Cryo Prison: Not something they use any more than any other kind of prison (although at least it would be more humane) – but in related concepts, they do keep uploaded mind-states of executed felons around in dead storage (places like the Aeon Pit) just in case. Turns out that with sufficient technology, you actually can undo the death penalty.
prison
Trope-a-Day: Prison Ship
Prison Ship: The Empire doesn’t use them as prisons – it finds the whole notion of prison really quite unspeakably barbaric – but lots of other polities have figured out the advantage of a prison surrounded in every dimension by a few million miles of vacuum. And enabling the threat of letting the vacuum in for riot control.
The Empire has been known to use purpose-built prison ships for internment and POW camps in time of war, though – pointing out that they can make them very comfortable prison ships and not resort to harsh levels of security and discipline to prevent escape because, well, unless you can breathe vacuum, where are you going to go? The propulsion bus disconnected and left, the guards’ section is physically isolated and can’t be got to without taking a walk outside, and so on and so forth.