
How Quantum Teleportation Works - Hooke
https://quantum.country/teleportation?access=patreon
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cousin_it
Quantum teleportation isn't a very good name. The best name I can come up with
is "sending quantum information over a classical channel using a quantum one-
time pad".

More precisely, let's say Alice has a qubit X in some unknown state, and also
Alice and Bob have a pair of qubits A and B in a certain known state with a
preexisting mystical link between them. Now Alice can apply a certain quantum
gate to X and A, measure them, and send the two classical bits to Bob over
ordinary internet. Then Bob can use these bits to choose which quantum gate to
apply to B, making it end up in exactly the same state that X started with.
The original state of X is "spent" in the process, Alice can't access it
anymore. The mystical link between A and B is also "spent", they can't be used
the same way again, unless Alice and Bob bring them physically together and
reestablish the link. The coolest part is that if X was mystically linked to
something else, then Bob's qubit ends up mystically linked to the same thing.

The math details are pretty simple (some reflections and 45-degree rotations
in 4-dimensional space) but involve some unusual notation that this comment is
too small to explain.

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roywiggins
I guess a handwavy worked version of the one-time-pad analogy would be 1) a
shared random pad that you can't read without destroying it, 2) some data to
send that you can't read either and 3) a destructive XOR operation that
destroys both inputs

Alice runs the XOR operation (destroying the data and her copy of the pad),
send the result to Bob, who runs it against his copy of the pad, producing the
original data.

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umvi
Maybe I'm just dumb, but I think quantum mechanics needs either a paradigm
shift or another Feynmann who can come along and make this stuff more
grokkable. Every explanation I read about entanglement or teleportation or
whatever is always amazingly long and complex, no matter what.

I'm able to understand other physics concepts like relativity, but quantum
mechanics seems too entrenched in mathematics at this point in time. I have a
very hard time visualizing what is going on.

~~~
AcerbicZero
I agree. I read this 2 or 3 times, and while I've read about the concept
before and I'd like to think I have at least somewhat of an understanding of
the "idea" at least, this was a total loss for me. If something can't be
explained without resorting to actual Greek characters and irrational numbers,
it might be worth trying again.

I don't know if I was just grumpy from the struggles of trying to parse this,
but having the author interrupt every other paragraph with a tangent about how
I shouldn't feel bad for not understanding this immediately didn't
particularly help matters.

~~~
klank
> If something can't be explained without resorting to actual Greek characters
> and irrational numbers, it might be worth trying again.

Can you expound on this thought a bit? It's not entirely obvious to my why
using math, or mathematical concepts, to explain something necessarily
invalidates the explanation.

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echelon
Can someone explain the following:

> _Before we verify that the teleportation circuit works, let 's briefly
> discuss one of the most common questions about quantum teleportation: does
> it enable faster-than-light communication?_

> _At first, it looks as though it may – after all, Alice is able to transmit
> her state |\psi\rangle∣ψ⟩ to Bob, even if he 's very distant from her. It'd
> be quite marvelous if it enabled faster-than-light communication, since that
> in turn would give rise to many incredible phenomena, including the ability
> to send information backward in time._

> _But while it would be marvelous, it is not possible. You can see the
> trouble if you think closely about the protocol. Remember, for Bob to
> recover the state |\psi\rangle∣ψ⟩, Alice must send Bob two bits of classical
> information. The speed of that transmission is limited by the speed of
> light. Without that classical information, Bob can 't guarantee that he
> recovers |\psi\rangle∣ψ⟩. Instead, what he has is a distribution over four
> different possible states. And while I won't prove it here, it turns out to
> be possible to prove that with only that distribution over states, no
> information is transferred from Alice to Bob. It's a pity, but that's the
> way the world seems to work._

Specifically,

> Without that classical information, Bob can't guarantee that he recovers
> |\psi\rangle∣ψ⟩.

and

> it turns out to be possible to prove that with only that distribution over
> states, no information is transferred from Alice to Bob.

Why is no information encoded in the distribution? Why can't it be
statistically gleaned?

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lswainemoore
[http://www.smbc-comics.com/comic/fossils-3](http://www.smbc-
comics.com/comic/fossils-3)

