That's the trick of designing a quantum algorithm. You set things up so that the "right" answer results in constructive interference, while the "wrong" answers interfere destructively. At the end of the quantum computation, all the probability rests in the answer, but in the course of the computation the system has explored many more possibilities through superposition.
You've hit on the basic power of quantum computing. In rough terms, while we have to go from 32 to 64 bits to double the processing power of a classical computer, we double the power of a quantum computer by adding just one more bit. This change in scaling allows you to do things like search a database in O(Sqrt[N]) time and factor a large number in polynomial time.
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That's the trick of designing a quantum algorithm. You set things up so that the "right" answer results in constructive interference, while the "wrong" answers interfere destructively. At the end of the quantum computation, all the probability rests in the answer, but in the course of the computation the system has explored many more possibilities through superposition.
You've hit on the basic power of quantum computing. In rough terms, while we have to go from 32 to 64 bits to double the processing power of a classical computer, we double the power of a quantum computer by adding just one more bit. This change in scaling allows you to do things like search a database in O(Sqrt[N]) time and factor a large number in polynomial time.