[tetris11]

I always feel I'm missing something every time this is mentioned.

Isn't this more of a storage/caching issue than anything to do with quantum?

We have emulators where we can record every flipped bit should we wish to. We just don't because of the insane storage requirements, no?

[mac3n]

There's also thermodynamics - the more irreversability, the more energy cost. The lower bound approaches zero cost, for computations that jitter forward and backward, where you apply energy to nudge them more often forward.

[mac3n]

Quantum computations conserve information. If your gates (transformations) don't conserve information, then you'll break the entanglement and superpositions. At the end of a computation, you do this in a measurement to get an answer.