From: Hal Finney (hal@finney.org)
Date: Sat May 11 2002 - 16:41:24 MDT
Serafino writes:
> Of course. They must discard every event wich has nothing to do with
> entangled photons. I also wrote "It is not easy to find a big good
> source of (just) entangled photons! There is always some noise in the SPDC.
> In these experiments they have to use filters, polarizers, and detectors
> of coincidences to be sure that they are recording just entangled
> events."
and:
> Hal Finney wrote:
> > But the point is that we can only identify coincidence events via
> > ordinary communication.
>
> Yes. (Unless we have a perfect big source of entangled photons).
No, I don't believe this is correct, if I am understanding you. Even
if the source were perfect and produced nothing but entangled photons,
there would still be a problem.
What we are doing on the left side is trying to measure the photon's
position by passing it through a narrow slit. Only some photons get
through the slit; most are absorbed by the mask. Only the ones which
get through the slit have been measured as having a narrowly localized
position. The remaining photons, even perfect EPR photons, are not
localized by this measurement.
It is the photons which have been measured, which have passed through
the slit, which produce the special interference effects viewed at the
other side. It's not a matter of just identifying good EPR photons,
it's a matter of knowing which of those EPR photons got measured as
having that localized position.
The coincidence detector identifies the photons on the right which
correspond to photons at the left which made it through the slit.
When we restrict our attention to this subset of the photons, we can
see effects relating to the slit size. But the only way we can know, at
the right side, which subset of photons to consider is via conventional
information from the left which tells which of the photons made it through
the left slit. So the special effects are only noticeable with the aid
of ordinary communication, regardless of the perfection of the EPR source.
Hal
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