PHYS; Quantum Teleportation

John K Clark (
Thu, 11 Dec 1997 22:54:55 -0800 (PST)


On Thu, 11 Dec 1997 Brent Allsop <> Wrote:

>>If I want to send a message to a receiver 2 things must happen:
>>1) I must change something in the receiver.
>>2) The receiver must be able to compare that change with something
>>else, such as the voltage on a wire now compared to what it was a
>>nanosecond ago.

>What? If you can't detect any change (#2) how could one claim a
>change (#1) has even occured?

You can't, not then, only when records are compared is the change obvious.
There is absolutely no doubt that it is possible to change things far away
much faster than light, with infinite speed in fact. This is not just theory,
it has been proven experimentally, but unfortunately that's not enough to
sent a message. A message must not only make a change but also provide a
standard the change can be measured against, otherwise the message will look
random. Let me give a specific example.

Some physical processes produce 2 photons that have the same polarization but
move in opposite directions. A billion years before I was born somebody in
the Virgo Cluster started making pairs of 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 also a billion light years away.

A billion years later on Earth I spin my polarizer to a random direction,
record its position, observe if the photon made it through the polarizer or
not and record that too, the exact time also. Now I spin the polarizer again
and do the same thing for the next photon and then for the next several
thousand photons. I know that at the same time I'm doing this a friend of
mine in the Coma Cluster is performing 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
pair 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. But this effect
can not be used for faster than light communication because before I arrived
with my records the results of my friend's experiment looked random to him,
half the time a photon made it through his detector and half the time it did
not. 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. In a way you could say that I did send him a message faster than
light, but it was encrypted and can only be read when I send him the key at
light speed or less.

Actually cryptography is one area where this effect does have a practical
value, you can use it to make a communication link that is as secure as the
laws of physics, it's already been done. I wrote about Quantum Cryptography
in a post about a year ago, if anybody's interested I'll dig it out.

John K Clark

Version: 2.6.i