[ExI] ftl communications again

scerir scerir at alice.it
Sat May 5 07:51:37 UTC 2012

>I vaguely recall a thread here on the possibility of faster than light
> communications based on quantum entanglement.  Did I dream that?  What was
> the name of the thread please and when?
> spike

It was based on the experiments by B.Dopfer.
Given the position-momentum entanglement
between photons (biphoton A and biphoton B)
biphoton A shows or doesn't show a two-slit 
interference pattern on a screen, depending
on what you measure on biphoton B (via Heisenberg's
 read from page S290

It is easy to imagine a FTL communication
via Dopfer's experiment. In example Cramer 
is trying such an experiment, since long time

It is not so easy to realize why it doesn't work.
It doesn't work because there is a sort of
second order complementarity principle.

The source of entangled photons we (must) use to perform
a two-photon interference experiment has a 'large'
size (divergence of the beam).

In other words it is known, since long time, there is a weird
'complementarity' principle between the usual one-photon
and the two-photon (between two entangled biphotons) interference. 
In the sense that the more you can see the first interference, the less
you can see the second interference, and viceversa.

So you cannot use the entanglement and Dopfer's
experiments to send FTL signals. (That is to say
you cannot have the right source of entangled photons).
See, i.e., these papers:

M.A.Horne, A.Shimony, A.Zeilinger, 'Two-Particle Interferometry',
Phys.Rev.Lett. 62, 2209 (1989).

M.A.Horne, A.Shimony, A.Zeilinger,
'Two-Particle Interferometry', Nature, 347, 429 (1990).

D.M. Greenberger, M.A. Horne and A. Zeilinger,
'Multiparticle Interferometry and the Superposition Principle',
Physics Today 46 8, (1993).

and these specific experiments ...

For new, different concepts read here:
http://arxiv.org/pdf/1011.3440v1.pdf (appendix B)

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