I have seen various explanations for the power of quantum computers:
Quantum computers perform operations in parallel universes
Quantum computers can use quantum tunneling to reach a global extremum of a function
Quantum computers can manipulate quantities of information smaller that 1 bit independently and perform logical operations on such quantities.
Constant entropy of unitary evolution allows quantum computers to roll back in time, when the answer is wrong, so to perform multiple calculations during the same time interval.
Which of these explanations is more correct or do they supplement each other?