Re: Has FTL communication really never been tested in this way?

Andreas wote:

Signal photons cannot cause their interference pattern
on the screen because their momentum uncertainty is large.
And their momentum uncertainty is large because the
source of entangled photons we (must) use to perform
a two-photon interference experiments has a 'large'
size (divergence of the beam).

This has nothing to do with our poor set up of the
experiment.

Since we were talking about the possibility of 'signaling',
at a distance, using a two-photon interference set-up,
having removed the coincidence detection unit, I was only
pointing out that the usual set-up, i.e. the usual SPDC
source itself, might not allow any single-photon interference
pattern, at the signal wing, for essential reasons (divergence
of the beam). More below.

Imagine a quantum eraser experiment with two slits where the
left slit is covered with a horizontal polarization filter and
the right slit is covered with a vertical one. If you shoot linearly
polarized photons (at an angle of 45°) through it there will be no
interference pattern on the screen because left and right linearly
polarized electromagnetic waves do not interfere.
Now, If you chose to measure circular polarization on the idler photon
and select those events where the idler photon has e.g. right circular
polarization then you would see the interference pattern for the
selected signaling photons on the screen.
(By choosing the left circularly polarized idler photons you would
see a shifted interference pattern).

Yes, I know these interesting experiments.
http://www.arxiv.org/abs/quant-ph/0106078
http://icpr.snu.ac.kr/resource/wop.pdf/J01/1998/033/R04/J011998033R040383.pd
f

You could argue now that it is possible to use circularly polarized
photons from the very beginning in which case you would see an
interference pattern on the screen without the need of any
coincidence unit. The question is then, what is the use of the idler
photon if not to decide on the type of measurement. And, does the type
of measurement actually change the interference pattern?
The answer by QM and experiment is definitely: No.

Again, an important distinction is to be made.
Two-photon interference and one-photon interference
are obviously different phenomena. In the first case
you need a coincidence detection unit of some sort
(two clocks at least). In the second you do not need
any coincidence device.

It seems to me (I may be wrong of course) that
these position/momentum correlated photons
'signaling' machines are based on a sort of ... fusion :-)
of the one-photon and the two-photon interference
phenomena (you perform a specific measurement on the idler
photons and, at a distance, without checking the coincidences,
an interference pattern would appear, or disappear,
at the signal wing).

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

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 ...
http://www.arxiv.org/abs/quant-ph/0112065
http://josab.osa.org/abstract.cfm?id=35389

Since the 'complementarity' principles, in general,
presuppose a 'smooth' transition from the visibility
of a phenomenon to the visibility of the other,
here we can also expect (imo) a smooth transition
from the visibility of a single-photon interference
to the visibility of a two-photon interference,
and viceversa. If there is an intermediate situation
in which both interferences are (badly) visible,
and if - in this intermediate situation - it is possible
to perform 'signaling' (in principle) experiments,
I cannot say.

Summing up. I think WE CAN AGREE that in the two-photon
interference we need a coincidence detection unit,
and in the usual single-photon interference we do not
need such a device. I am pointing out that it is not
just about the use of the coincidence unit, or the use
of specific detectors. There is much more physics beyond,
there are many essential principles involved here.

Regards,
s.




.

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