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I don't understand how quantum teleportation is considered teleportation if no information is transfered (in q-bits). Wouldn't it be less misleading calling it "quantum encrypting" or something similar? For what I understand, you interact information with an entangled system, get information back, and use that with the other part of the system to retrieve information. No information was teleported. A two q-bit system "teleporting" one bit of information requires two bits to recover the information, that's exactly 4 possibilities, the exact amount of possibilities in the system, it's pretty much saying which combo it was, that's not teleportation... Am I wrong, or is the name just misleading?

Clarification 2.0: Encryption makes information "hidden" and then recreated later. Teleportation is movement of an object without translation. Is quantum teleportation really encryption, or is there no difference between encryption and teleportation? Does recreating a destroyed object count as time-travel (rhetorically using the same logic)? Hope that clarifies...

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    $\begingroup$ Possible duplicate of What are the benefits of quantum information "teleportation"? $\endgroup$ Commented Jan 9, 2018 at 22:00
  • $\begingroup$ This appears to be asking about our opinions of the phrase, which makes it off-topic as primarily opinion based. $\endgroup$
    – Kyle Kanos
    Commented Jan 10, 2018 at 11:13
  • $\begingroup$ I re-clarified, as my question is a real question on physics, but the solution might be found as a fault in definitions, I wasn't sure... $\endgroup$
    – Terran
    Commented Jan 10, 2018 at 18:26

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I find the claim that in quantum state teleportation "No information was teleported" to be pretty much flat-out wrong. This much is true,

you interact information with an entangled system, get information back, and use that with the other part of the system to retrieve information,

but the information that Alice gets back from interacting with and measuring the system she intends to teleport does not give her any information about that state - and, even if Alice tried, she would be completely unable to obtain enough information about the state to transmit it classically.

The core point of the issue is that quantum teleportation involves the transport of (i) a quantum state, and moreover, (ii) the quantum state of a single instance of the given system.

If the system were classical, then Alice could just measure the system, transmit the results, and Bob would use those as instructions to reinstate the system, but quantum states don't work like that: given a single instance of the system, there is provably no way to fully characterize its state. If you have an ensemble of copies, you can perform quantum state tomography on them, but if you have a single qubit, say, there are a bunch of non-commuting relevant observables, and measuring one of them will collapse the state and destroy all the information about what would have happened if you measured the others.

When seen like that, it looks downright impossible that any protocol short of physical transport of the system will be able to transmit that full quantum state to Bob, but that is precisely what quantum state teleportation achieves. In that protocol, Alice actively relinquishes her chance to make a measurement that would give her information about the state, and instead she performs an entangled Bell-basis measurement with a second particle (itself entangled with a counterpart controlled by Bob) the results of which describe the type of correlation that would be induced on the 1-2 pair (were the second particle not entangled; since it is, the results describe how the state to be teleported has been encoded in Bob's particle) but they yield no information at all about what that state was.

This is what's transmitted in the two classical bits you mention,

A two q-bit system "teleporting" one bit of information requires two bits to recover the information, that's exactly 4 possibilities,

$-$ yes, four possible encodings of the original state into Bob's system. However, your assertion that these two bits of information are somehow

the exact amount of possibilities in the system

is flat-out wrong. The information that Bob acquires in the protocol isn't one bit and it isn't two, it is one qubit, and that is a special kind of information all its own, which just isn't comparable to bits or measurable in those units. (Indeed, if you want to make such a case, then a qubit contains an unbounded amount of information, and thus there is a (fairly useless) sense in which the protocol transmits an infinite (or at least arbitrarily large) amount of information.)

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  • $\begingroup$ Am I missing something or is there still no transfer of information aside from the classical component? If knowing the state of the q-bit is only necessary at the end then the knowledge of the correlation is the information in the quantum system, the encryption method, I still don't see how information is being "teleported"? $\endgroup$
    – Terran
    Commented Jan 12, 2018 at 2:20
  • $\begingroup$ The quantum state starts off at Alice's location and by the end of the protocol it's at Bob's, without any information about the state itself being contained in the classical message. Plenty of people find that to be sufficient grounds for the name 'quantum state teleportation' to be justified. If you don't, no one's forcing you to. $\endgroup$ Commented Jan 12, 2018 at 8:48
  • $\begingroup$ Got it, did make me realize though, there is very little difference between encryption and teleportation, too little... We need better terms for things, keep running into not having words that describe things and having other words be too vague... So I suppose you're right. $\endgroup$
    – Terran
    Commented Jan 12, 2018 at 18:26
  • $\begingroup$ @Terran Naming stuff is hard, and even more so in science. Do not begrudge people their choices of terminology unless you have an alternative which is clearly superior in every respect (which you don't in this case) - and even then, you still shouldn't do it. $\endgroup$ Commented Jan 12, 2018 at 18:35
  • $\begingroup$ I'm not going after people because they called it teleportation, it just seemed to me that it was not teleportation, since it seemed to be only encryption. Oh well, now I have to go on wondering what the difference is because now the line just seems blurred. :/ $\endgroup$
    – Terran
    Commented Jan 12, 2018 at 18:50

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