
Quantum teleportation demystified - lisper
http://blog.rongarret.info/2014/12/quantum-teleportation-demystified.html
======
tjradcliffe
The first step in demystifying quantum "teleportation" is to stop calling it
"teleportation", which is a term that is made to be misleading.

The second thing to do is to stop reading any article that ever says anything
remotely resembling, "the photon/atom/whatever was teleported..." since that
is a flat-out falsehood that only a ignoramus or a liar would publish.

No photon, no atom, no _thing_ has ever been "teleported" nor ever will be.
Only the quantum state gets "teleported", and the quantum state most decidedly
does not have the ontology of a "thing". That is why the quantum world is so
weird.

Quantum "teleportation" ought to be called something like "entanglement
transfer", which has the advantages of being a) fairly accurate and b) hard to
use in the misleading and dishonest ways "teleportation" gets used.

~~~
Xcelerate
> The first step in demystifying quantum "teleportation" is to stop calling it
> "teleportation", which is a term that is made to be misleading.

> No photon, no atom, no thing has ever been "teleported" nor ever will be.
> Only the quantum state gets "teleported", and the quantum state most
> decidedly does not have the ontology of a "thing".

I used to think the same thing as you, but if you accept quantum mechanics as
complete and accurate, then the quantum state describes the totality of the
system. There is nothing more than the state -- no other hidden variables.

So it comes down to the philosophical question: if you can make a perfect copy
of something, except you have to delete the original version in the process
(see the no-cloning theorem), is the new version the original "thing"? I would
argue yes, it is. It's completely indistinguishable in every way.

(Now do I actually believe quantum mechanics is complete? I'm not sure I do.
But if I _assume_ that it is, then quantum teleportation is really
teleportation.)

~~~
michael_nielsen
Asher Peres, one of the coinventors of teleportation, had an amusing story
(with a serious subtext) about the question of what is teleported: "Later,
when a newsman asked me whether it was possible to teleport not only the body
but also the soul, I answered “only the soul.” Even that is a gross
oversimplification."

[http://arxiv.org/pdf/quant-ph/0304158.pdf](http://arxiv.org/pdf/quant-
ph/0304158.pdf)

In teleportation experiments to date only some degrees of freedom are
teleported. So it doesn't usually make a whole lot of sense to say "the atom"
was teleported. Rather, it'll just be the quantum state associated to nuclear
spin states, or electronic degrees of freedom, or whatever.

~~~
Xcelerate
Cool article. I wonder what the maximum number of degrees of freedom that has
been teleported so far is. I also wonder if it gets exponentially harder to
transmit larger and larger quantum states (the only reason I posit this is
because it always seems like Nature prohibits us from doing all kinds of cool
things in an exponentially hard way).

------
abdullahkhalids
"when Bob receives Alice’s bits, he uses that information to apply one of four
transformations to his photon. It is Bob’s action that changes his photon’s
state into the state of Alice’s original photon. And that is the _only_ time
that Bob’s photon changes state."

I disagree with this particular bit. The state of Bob's photon changes exactly
when Alice does her measurement [1]. Why? The state of a photon is given by 2
real numbers (infinite bits). Bob receives 2 bits of information from Alice so
his 1/4 operation can't change the state of the photon to an arbitrary state.

The 'magic' does happen when Alice measures her state. After her measurement
Bob's photon is in one of four possible states, which are the same up to 2
swaps. The 2 bits of information are needed to find out which swaps to
perform. This swapping step nicely prevents faster-than-light travel.

[1] Caveat. In a relativistic universe, ordering of events depends on the
frame of reference. All arguments carry through when this is taken into
account.

Edit: I should explain how this saves relativity. No measurement (weak or
strong) can distinguish which pre-swap-state Bob has. You can try to be clever
and ask Alice to teleport millions of photon states to Bob. But that doesn't
help either because each of the four states are equally likely in the whole
ensemble. So you need 2 bits of information per teleported photon state to get
the correct final state each time.

~~~
lisper
> The state of Bob's photon changes exactly when Alice does her measurement
> [1].

No, it doesn't. If it did, then this would lead to faster-than-light
communication. The whole concept of "exactly when Alice does her measurement"
isn't even well defined!

> Why? The state of a photon is given by 2 real numbers

Actually they are complex numbers. But this part of your point is valid
nonetheless.

> The 'magic' does happen when Alice measures her state.

No, the "magic" happens when the entangled pair is first produced. Alice's
measurement changes nothing on Bob's side.

------
aidenn0
Ron attributes to malice here that which can be more easily explained by
indifference. I think Physicists have largely given up on trying to correct
media's portrayals of QM as they either see it as not being important or
because they see it as tilting at windmills.

The majority of experts I know feel like it's futile to try and correct the
media's representation of their field. I think QM has it doubly hard as it has
all the issues of other technical field, while suffering from both the problem
of its ontology being foreign to the lay-person and certain key points of its
ontology are still debated within the field.

~~~
lisper
In general I agree with you. But this case is different. This is not the
popular press publishing this false information, it's NASA itself. I think
they ought to hold themselves to a higher standard.

~~~
aidenn0
I guess I mentally put whatever part of NASA published this in the same
category as university PR departments. You almost certainly know more about
how NASA works than I do.

~~~
lisper
Your analogy is not wrong, but I think university PR departments ought to be
held to a higher standard too. If a university PR department issued a press
release that denied anthropogenic climate change or evolution there would be
an uproar. Why should it suddenly be OK to play fast-and-loose with the truth
just because the topic is physics rather than biology or climate science?

~~~
aidenn0
I had to think a bit before responding, but university PR departments
regularly report questionable findings. The two you picked are obviously
highly politicized topics, so it's obvious why _that_ would cause an uproar.

I agree that they ought to be held to a higher standard, but I think that the
motivation of scientists for not doing so is more out of apathy, and less out
of a desire to seem more mysterious.

