Quantum teleportation

  • #26
selfAdjoint
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A Bell measurement (after the late John Bell, a physicist at CERN) is a measurement of a Bell state. It collapses the state.

A Bell state is an entangled quantum state of two particles such that if the state of one particle is measured (Bell measurement) the state of the other can be determined, even though the particles are separated.

qubit I think this is what you meant. It's a coined word, made up from "quantum" and "bit". It's pronounced "Q-bit" or cubit, like the Ark measurement.

In quantum mechanics the states of a particle can be superimposed. This does not mean added together, exactly nor does it mean mixed, the closest analogy I can think of is the notes in a chord. The notes are not added or mixed, but you hear them together, as a chord, which is something different from playing the notes one at a time (off topic, Schoenberg never seems to have really go this point). A superimposed state of values for one particle is not the same thing as the entangled state of two particles (Bell state).

So now think of "quantum information. The smallest unit of classical information is the bit. The bit can have one of two state, 0 or 1 (or on or off or many different possibilities). A bit is the amount of information you acquire in answer to a question that can be answered yes or no. For example the sex of a person (ordinarily) is coded in one bit.

Now let the two values or alternative of a bit be superimposed, quantum fashion. This is the minimum unit of quantum information: a qubit.
 
  • #27
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Thanks SA for your accurate explanations (like always), but I knew before what is a qubit. Qudit is another different thing, it appears for example in the technique called Qudit quantum state tomography
 
  • #28
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That was a new one on me, so I did what I always do, I googled. And I found http://www.physics.uq.edu.au/people/andrew/publications/2002/qudittomo.pdf [Broken] , with a definition. Qubits are mathematically determined by the spin-2 matrices SU(2), and the paper includes that definition with examples. SU(2) consists of 2x2 matrices of complex numbers that are Special in having determinant 1 and Unitary by their adjoints being their inverses. The adjoint is the conjugate transpose, if you follow me.

So then qudits are defined in the analogous way be dXd special unitary matrices SU(d). And where qubits have 2 superposed states it appears qudits can have d superposed states.
 
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  • #29
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Ansible communication?

I have heard the Ansible method discussed in past years where via the splitting of a particle(such as a photon)results in 2 halves which think they are still a whole particle.By taking one half of the particle to a different location it would be possible to use the link between the 2 halves as a conduit for messages.Is this a feasible theory or does the Uncertainty Theory preclude this?As to the QT of living humans i think it would be a long while before we have the computing power to capture a persons holistic wavefront and store along with the relative positions and states of every molecule needed for to translocation.
 
  • #30
selfAdjoint
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The method you are referring to depends not on splitting a particle, but splitting an entangled state of two particles. It is possible to produce two particles in a state such that if one is polarized this way, the other is necessarily polarized that way. So if you were to separate the particles, and measured the polarization state of one, you would automatically know the polarization state of the other.

This has been used by science fiction authors to instantiate the old sf idea of the ansible, or instant communicator. The word ansible goes back to the science fiction of the 1960's and has been used continuously ever since.

Would it work? No. The idea is that if you flip the particle here from the orientation you brought it with, to the opposite one, then the other particle, over in the Alpha Centauri system, will also flip - in the opposiite direction, and thus you have sent a bit from here to Alpha, and if you can send bits then you can send messages.

But the particles come to you with every possible orientation. If you flip a random one, the other one is random too, and your message is lost in noise. And how do you know your own particle flipped. You can't know what state it's in - remember they come to you in all possible states - unless you look at it. And here we come to the killer fact:

If you look at either of the particles, if you interact with it in any way whatsoever, you destroy the entanglement

It's called collapse of the wave function. It was the wave function that carried the entanglement.

So you're hoist on the horns of a dilemma. Either you check the particle to see how it's oriented so you can flip it - in which case you have no more entanglement and the bit won't be sent, or you flip it blindly in which case the receiver at the other end can't tell which particle you flipped or which way is a flip.

But maybe you're clever. You say, I won't use random particles, I'll use particles all oriented in a certain direction, by agreement. So I won't have to look first, and my friend on Alpha will know what a flip is. But the process that produces entangled particles produces them with all possible orientations, and there are symmetry arguments that say this has to be. So to get the particles to line up in some fixed way you'ld have to act on them - ooops! there goes your entanglement again!

Bottom line. There ain't no such thing as a working ansible.
 
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  • #31
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The person we were 1 minute ago, in our physical world as we percieve it, is no longer
the person we are now. (t- n, may never equal t) although I know our set theories tell us that
this should be a part of the set, (if n=0), but we have excluded n=0 many times in the past (boundaries?).

I don't see how this would affect what you are looking for, just might make some old human predjudices obsolete. We have to look at "n" approaching "0" to get as close as we are able.

I agree with Selfadjoint, it is unpalatable, but can not be proven to be incorrect. ( I was about to say is right, but this would not be true.)
 
  • #32
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It's worth to mention that there are various schemes, various arrangements of entangled particles and procedures to perform the teleportation. Each of these schemes is called a teleportation protocol. The first of these protocols was proposed in 1993 by Bennett, is called Bennett protocol. The protocol known as quantum scissors, was proposed in 1998 by Pegg, Phillips and Barnett. Kak teleportation protocol was proposed in 2003 in this paper
http://arxiv.org/abs/quant-ph/0305085
 
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  • #33
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I just notice this thread and will have a go through. I would not like disseminate what has already been hashed out logically and consistently.

I think this well sets the definitions for the Philosophy of Teleportation Thread.

Can these threads be joined under this heading?

http://wc0.worldcrossing.com/WebX?14@40.fznLbdAkWcL.9@.1ddf77cf [Broken]

regards,
 
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