Talk:Quantum teleportation

So far, the article mentions three "people" who are hypothetically examining and "teleporting" quantum state information, "Alice," "Bob," and "Carol."

Those of us who were at least teenagers in 1969 remember Bob & Carol & Ted & Alice, which its wikipedia article describes as a "1969 comedy-drama film" about two married couples who almost have a foursome in bed.

Does the inclusion of "Carol" in the description reflect wider usage in the QM community (I've seen "Alice" and "Bob" used throughout papers on quantum teleportation to identify hypothetical observers of quantum states in discussions of quantum teleportation, and the authors of the June 3 TU Delft paper on "unconditional quantum teleportation" actually name their quantum teleportation devices "Alice" and "Bob" ^[1]), or was it just a bit of humor on the original editor's part, alluding to the film I described? loupgarous (talk) 02:15, 10 June 2014 (UTC)

They are common names used in cryptography and physics. Carol is the classic third participant. Eve is commonly the eavesdropper. See Alice_and_Bob. 134.134.137.73 (talk) 21:35, 11 June 2014 (UTC)

A general question about the experiments that verify EPR, quantum teleportation, entanglement, non-locality etc.

When the experiment is done, is just one system and outcome at a time being observed, in an individualized fashion, with pauses between?

Or are the systems being produced in a rapid-fire way, one after another, and only final intensities (or perhaps statistics) being measured?

In other words I'd like a clearer picture of the individualization of the systems in the experiment. I've not seen this treated, so far, in the Wikipedia articles I've read on the subject. If it is already appears somewhere in Wikipedia, I'd be interested in knowing where. 178.38.115.176 (talk) 07:52, 5 May 2015 (UTC)

These question pertain to the real-world particles that might occur in the section called "Formal presentation".

(1) In a real experiment corresponding to this mathematical setup, what kinds of particles are A,B,C? Electrons? Photons?

(2) I noticed that the joint AB state is symmetric under exchange of the two particles. Does that mean A, B are identical and are bosons?

(3) On the other hand, the AC states are neither symmetric nor antisymmetric under exchange. Does that mean A and C are necessarily not the same kind of particle?

(4) Bob would send his qubit through the unitary quantum gate given by the Pauli matrix ${\displaystyle \sigma _{3}}$. How does he execute this unitary operation with, say, photons or electrons in a real experiment?

178.38.90.0 (talk) 18:49, 5 May 2015 (UTC)

The above-mentioned three gates correspond to rotations of π radians (180°) about appropriate axes (X, Y and Z).

Rotations of what? Is the spin representation of SU(2) being used? The named rotations all have order two and general an abelian group G of order 8 in SO(3). But the listed gates have orders 2, 2, and 4 and generate a nonabelian group of order 8 in O(2), namely the symmetries of the square. They lie in U(2) ∩ GL(2,R), but not in SU(2) which projects to SO(3). What is the relationship to X, Y, Z?

Something is wrong here?

178.38.90.0 (talk) 18:49, 5 May 2015 (UTC)

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Hello Jim1138:

As it is, the following paragraph appears to me off-topic:

"In October 2015, scientists from the Kavli Institute of Nanoscience of the Delft University of Technology reported that the quantum nonlocality phenomenon is supported at the 96% confidence level based on a "loophole-free Bell test" study.[10][11] These results were confirmed by two studies with statistical significance over 5 standard deviations which were published in December 2015.[12][13] "

1. First of all, these are not the first works supporting the existence of non-locality. Early experiments where conducted by John Clauser and Stuart Freedman in 1972, and later by Alain Aspect in 1982.
2. Second, should we cite these experiments without explaining what they are, or at least what they show? I mean here the above-mentioned paragraph talks about non locality but does not relate it with teleportation. The only relation somehow is that we need entanglement to perform teleportation, but there is already a pointer to the Wikipedia page to "quantum entanglement" in the first sentence. There the reader will find all the information about entanglement and about how to test for entanglement.
3. Thirdly, even though Bell experiment are somehow related to entanglement, the goal of these experiments has nothing to do with teleportation. For example, Bell's inequalities are known from the 60's while the teleportation has devised only from the 90's. In fact the relation between these two things is highly non-trivial.

For these reasons I think that, as it is, the above paragraph is uninformative and appears to be off-topic, therefore it should be removed. BooBoo314159 (talk) 11:32, 10 October 2017 (UTC) ; edited 09:28, 11 October 2017 (UTC)

It's about 5 years old and many new sources have been added since then. (The "arxiv.org" link below wasn't added by me, it's just listed here for some reason.) Mr. Dodo'sss (talk) 12:42, 12 November 2017 (UTC)

Since quantum teleportation depends on classical communication as one of its components, and that can proceed no faster than the speed of light, there's no way quantum teleportation can communicate faster than light unless there's already a classical way of doing so, e.g. involving exotic distortions of spacetime like wormholes. Even so, it would be the special spacetime topology responsible for the FTL communication, not the teleportation protocol itself. Therefore I think we should not suggest, as the previous wording "cannot currently be used" did, that quantum teleportation might some day might enable FTL communication.CharlesHBennett (talk) 21:53, 18 January 2018 (UTC)

recently different IPs have repeatedly added a dubious section on a paper about "3-particle teleportation". The section is written to promote the author of that paper and his institution. Moreover, what is discussed in that paper (among rather weird mentions of Planck length, topology, a "Physical System Theory Model and its counterpart as consciousness system model" that seem to have no bearing on the actual protocol) is the old idea that a multi-partite entangled state can be used to teleport from A to B with the assistance of the remaining parties, that was first published in more general form in W. Dür & J. I. Cirac (2000). "Multiparty teleportation". J. Mod. Opt. 47 (2–3): 247–255. doi:10.1080/09500340008244039.. The paper linked to in this section also discusses a generalization of teleportation to d-level systems, that was in general discussed by Werner in 2001 (Werner, Reinhard F. (2001). "All teleportation and dense coding schemes". J. Phys. A: Math. and Gen. 34 (35): 7081. arXiv:quant-ph/0003070. doi:10.1088/0305-4470/34/35/332.). I support @Dmr2:, who removed the section as "questionable"; if one wants to write a section on multiparty teleportation, it should cite the original sources (and be written in a neutral way). In order not to stat an edit war by simply removing it again i'd like to discuss the issue here. --Qcomp (talk) 17:27, 6 October 2018 (UTC)

having had another look at the literature, I propose to merge the two sections on teleportation using multipartite states and teleportation using higher dimensional systems into one section called "Generalizations of the Teleportation Protocol". The rational is that both are, in my view, rather technical and do not add conceptionally new aspects. Especially in the multipartite case there are quite number of different directions to generalize: (i) controlled teleportation (where C can control whether A can teleport to B) (ii) "shared teleportation" (where B and C receive the state jointly and only can access it in a cooperative fashion), (iii) genuine multi-receiver teleportation (where A wants to send a state to a group of receivers) (iv) imperfect versions (where the available states do not allow single-copy ideal teleportation, but still provide a finite quantum capacity). I propose the following replacement:

Generalizations of the Teleportation Protocol

The basic teleportation protocol for a qubit described above has been generalized in several directions, in particular regarding the dimension of the system teleported and the number of parties involved (either as sender, controller, or receiver). A generalization to ${\displaystyle d}$-level systems (so-called qudits) is straight forward and was already discussed in the original paper by Bennett et al. [REF]: the maximally entangled state of two qubits has to be replaced by a maximally entangled state of two qudits and the Bell measurement by a measurement defined by a maximally entangled orthonormal basis. All possible such generalizations were discussed by Werner in 2001 [Werner, Reinhard F. (2001). "All teleportation and dense coding schemes". J. Phys. A: Math. and Gen. 34 (35): 7081. arXiv:quant-ph/0003070. doi:10.1088/0305-4470/34/35/332.]. The generalization to infinite-dimensional so-called continuous-variable systems was proposed in [Braunstein, S. L.; Kimble, H. J. (1998). "Teleportation of Continuous Quantum Variables". Phys. Rev. Lett. 80: 869. doi:10.1103/PhysRevLett.80.869.] and lead to the first deterministic teleportation experiment. [Furusawa, A.; Sørensen, J. L.; Braunstein, S. L.; Fuchs, C. A.; Kimble, H. J.; Polzik, E. S. (1998). "Unconditional Quantum Teleportation". Science. 282: 706. doi:10.1126/science.282.5389.706.]

The use of multipartite entangled states instead of a bipartite maximally entangled state allows for several new features: either the sender can teleport information to several receivers either sending the same state to all of them (which allows to reduce the amount of entanglement needed for the process) [W. Dür and J. I. Cirac (2000). "Multiparty teleportation". J. Mod. Opt. 47 (2–3): 247–255. doi:10.1080/09500340008244039.] or teleporting multipartite states [Yeo, Ye; Chua, Wee Kang (2006). "Teleportation and Dense Coding with Genuine Multipartite Entanglement". Phys. Rev. Lett. 96: 060502. doi:10.1103/PhysRevLett.96.060502.{{cite journal}}: CS1 maint: article number as page number (link)] or sending a single state in such a way that the receiving parties need to cooperate to extract the information [Karlsson, Anders; Bourennane, Mohamed (1998). "Quantum teleportation using three-particle entanglement". Phys. Rev. A. 58: 4394. doi:10.1103/PhysRevA.58.4394.]. A different way of viewing the latter setting is that some of the parties can control whether the others can teleport.

In the end, it said (quote): Challenges faced in quantum teleportation include the no-cloning theorem which sets the limitation that creating an exact copy of a quantum state is impossible, the no-deleting theorem that states that quantum information cannot be destroyed, the size of the information teleported, the amount of quantum information the sender or receiver has before teleportation, and noise that the teleportation system has within its circuitry. Quantum teleportation, like any other sort of communication, cannot transfer information faster-than-light (no-communication theorem).

1. The protocol here in this text is for sending any quantum state, even ones that are unknown to the sender, and it does this by sending - not copying - the state. So it is meaningless to talk about the no-cloning theorem? It even says in the first reference cited, in the introduction, that it is possible to teleport an unknown state. (Also if the state is known to be orthogonal to some known basis then its possible to "side-step" the no-cloning theorem. For example it is possible to copy a known boolean value with CNOT since we know all boolean states are orthogonal to the {0,1}^n basis/comutational basis)

2. "the no-deleting theorem that states that quantum information cannot be destroyed, the size of the information teleported, the amount of quantum information the sender or receiver has before teleportation" ← this sounds weird to me. The size of the information sent is "one qubit" for the basic protocol, and it is known, and the qubit can be in any state (see above).

I ended up removing the paragraph. · · · Omnissiahs hierophant (talk) 16:43, 8 November 2021 (UTC)

I think you are right. The no-cloning theorem is not a challenge for quantum teleportation. It is rater one reason why quantum teleportation is necessary for certain tasks, and cannot be replaced by classical communication. Similar for the no-deletion theorem. Also, the sentence about faster-than-light communication should be preserved in the introduction (as you did), because it is a very common misunderstanding that quantum teleportation would achieve such a thing. --Geek3 (talk) 17:26, 8 November 2021 (UTC)

If we have an article on this, I can't find it. This is the (fairly) old idea of extracting energy from the vacuum, and there's an interesting new article on this here: https://www.quantamagazine.org/physicists-use-quantum-mechanics-to-pull-energy-out-of-nothing-20230222/ Excerpt: "... 15 years ago, Masahiro Hotta, a theoretical physicist at Tohoku University in Japan, proposed that perhaps the vacuum could, in fact, be coaxed into giving something up ... The first skeptic of quantum energy teleportation was Hotta himself. ... Hotta found, to his surprise, that a simple sequence of events could, in fact, induce the quantum vacuum to go negative — giving up energy it didn’t appear to have. “First I thought I was wrong,” he said, “so I calculated again, and I checked my logic. But I could not find any flaw.”

The trouble arises from the bizarre nature of the quantum vacuum, which is a peculiar type of nothing that comes dangerously close to resembling a something. ..."

Anyway, interesting stuff. Hotta himself has no Wikibio, and perhaps merits one. --Pete Tillman (talk) 20:06, 27 February 2023 (UTC)