# Can quantum computers really compute a vast number of possible solutions simultaneously?

They are not constrained to stepwise calculations but, rather, can compute a vast number of possible solutions simultaneously—and at a speed that is far beyond anything we can imagine.

Is this really true? AFAIU quantum computers cannot compute a vast number of possible solutions simultaneously. A quantum computer is not a parallel computer. It is true that the wave function will exist in a superposition of states before measurement, but when we make a measurement it will collapse to a definite state. And measurements is all we can do.

From scott aaronson’s blog (https://www.scottaaronson.com/blog/)

If you take just one piece of information from this blog: Quantum computers would not solve hard search problems instantaneously by simply trying all the possible solutions at once.

curious to know what experts think

• Does this answer your question? Quantum computers, parallel computing and exponential time – Kyle Jones Jan 30 '20 at 4:28
• It helps but I am afraid it does not answer my question. There ought to be a simple yes or no answer to the question and that is what I am looking for with a modest explanation. – morpheus Jan 30 '20 at 17:55
• there is actually a comment by yuval-filmus on that page which basically echoes what I have said in my question and that comment I think answers my question. – morpheus Jan 30 '20 at 18:17

It is not exactly wrong to think of superposition $$\frac{|0\rangle + |1\rangle}{\sqrt{2}}$$ as “both $$|0\rangle$$ and $$|1\rangle$$ at the same time”, but it’s confusing. It might make you think that both things are simultaneously available for arbitrary processing, which you can fully control. As if preparing a superposition is something like creating clones.
A more precise way to think about superposition is interpreting it as uncertainty. So, less confusing statement would be “neither $$|0\rangle$$ nor $$|1\rangle$$, until the setup forces it to be definite”.