really work. Without reading the paper, I'd guess that the crystal enters a different energy state. A bit like a jet of water that's been 'absorbed' by a sponge, rather than sloshing around in a hollow ball [1].
[1] I know that's a bad analogy but I can't think of anything better off the top of my head right now.
For intended purposes like the "Quantum repeater" I believe it does not matter if the photons really stopped or wobbling in some very small space. For the repeater they need to time shift photons. (IMHO, I have no idea what I'm talking about :) )
If they are not halted and then let go, but instead bouncing back and forth, there's a chance that when you open the Cristal it may exit by the other side.
I guess the question could be phrased - is the light really stopped, or just confined to a REALLY small area (but the light still vibrates, wobbles, spins, or whatever else light does when trapped in a dark place)?