What is the difference between quantum computing and parallel computing?
For a parallel computer, we need to have one billion different processors. In a quantum computer, a single register can perform a  billion computations. This is known as quantum parallelism. Also a quantum computer returns a single randomly-selected correct result, while a parallel computer can directly return all valid results. That's the reason why reading results out of quantum computers generally is slow and getting all results requires running the quantum computation enough times to be relatively certain that the random sampling has seen all possible results.
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