
High-dimensional orbital angular momentum entanglement over a 1km fiber - bookofjoe
https://www.osapublishing.org/optica/abstract.cfm?uri=optica-7-3-232
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tyingq
_" future OAM-based high-dimensional long-distance quantum communication"_

As a layperson, this is confusing. I thought you couldn't use entanglement for
communication.

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jerf
As others say, it doesn't enable FTL communication.

But if you have an entangled set X and Y, you can send your partner X while
you keep Y. When you both look at X and Y, you will randomly get 0 or 1 while
they get the opposite. While you can't control which you get, you _can_ take
advantage of the fact that only you and your partner know what these values
are and it is guaranteed that nobody else knows what they are. You don't know
how your partner has used this info until you get the conventional
communication.

It's more complicated than that in reality but that's the principle of why
it's useful.

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echelon
Can you "nudge" the system without collapsing the entanglement? And if so,
could you do so in a way that predictably impacts the value of the entangled
property?

I find it hard to grasp that there's no clever way to impart information over
entanglement (without relying on classical messaging).

Is the state of entanglement fragile beyond our limits to record or alter?
What about an ensemble of thousands of entangled particles - could we
statistically sample some of them and force how they resolve?

What are the different properties that can be entangled? Angular momentum
(spin, orbital) ...? Surely there's something we can clock and predict in the
real world without requiring us to collapse the wave function to know what the
state would be.

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jerf
"Can you "nudge" the system without collapsing the entanglement? And if so,
could you do so in a way that predictably impacts the value of the entangled
property?"

You can nudge it in a couple ways, and you can _statistically_ predict what
may happen as a result. However, none of the ways you can do that result in
communication, because you can't get around the way you can't _choose_ the
collapse. There is no way to communicate because you can't force the decision.
If you do force the decision you break the entanglement.

I _think_ this is one of the main errors people make on this matter; you hear
"ah, if mine is "up" then theirs is "down" automatically, so surely if I set
mine to 'up' then I can communicate with them?" But you don't get to "set" it.
You have no choice in the matter. It just happens. In communication, you are
ultimately communicating your choice to have a 0 or 1, and you can't
communicate when you have no choices to make. In more classical terms, it'd be
like me trying to send you a message with _next week 's_ lottery numbers. I
can't; I have no control over those numbers. (Not a precise analogy since
there's no "quantum" there, but I think the intuition is reasonable.)

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echelon
> If you do force the decision you break the entanglement.

"Measurement" causes collapse, correct? What is different about measuring the
system in a way that forces how the system collapses versus a way that results
in a random result?

Does the other entangled system yield random results if you try to force the
collapse of its entangled sibling?

Can you describe some experimental setups? I'd like to know how we found that
out.

~~~
jerf
You can't "force" collapse in a particular way. You can read UP vs. DOWN, or
you can fiddle it around so you read LEFT vs. RIGHT, but there simply is no
way to "measure" an outcome to be UP in the UP/DOWN direction,
unconditionally, thus forcing the other side to get DOWN.

The operation you are depending on to communicate doesn't exist.

It's a bit weird that you can choose to read UP/DOWN and your counterpart can
read LEFT/RIGHT, or even a bit of both by reading at an angle, but you still
can't move classical bits around that way, because you can't read "RIGHT". If
you do fiddle with things to force a result, the fiddling will erase the
entanglement, because under the hood, in math, those two things are basically
equivalent statements. If you _do_ force something, your forced result will
have no bearing on what your counterpart gets; if you force "RIGHT" all that
will happen is that your counterpart will still get a 50/50 in the LEFT/RIGHT
basis. You didn't communicate anything.

~~~
echelon
That makes sense.

Thanks so much for explaining this!

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sabujp
i thought this was about space elevators until i read the abstract

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sebastianconcpt
I'm fascinated by the contrast of conceptual abstraction and concreteness in
that title (and work of course)

