What do you mean by "an impossible gap?" If it wasn't possible, it wouldn't happen. Even beyond that, "impossible" does not entail great magnitude, but simply something outside the set of possibilities, which could be good or bad.
In quantum physics, a particle can be found at energy level 0 or energy level 1, but cannot be found at any level in between. (for example, this is a result of the infinite 1D quantum well). That is the impossible that I was talking about.
It's a very tiny jump in energy according to human perception - the interesting part is that the particle is only in state 0 or 1, never "in between".
It has nothing to do with size of the jump and everything to do with continuity along the path. A "quantum leap" from A to B implies that the path from A to B was discontinuous and therefore completely unpredictable given the knowledge you had at point A.
In other words, You went from A to B without going through any intermediate steps.
Said yet another way: B is a post singularity relative to A.
To my reading, OP didn't intend to convey "unpredictable" so much as "large," and if he did, then I'm not sure what he was thinking; "quantum leap" isn't even colloquially understood to mean "unpredictable change".
