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Quantum Computing

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    $\begingroup$ The issue is, effectively, knowing which bits of communication have succeeded. If I have a long string and I know a random (approximate) quarter of the bits are correct, how does that help me? You need to wait for a classical message to tell you which ones the correct ones are. $\endgroup$ Commented May 14, 2020 at 7:12
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    $\begingroup$ Even if you could do that (you can't get the information back out again. there are other stack exchange questions about this), the fundamental issue is still the same: the receiver doesn't know whether the data they have received is correct or not, so it's no use to them. $\endgroup$ Commented May 14, 2020 at 12:49
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    $\begingroup$ “ However, as I understand it, this does not prevent one from storing 𝑛 classical bits into a single qubit, imperfectly cloning it 𝑛−1times, and thus retrieving 𝑛 classical bits out”. That is exactly what it prevents. When you clone, roughly speaking, it’s the same part of the state that gets copied many times, not different parts. $\endgroup$ Commented May 14, 2020 at 16:47
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    $\begingroup$ To put another way: the clones are highly entangled. You measure one, and they all collapse. $\endgroup$ Commented May 14, 2020 at 17:39
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    $\begingroup$ Random guessing works even 50% of the time! $\endgroup$ Commented May 16, 2020 at 10:28