More on entanglement signals possible or not?

Begin forwarded message:

From: Jack Sarfatti <This email address is being protected from spambots. You need JavaScript enabled to view it.>
Date: July 3, 2011 4:13:33 PM PDT
To: Ruth Elinor Kastner <This email address is being protected from spambots. You need JavaScript enabled to view it.>

Subject: Re: The issue of small n - FTL signals via over-complete non-orthogonal coherent states?

No I don't think it's that simple.
Sure, if you use spins then the eigenstates are orthogonal and no FTL signal. We all agree on that.

Simply because we maybe cannot do it with polarization entanglements does not mean we can't do it with other degrees of freedom.

The Glauber states are a different story. Suppose mean n i.e. <n> is small order of 1 or 2

z = <n>^1/2e^i@

the point is that there is coherence between the vacuum |0>, and all states |1>, |2> ....

but the peak is at <n> with a Poisson distribution. The coherence properties are qualitatively different from that of a sharp n Fock state - though not so different when <n> is small agreed.

Even when <n> >> 1 it is a mistake to think of it as a classical EM wave in the sense of Maxwell theory prior to laser beams.

I think Roy Glauber's original papers explain why. I need to refresh my memory on this from the 1960's.

Coherent microwaves are close to Glauber states, but not ordinary optical sources that are not lasers.

Glauber states and their squeezed variations have properties above and beyond their non-laser cousins e.g. from an incandescent lamp etc.

From: Ruth Elinor Kastner <This email address is being protected from spambots. You need JavaScript enabled to view it.>
To: Jack Sarfatti <This email address is being protected from spambots. You need JavaScript enabled to view it.>
Sent: Sun, July 3, 2011 12:38:36 PM
Subject: RE: The issue of small n

RK: OK then the issue is just the non-orthogonality itself ; this yields apparent FTL signalling trivially in cases where we're pretty sure it can't be done.  R.

JS:  All you sort of showed was that there was allegedly no way to prepare an over-complete basis of spin states of sharp n. Lasers show how to prepare many-photon states as Glauber states all with same polarization. The issue is then how they can entangle. Can we entangle laser beams with opposite momenta Fourier components as in the original EPR all with same polarizations? That's what I assumed in the toy model.
________________________________________
From: Jack Sarfatti [This email address is being protected from spambots. You need JavaScript enabled to view it.]
Sent: Sunday, July 03, 2011 3:00 PM
To: Ruth Elinor Kastner
Cc: nick herbert; david kaiser; Saul-Paul Sirag
Subject: The issue of small n

Assuming I did not make an error

Large <n> is not needed

Even small <n> works in principle, though perhaps with a much smaller signal to noise ratio?

If there is large n then only the constructive peaks contribute to the non-orthogonality
If n is small then more relative phases @ contribute

<A1'|x'><x'|A2> = e^-<n>(1-cos@(x'))  
this formula of mine may be wrong, I am not sure. It has nice properties however as a comb of Dirac delta functions when <n> ---> infinity at @ = integer multiples of 2pi (constructive interference of the Young double slit fringes).

where

<A1'|A2'> = x' integral over the screen of <A1'|x'><x'|A2>

we also need

<A1'|e^i@|A2'> = x' integral over the screen of <A1'|x'><x'|A2>e^i@(x)


On Jul 3, 2011, at 9:03 AM, Ruth Elinor Kastner <This email address is being protected from spambots. You need JavaScript enabled to view it.> wrote:

RK: Hi all, I was just addressing the point stressed by Jack about using a non-orthonormal basis to compute the partial trace, since that's crucial to Jack's goal of obtaining a nonlocal signal. This is independent of whether or not there's a detailed picture of the experimental apparatus. I just take it as a conceptual thought experiment.

JS: Exactly. For some reason Nick Herbert took umbrage at that.

RK: The point is that if you allow partial trace to be done in a non-orthonormal basis you can get trivial nonlocal signalling in an ordinary EPR -spin experiment. So the question is whether using a nonorthonormal basis has any genuine physical application -- which seems doubtful, but the point is that IF you could use such states for your experiment, and maintain the entanglement you apparently get nonlocal signalling. It appears to me from at least one of the refs Jack sent on entangled coherent states that the entanglement depends on having a very small avg photon number (< or = 2) so that the partial trace does not depart from the standard def., and that one cannot use those for nonlocal signalling (which in Jack's analysis requires large avg photon mumber). Again I think the crucial point is that large n coherent states approximate classical waves which can't be entangled at the individual photon level. I would love to try to work this out quantitatively but too busy right now...

JS: It's too simple to say large <n> means classical waves. That's like saying the superfluid is a giant classical wave. In other words there is a distinction between macro-quantum coherent waves and classical waves of the same <n> that shows up in the coherence correlation functions e.g. see modern books on quantum optics (or even Wikipedia).

In any case we agree this is a valid question that deserves careful investigation. I am not aware if any such work has been done?