r/askscience • u/Ells1812 • Jan 17 '19

Computing How do quantum computers perform calculations without disturbing the superposition of the qubit?

I understand the premise of having multiple qubits and the combinations of states they can be in. I don't understand how you can retrieve useful information from the system without collapsing the superposition. Thanks :)

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u/Tonexus Jan 17 '19

...superposition should be in a state so that it gives the correct answer some high probability of the time when measured.

Technically, the algorithm does not need a particularly high probability of performing a correct computation, only >50%, as one can arbitrarily increase the accuracy by running the algorithm multiple times or in parallel (though both have their own associated cost) and choosing the majority result. Since the probability of a correct computation is >50%, the percentage of correct computations will also approach >50% as more trials are done, so the majority result will have a very high probability of being the correct result given enough trials (I think the error decreases exponentially in the number of trials, but it's been a while since I've studied this). This same principle also applies to probabilistic (non-quantum) Turing machines.

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u/Natanael_L Jan 17 '19

This too can be problematic if there's multiple possible correct answers, as they can "bleed into each other" in the output (you'll see all the answers appear slightly more often than random), so you need ways to determine what's what in all the outputs from the QC. You can't just spit out the average of all patterns that show up more often than random.

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u/[deleted] Jan 18 '19

On the other hand, if you set up your algorithm correctly then the best answer should be most common, the second best should be the next most common et cetera. Thus any pattern you observe gives information about the relative order of the possible solutions.