Re: Quantum communication might be possible?


primus wrote:
> This interesting thought experiment is based on the classical double
> slit experiment.
>
> First you record (without observing) the which-way information of each
> particle at the slits, and the particle landing point information at
> the back wall separately. You store the information in two separate
> memory storages, after which you move the second memory storage far
> away from the first one.
>
> Then you make a delayed choice about erasing or not touching the
> which-way information recorded at the slits particle by particle and
> stored in the first memory storage. This will "affect" the landing
> point information (of the respective particles) recorded at the back
> wall and delivered over a long distance as the second memory storage.
> In other words, you have quantum communication, do you?
>

The way it was explained to me was: suppose you have two boxes, one
with a black marble and the other a white marble. You mix the boxes so
that it is impossible for any human to know which is which. Send one
of the boxes to Alpha Centari and open it. You will instantly know the
color of the marble in the other box on Earth. This could be useful to
humans.

It is true that the quantum situation is different. You could have two
entangled qbits, each in its own box. We know that the q-bits stay in
an undefined state until observed, then the states will be
complementary. That is amazing, but it is not any more useful to
humans than the two boxes with marbles. So why bother with qbits?

To sum up: The entanglement propagates at the speed of light. So you
can't use the entaglement to produce faster-than-light communication.
If it were possible to have your cake and eat it too, to observe the
state of the photon and have the other guy be able to tell whether you
have observed it or not, then that could be used for communication.
But you can't. I say that the concept of "observing the photon first"
has no meaning, and that this experiment provides proof of that.
Correct me please if this is wrong.

.

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