# Re: Coherent states of a superposition

```> Il 4 dicembre 2018 alle 16.36 agrayson2...@gmail.com ha scritto:
>
>
>
>     On Tuesday, December 4, 2018 at 10:13:38 AM UTC, Bruno Marchal wrote:
>
>         > >
> >
> >             > > >             On 3 Dec 2018, at 20:57, agrays...@gmail.com
> > wrote:
> > >
> > >
> > >
> > >             On Sunday, November 18, 2018 at 1:05:26 PM UTC,
> > > agrays...@http://gmail.com wrote:
> > >
> > >                 > > > >
> > > >
> > > >                 On Saturday, November 17, 2018 at 7:39:14 PM UTC,
> > > > agrays...@gmail.com wrote:
> > > >
> > > >                     > > > > >                     If you write a
> > > > superposition as a sum of eigenstates, why is it important, or
> > > > relevant, or even true that the component states are coherent since
> > > > eigenstates with distinct eigenvalues are orthogonal. This means there
> > > > is no interference between the components of the superposition. AG
> > > > >
> > > > >                 > > > >
> > > >                 Put another way; from what I've read, coherence among
> > > > components of a superposition is necessary to guarantee interference,
> > > > but since an eigenstate expansion of the superposition consists of
> > > > orthogonal, non interfering eigenstates, the requirement of coherence
> > > > seems unnecessary. AG
> > > >
> > > >             > > >
> > >             For decoherence to occur, one needs, presumably, a coherent
> > > superposition. But when the wf is expressed as a sum of eigenstates with
> > > unique eigenvalues, those eigenstates are mutually orthogonal; hence,
> > > IIUC, there is no coherence. So, how can decoherence occur when the state
> > > function, expressed as a sum of eigenstates with unique eigenvalues, is
> > > not coherent? I must be missing something, but what it is I have no clue.
> > > AG
> > >
> > >         > >
> >
> >
> >         Decoherence never occurs, except in the mind or memory of the
> > observer. Take the state up + down (assuming a factor 1/sqrt(2)). And O is
> > an observer (its quantum state).
> >
> >     >
>         > >
> >         O has the choice to measure in the base {up, down}, in which case
> > the Born rule says that he will see up, or down with a probability 1/2. He
> > will *believe* that decoherence has occurred, but if we long at the
> > evolution of the whole system O + the particle, all we get is
> >
> >         O-up up + O-down down,
> >
> >         And some other observer could in principle test this. (O-up means O
> > with the memory of having seen the particle in the up position).
> >
> >         But O could measure that particle in the base {up+down, up-down).
> > He has just to rotate a little bit its polariser or Stern-Gerlach device.
> > In that case he obtains up+down with the probability one, which souls not
> > be the case with a mixture of up and down. In that case, coherence of up
> > and down do not disappear, even from the pot of the observer.
> >
> >         Decoherence is just the contagion of the superposition to anything
> > interacting with it, including the observer, and if we wait long enough the
> > whole causal cone of the observer.
> >
> >         Bruno
> >
> >     >
>     Thanks, but I'm looking for a solution within the context of interference
> and coherence, without introducing your theory of consciousness. Mainstream
> thinking today is that decoherence does occur, but this seems to imply
> preexisting coherence, and therefore interference among the component states
> of a superposition. If the superposition is expressed using eigenfunctions,
> which are mutually orthogonal -- implying no mutual interference -- how is
> decoherence possible, insofar as coherence, IIUC, doesn't exist using this
> basis? AG
> ```
```
There are instruments like the MZI (Mach-Zehnder Interferometer).. In this
insrtrument one (spli)amplitude goes through path A, the other (plit)amplitude
goes through par'th B. At the end of their travef both amplitudes recombine
interferentially giving *always a single* outome. As for the de-coherence
frankly i did not realize its conceptual meaning.

>
>         > >
> >             > > >             -- 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 everything-li...@googlegroups.com.
> > >             To post to this group, send email to
> > >             Visit this group at
> > >             For more options, visit https://groups.google.com/d/optout
> > >
> > >         > >
> >
> >     >
>
>
>     --
>     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
>     To post to this group, send email to everything-list@googlegroups.com
>     Visit this group at https://groups.google.com/group/everything-list.
>     For more options, visit https://groups.google.com/d/optout.
>

--
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