> On 7 Aug 2019, at 14:41, Bruce Kellett <[email protected]> wrote: > > On Wed, Aug 7, 2019 at 7:31 PM Bruno Marchal <[email protected] > <mailto:[email protected]>> wrote: > On 7 Aug 2019, at 00:37, Bruce Kellett <[email protected] > <mailto:[email protected]>> wrote: >> On Wed, Aug 7, 2019 at 4:00 AM 'Brent Meeker' via Everything List >> <[email protected] <mailto:[email protected]>> >> wrote: >> >> In something like Shor's algorithm there is only one final state with >> non-vanishing probability. >> >> Exactly. So there are no other components in superposition. Bruno wants to >> characterize this as a collapse. But without this happening, the QC would >> be unreliable -- there would be some finite probability that you would get >> the wrong result. As I understand it, there are some QC algorithms that do >> not lead to definite results, so you have to run them several times to >> reduce the probability of a wrong answer to some acceptable level. >> >> As usual, Bruno is introducing irrelevancies in order to distract attention >> from the fact that he cannot answer the central contention of my argument -- >> which is that quantum computer do not provide any evidence for the existence >> of parallel worlds. They can work perfectly well in just one world -- >> without any collapse whatsoever. > > I can’t explain even just the two slits without superposition. > > Superpositions are fine. It is just that they do not consist of "parallel > worlds”.
But then by QM linearity, it is easy to prepare a superposition with orthogonal histories, like me seing a cat dead and me seeing a cat alive, when I look at the Schoredinger cat. Yes, decoherence makes hard for me to detect the superposition I am in, but it does not make it going away (unless you invoke some wave packet reduction of course) > > “Parallel worlds/histories” are just a popular name to describe a > superposition. > > In your dreams, maybe. There is a clear and precise definition of separate > worlds: they are orthogonal states that do not interact. The absence of > possible interaction means that they are not superpositions. That is weird. The branches of a superposition never interact. The point is that they can interfere statistically, if not there is no superposition, nor interference, only a mixture. > > Then nothing in the QM linear evolution can explain how the superposition > disappear, > > Yes there is. If the dynamical evolution leads to zero coefficient for some > branch, then that branch drops out of the superposition. If all branches but > one go to zero in this way, then the superposition disappears. In some case that happens, but in the usual superposition, not tamed for computation purpose, the superposition remains. In fact they always remain, but disappear when we look in the base which admits those superposition as (dual) basis state. > > although the decoherence theory explain why it is hard to see a > macro-superposition. > Shor algorithm does not make them disappear, even if it manages to get the > result with high probability in the branch we are situated in. > > If the coefficient of a branch in the wave function vanishes exactly, then > the probability of obtaining that result is zero. The branch with zero > coefficient is no longer present in the superposition -- exactly, and not > merely FAPP. All pure states are superpositions of some other states. > The effective quantum computer sets all branches to zero except the branch > corresponding to the required result. There is no residual superposition. That’s true for some quantum computation, but in no case could a superposition disappear. A particle with a precise position is a superposition of all momentum, and vice versa. So depending of the base, you are in many similar world, or in many different world, but no superposition has gone away. That would need non linearity or non unitarity. Bruno > > Bruce > > -- > You received this message because you are subscribed to the Google Groups > "Everything List" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected] > <mailto:[email protected]>. > To view this discussion on the web visit > https://groups.google.com/d/msgid/everything-list/CAFxXSLSJgEiNLM6kAhfh3LxYZQOYfUp00Ftp2%3DN%2BO4JOmcoGrA%40mail.gmail.com > > <https://groups.google.com/d/msgid/everything-list/CAFxXSLSJgEiNLM6kAhfh3LxYZQOYfUp00Ftp2%3DN%2BO4JOmcoGrA%40mail.gmail.com?utm_medium=email&utm_source=footer>. -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/829142AD-90FE-4EE0-8D70-F3CF48A196F8%40ulb.ac.be.
