# Polarization is digital, sort of

John K Clark (johnkc@well.com)
Sun, 27 Apr 1997 21:50:59 -0700 (PDT)

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On Sun, 27 Apr 1997 Michael Lorrey <retroman@tpk.net> Wrote:

>When you use a polarizing filter, all it does is filter out the
>amount of polarization in a photon that is at right angles to it.

No, the photons are unchanged, the filter just reduces their number.

>If you have a photon with 45 degrees of polarization and an amplitude
>of 1.414, its amplitude is reduced to 1 as it loses one or the other
>leg of polarization to become oriented at either 90 degress or 0
>degrees.

You can't change the amplitude (color) of a photon, the energy of a photon
in a vacuum is determined by its wavelength and nothing else.

If a beam of light is polarized at 0 degrees and you set your polarizer at
X degrees then the number of photons in the beam will be reduced by
[cos(X)]^2, for 45 degrees that is 50%. For an individual photon there is a
50% chance it will make it through and a 50% chance it will not, if it does
make it through the amplitude has not been changed at all, if fact nothing at
all about the photon has been changed because it has ALWAYS been polarized at
exactly 45%. I don't claim this is a sensible way to run a universe, but it
has been conformed experimentally, some call it "Quantum Weirdness".

Some atoms produce 2 photons of the same polarization but traveling in
opposite directions, so if you measure the polarization of one you know the
polarization of the other. A billion years before I was born somebody in the
Virgo Cluster started making pairs of such photons that have identical but
unknown polarization. He sent one stream of photons to the earth, a billion
light years away, he sent another stream of photons to the Coma cluster in
the opposite direction from the earth a billion and one light years away.
A billion years later on Earth I spin my polarizer to a random direction,
record the position, observe if the photon made it through the detector or not
(a 50, 50 chance) then record that too. Now I spin the polarizer again and do
the same thing for the next photon (again a 50, 50 chance) and then for the
next several thousand photons. I know that a year from now a friend of mine
in the Coma Cluster will perform the exact same experiment on his stream of
photons and I decide to visit him. I get in a space ship with my records and
blast off for the Coma Cluster at 99% of the speed of light.

After 2 billion years I arrive in the Coma Cluster and compare notes with my
friend. I notice that the direction I had my polarizer turned to and the
direction my friend had his turned to were different, not very surprising
since both were picked at random, but then I find something astounding,
the square of the cosign of the angle between the 2 detectors for each photon
is proportional to the probability that a photon will make it through my
friend's detector.

I have instantly changed something that is 2 billion light years away and was
made long before dinosaurs walked the earth. Pretty weird. Unfortunately this
effect can not be used for faster than light communication because before I
arrived with my records the results of my friends experiment looked random
to him, it's only when he compared his results with my records, and that can
only be done at the speed of light or less, did it become obvious that
turning my polarizer and observing the results instantly changed his photons
far away. Nevertheless this does have a practical benefit, you can use it to
make a communication link that is absolutely secure, it's already been done
and not just in the lab. Recently two banks in Switzerland used Quantum
Cryptography to send secure financial data to each other.

John K Clark johnkc@well.com

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