How do I perform silent takedowns?

In Skyrim, when I backstab an enemy his allies notice, even if they're facing the opposite direction and I kill the target in one hit. This makes it difficult to ambush a column of soldiers escorting a prisoner.

Is it possible to kill the target in one hit without anyone noticing?

No kill is silent, because the target always makes a noise when they die. You can however (if your sneak is high enough and you're far enough away) get away from the body so you remain hidden, and they search for a few seconds before saying something like "My mind's playing tricks on me" (because you know, your friends always just die for no reason).

If you're far enough away from other enemies then they won't hear the target dying so it will seem to be silent (they don't have to be that far away).