Talk:Quantum teleportation

References removed

On 20:08, 16 Jun 2004, the following user : 130.39.223.46, removed the references from this article. Does anyone have a clue why he did this ? I am a new user, so there might be a rule that I am not aware of. Maybe it was because the references had no direct connection with the content of the article ?

Also, now that teleportation has been demonstrated with atoms, maybe we should remove " It is not clear if such a procedure can be scaled up to larger systems"

user michaelmestre

it would seem this was Subhash Kak (see my comment on his Talk page; he had his own reference removed (for unnamed reasons) on 18 Mar 2004, (equally anonymously, by 218.22.21.27). We may be witnessing some sort of auctorial rivalry here ;)

Dbachmann 17:48, 28 Jul 2004 (UTC)

Rewrote and added details

I just went to a quantum computation seminar, and part of it was on teleportation. Rewrote part of the article and added more technical detail. I don't really know how to do LaTeX formatting though, so if someone can please help clean up the equations to make them look nicer, it would be appreciated.

Pkeck - teleporting someone's brain will involve getting information about both the electronic quantum states and the position / momentum quantum states of all the particles. Right now quantum teleportation only transfers electronic quantum states.

24.17.245.100 09:47, 20 May 2006 (UTC)[reply]

Is this true?

Is this part of the Star Trek beaming section true?

- So, we cannot move matter from one place to another with quantum teleportation. Biological functions, especially the thoughts in the brain (the mind), depend not only on the right positioning of the atoms but also on their correct internal quantum state. These, we cannot copy, due to the no-cloning theorem. But, we can teleport them from the original onto the replica, and afterwards, the replica would live and think, and the original would maybe "crumble to dust." So, we did not replicate the human being, but teleported it. -

As far as I know, the quantum state of particles in tissues has no known impact on thoughts or other biological functions, or at least not to the extent that they affect consciousness or anything like that. (Of course, I'm overlooking simple things like the ionic state/electron configuration of atoms so that they can bond correctly with other atoms, but I don't think this is what the passage is saying.)

Where/when has somebody teleported the contents of one persons brain onto another? I'm going to delete this section, and if someone can provide a more clear explanation of this subject along with some reputable references, maybe it can be added back in.Pkeck 16:59, 1 Apr 2005 (UTC)

I am a layman

Lets dumb this piece down a bit, I am a layman and I have only an average knowledge of mathematics (no more than community college level). I think this article is trying to say that we can transfer the state of one piece of matter to another over a distance by manipulating one of the entagled entities quantum state. I am probably wrong ,so if someone could dumb this down or provide a paragraph that gives us an anology to something more tangible I would appreciate it.--Mikeroodeus 17:16, 23 Jun 2004 (UTC)

Re : I am a layman

Sorry if I offend you, but you are not an idiot ; in fact, quantum teleportation has philosophical implications hardly understood by anyone. I doubt there is a good analogy for it. The summary, (without going into considerations of it being believable or not, or real or not), is that the state is indeed transferred from one particle to the other, over an arbitrary distance, at a speed limited by the speed of light (because of the mandatory presence of a classical communication channel). Entanglement is just the algebraic formalisation of the constatation that the event "measurement of a physical quantity in one element of the entangled pair" has, very often (and more often that what could be conceivable by just pure luck), a consequence on the event "measurement of a physical quantity in the other element of the entangled pair". Sorry, it is very hard to explain things simply when you (that is, the person who explains) don't understand them fully ! Hope this helps. michaelmestre 11:10, 24 Jun 2004 (UTC)

On 2 Sep 2004, I have replaced the whole article by a complete rewrite. Here is the old text: Simon A. 08:12, 2 Sep 2004 (UTC)

Quantum teleportation is a quantum information processing operation which can be summarized as follows:

Suppose Alice and Bob (arbitrarily named protagonists) are spatially separated. They have at their disposition a classical information channel and share a perfectly entangled bipartite quantum state. Alice has a quantum system in a particular quantum state which she wishes to transfer to Bob. She does not know what the state is. Because measurements disturb quantum information, she cannot just measure her state and send the result to Bob over the channel. She could simply send him the system, but this involves the use of a quantum information channel which she may not have.

However, there is a method which allows her to transfer the state over to Bob by performing a manipulation involving her quantum system and her part of the shared entangled state, then sending 2 classical bits over the classical channel. Once Bob has received the information, he knows how to manipulate his part of the shared state in order to recover the unknown state at his location.

Alice's manipulation destroys her copy of the unknown state (if it did not, it would violate the no-cloning theorem). Note that despite appearances, this scheme could not be used for superluminal communication, because a classical information transfer is an integral part of the procedure.

The first experimental verification of the teleportation of the polarization state of a photon was reported in 1997. It is not clear if such a procedure can be scaled up to larger systems.

As quantum teleportation of the quantum state of a qubit was expected to be a key element in quantum computing, it might be significant that BARETT et al. report (Nature 429, 737 - 739 (17 June 2004)) "unconditional teleportation of massive particle qubits using atomic (9Be+) ions confined in a segmented ion trap, which aids individual qubit addressing. We achieve an average fidelity of 78 per cent, ...).

Teleporting an atom

The following text has been removed from the article:

The New York Times June 17, 2004 p.A21, reports that two teams, one from NIST, Boulder, Colorado, another from University of Innsbruck, Austria, have teleported atoms of beryllium and calcium, respectively, as published in Nature, June 17, 2004. The setup involves triplets of charged atoms (A, B, C) which are trapped in magnetic fields.

B and C are entangled.
C is moved away.
B and A are entangled.
The state of A and B are read, which affected C at a distance.
When a pulse of laser light was aimed at C, then C was turned into an A (but which destroyed the A,B state, by the no-cloning theorem)

Here's the links, to newspaper: [1], to abstracts: [2] [3]. I think this should be included in the article, which does not say a word about existing implementations. Conscious 07:20, 27 June 2007 (UTC)[reply]

New teleportation experiments are conducted on a regular basis. So far none have made it into the article. On the process you outlined: A and B are destroyed as soon as they are read. Contrary to what your link #1 seems to be suggesting no atoms are teleported, only quantum information that happens to be encoded in the atoms is teleported. Skippydo 15:02, 27 June 2007 (UTC)[reply]

I'm only arguing for the inclusion of information about existing experiments in the article. It's not me who wrote the passage above, but the links I have found may be helpful. If there are any newer (and more successful) implementations of quantum teleportation, they should be mentioned in the article. Conscious 20:16, 27 June 2007 (UTC)[reply]

It's a good idea. But there have been many steps along the way. There would be a lot of information to collect. I come across a few every month. Skippydo 11:56, 28 June 2007 (UTC)[reply]

External links

Yet Another Question from a Layperson

A question for those more knowledgeable than me: why must a classical information channel - i.e. something at the speed of light - be involved in quantum teleportation? I understand that this is because information cannot be transmitted faster than the speed of light; what I don't understand is *why*. Thank you very much for your time. - Brasswatchman August 20, 2005. 12:55 PM EST.

Hah! I've been working in this field for quite a while, and I can tell you we'd all like to know *why*. Dave Kielpinski 07:06, 15 December 2005 (UTC)[reply]

Quantum channels can not transport information in the classical sense. They just designate the transformation process of the quantum information. If you ever need to transport that information elsewhere, you must use classical means of transportation, whose speed is limited by speed of light according to the special relativity. So, you have two measured bits in your hand in place A, and you need to send them to place B somehow. The speed of that kind of transportation of information is always limited by the speed of light. There are no other alternatives. SYS64738 23:33, 1 February 2006 (UTC)[reply]

Cleanup Tone

There has to be a formal discussion of the topic before we get into Alice and Bob. Superm401 | Talk 09:28, 13 November 2005 (UTC)[reply]

As it stands now Alice and Bob have no introduction at all - 128.243.220.42 removed the intro which discusses indistinguishability, saying it had nothing to do with teleportation. As I understand it, indistinguishability is in fact intimately related to the idea, because it sets up the problem of how we tell one thing from another, which is related to how we can tell something has "moved". At any rate, I don't think Alice and Bob are necessary for a clear and relatively accurate description of the concept. Pjrich 04:49, 1 March 2006 (UTC)[reply]

Why is this non-formal?

May i suggest this thing be taken off the non-formal list? As far as I can see, it is fairly formally written.

Nazgjunk||(talk) 20:56, 18 November 2005 (UTC)[reply]

I don't agree. I write and review academic papers and this text is very informal according to those standards. The text is filled with subjective adjectives and adverbs, and the manner of explanation is very informal. SYS64738 23:44, 1 February 2006 (UTC)[reply]

Replicators

I was wondering: Would it be fair to say that Replicators would use a system much like this? Let's say I wanted a cup of coffee and I went over to the replicator and told it I wanted a cup of coffee, inside the replicators database is a large file on pre-programmed foods (or other objects) that the replicator has examined and stored. The Replicator has an ability to recycle as well, by "dis-assembling" un-needed objects such as garden rocks or dirt, then converting that energy into raw energy which would be used to assemble/produce your coffee (or whatever object). If this were possible, wouldn't it be basically the end of currency? If one could simply replicate their produce or product by using recycled energy what need would there be to go to the store? Davethewave 13:33, 15 February 2006 (UTC)[reply]

Landroo 08:19, 9 June 2006 (UTC) I think this type of "teleportation" needs to be reassigned to a science fiction discussion article. Quantum teleportation and Star Trek teleportation really don't belong in the same article.[reply]

Amen to that. If you just want a cup of coffee, it doesn't matter if it's in the same quantum state as some other cup of coffee used to be. —Keenan Pepper 14:43, 9 June 2006 (UTC)[reply]

No-communication vs. no-broadcast

This article mentions both a no-communication theorem and a no-broadcast theorem. I don't know enough about quantum teleportation to decide, but it sounds like these might be the same theorems. Are they? --Manscher 07:55, 9 August 2006 (UTC)[reply]

no, not quite the same. no-communication says communication can not be achieved via shared entanglement alone. on the other hand, no-broadcasting is a corollary of the no-cloning theorem, (quantum states can not be copied in general, therefore can not be broadcasted). Mct mht 08:12, 9 August 2006 (UTC)[reply]

I started no-broadcast theorem based on your answer. Feel free to expand it :-) --Manscher 07:27, 11 August 2006 (UTC)[reply]

Writing 3 particle state with new basis?

Could someone give a few lines of derivation of how this is done, or give a link to explain how this transformation is done? Thanks

try substituting according to the identities provided and check that the claim is true. it's straightforward. in linear algebra terms, this is a change of basis unitary transformation. Mct mht 06:24, 5 October 2006 (UTC)[reply]

Danes

http://www.sciam.com/article.cfm?chanID=sa003&articleID=000E9691-0261-1524-826183414B7F0000

vague statement

the following statement was recently added to article:

"teleportation...will allow quantum computing to be more secure and also function more quickly."

could the precise justifications for this claim to given (be it theoretical or experimenal)? Mct mht 01:39, 21 January 2007 (UTC)[reply]

Partial measurement

The idea of partial measurement needs to be explained more. It took me a while to understand that partial measurement is actually a technical term, and I had to look up elsewhere what it really meant. I feel that I am not qualified to explain it though. Could someone who is give it a shot?

130.126.39.182 04:53, 24 April 2007 (UTC)[reply]

i assume by "partial measurement" you mean that the measurement apparatus only interacts with part of the system. in the formalism, it means only certain factors in the tensor product are involved. it's stated in the article, including the section on general teleportation schemes (see the statements on the tensor factors). feel free to make the language more explicit. Mct mht 05:42, 24 April 2007 (UTC)[reply]

Undefined term

The term "Bell basis" is essential to the understanding of the article, yet defined nowhere. How have people who have gotten their understanding of teleportation from this article gotten past that hurdle? Has anyone actually tried to follow the article's math? --Vaughan Pratt 05:07, 1 August 2007 (UTC)[reply]

hello, Prof. Pratt. "Bell basis" refers to the four Bell states. they span

C^{2}\otimes C^{2}=C^{4}

; you get them by basically shuffling around +/- 1's and 0's between the four components in the obvious way. the article should probably point this out more explicitly. Mct mht 05:23, 1 August 2007 (UTC)[reply]

I've listed the four bell states. Thanks for the advice! Skippydo 05:40, 1 August 2007 (UTC)[reply]

Copying the four Bell states over from the Bell state article helps a little, but still leaves the term "Bell basis" itself undefined in both articles which was the main problem. Also the reader who actually tries verifying the identities for the rewrite of Alice's two qubits will come up with equations that seem to equate Alice's two qubits with the two qubits shared between Alice and Bob. Following "thus becomes", Bell states subscripted by A make their first appearance, without definition. In general there's a lot of moving around of subscripts that is neither adequately explained nor formally derivable from the present formulas---a lot of creativity is being asked of the reader. --Vaughan Pratt 06:42, 1 August 2007 (UTC)[reply]

In general, I think the article is a little 'too much' like a formal derivation, while it shouldn't be. This is an encyclopedia, not a quantum information manual. Maybe we should rewrite this article to include less formal equations and more wishy-washy hand-waving kind of explanations. Also, there should be more emphasis on the history of the discovery and what is does/might mean to science in general. UnHoly 07:03, 1 August 2007 (UTC)[reply]

As a metacomment, it's really too bad the quantum computing crowd has stuck with a notation better suited to analogue experiments than the digital world of qubits. The explanation would be so much easier to read if the Bell states were written ab + a'b' where a denotes ${\frac {1}{\sqrt[{4}]{2}}}(|1\rangle )_{A}$ and a' denotes ${\frac {1}{\sqrt[{4}]{2}}}(|0\rangle )_{A}$ . Having distinct variables for distinct qubits and treating them like literals in propositional calculus allows you to use ordinary high school algebra instead of having to invent a whole new set of rules for sound manipulation of subscripts. The need for Greek variables goes away if we use t and r for transmit and receive instead a and b for Alice and Bob (imagine what you'd think if a handbook of digital electronics named all its ports after people as a pedagogical technique to get people to think of ports as people sending and receiving bits). The data qubit to be teleported becomes d. Rather than primes for the negative literals I'll follow the convention De Morgan used in his 1858 paper introducing relation algebra of upper and lower case for positive and negative literals (but by all means use whatever you prefer for literals). The account would then read as follows.

The qubit to be teleported is in the superposition aD + bd while the teleportation channel is in the superposition TR + tr. The collective superposition is then (aD + bd)(TR + tr) = aDTR + aDtr + bdTR + bdtr. Teleportation is accomplished by rotating a measurement apparatus suitably in the four-dimensional space corresponding to qubits t and r, measuring the two qubits at that angle to produce two (classical) bits, transmitting the bits to the receiver, and using them to choose one of four unitary transformations chosen to transform the receive qubit to the superposition aR + br, the definition of teleportation of aD + bd.

Writing w, x, y, z for the coordinates of the frame of the apparatus, the appropriate orientation for the apparatus is 2w = DT + dt, 2x = DT - dt, 2y = Dt + dT, 2z = Dt - dT, the so-called Bell states. Expressed in this new framework our old framework had axes DT = w + x, dt = w - x, Dt = y + z, and dT = y - z. Our system state in apparatus coordinates then becomes w(aR + br) + x(aR - br) + y(ar + bR) + z(ar - bR). Measurement of the data and transmit qubits nondeterministically projects the whole three-qubit state onto one of these four axes, leaving the component of the axes associated with r unchanged. This puts the three entangled qubits in one of the states w(aR + br), x(aR - br), y(ar + bR), or z(ar - bR), up to a constant factor. The knowledge of which state the system collapses to permits the appropriate unitary transformation to be applied to the receive qubit to put it in the superposition aR + br.

Classically speaking, a and b (more precisely, a/b) express infinitely many bits, all but two of which were transmitted instantaneously at the moment of measurement. Thus already the receiver possesses most of the state of the teleported data long before the last two bits arrive (delayed by the speed of light) to complete the process. In the process the two qubits at the transmitter end have been put in a Bell state, thereby overwriting the original data and the transmit qubit with the two ends of a reusable but zero-length teleportation channel. Teleportation of a stream of qubits can be accomplished by keeping all four of the data, transmit, receive, and classical streams synchronized, bearing in mind that the transmit and receive streams must be prepared, and the receive stream physically transported, on an earlier schedule than the data and classical streams. --Vaughan Pratt 18:30, 1 August 2007 (UTC)[reply]

The types in the above should be declared as scalar for a and b, C² for D, d, T, t, R, and r, C^2ⁿ for terms of degree n (e.g. C⁴ for TR, aDT, w, etc. and C⁸ for DTR, wR, bdtr etc.), and (for completeness) Boolean for the two classical bits. Also it might be worth mentioning that since rotation of a rigid body has only three degrees of freedom (pitch, roll, yaw), any physical measuring apparatus needs to be articulated in order to have the requisite four degrees of freedom (the meaning of rotation in four dimensions). --Vaughan Pratt 20:26, 1 August 2007 (UTC)[reply]

You are confusing physical rotation and qubit rotation on the Bloch sphere. The Bloch sphere is an abstract space. Operations on qubits act like rotations, but they are not actual rotations. For example, if you use polarized photons as qubits, "rotations" are performed using fixed half-wave-plates and quarter-wave-plates. UnHoly 06:00, 2 August 2007 (UTC)[reply]

You make some good points. I just looked at the bell state article for the first time and it could use some cleanup. It should be pointed out, at the very least, that the bell states form a basis. But about the subscripts, perhaps they should be removed? Skippydo 01:07, 2 August 2007 (UTC)[reply]