Quantum computing -- How is the relevant solution selected?

[jeremyfiennes]

TL;DR

During a quantum computation, all possible solutions exist in a state of superposition. How is the relevant one selected?

During a quantum computation, all possible solutions exist in a state of superposition. How is the relevant one selected?

 

[Demystifier]

By measurement.

 

[jeremyfiennes]

I looked at it, but could not find an answer. As I understand it: when factorizing a large number, for instance, a normal computer takes pairs of numbers, multiplies them, and if the result is not the desired one, tries another pair. And so on, working sequentially.
Whereas a quantum computer generates simultaneously a superposition state of all possible products of all possible numbers, and then selects the desired one. My question is: how is this done in practice?

 

[DrClaude]

I looked at it, but could not find an answer. As I understand it: when factorizing a large number, for instance, a normal computer takes pairs of numbers, multiplies them, and if the result is not the desired one, tries another pair. And so on, working sequentially.
Whereas a quantum computer generates simultaneously a superposition state of all possible products of all possible numbers, and then selects the desired one. My question is: how is this done in practice?

The quantum computer doesn't select the desired result. In the final stage of the algorithm, there is a measurement performed, and that measurement has a high probability of giving the correct answer, but the result has to be checked to make sure the right one was obtained.

https://www.quantiki.org/wiki/shors-factoring-algorithm

 

[jeremyfiennes]

Thanks. Dashing my hopes for a simple answer!