# Quantum teleportation.

1. May 30, 2014

### Alex Jong

Hey, i hear about quantum teleportation on the news today. They said that they successfully teleported information. Can someone plz explain it to me in a simple way so that a 13 y/o can understand it?

2. May 30, 2014

### UltrafastPED

3. May 30, 2014

### m~ray

u bought a car and u need to call up a friend (in a different city) and describe the car to him in details. So you convey to him all classical information about the car - like its shape, size colour etc. with a good description your friend can even make a good replica of it at his location. this is like REMOTE STATE PREPARATION. teleportation is similar but u dont know the car's characteristics yourself but u still need to convey certain information so that your friend can reconstruct the car at his place. in quantum mechanics every thing is described by a quantum state. you too have a quantum state which changes all the time. theoretically it is possible to destroy this quantum state at your location and create it at another location after doing some quantum mechanical measurements at your place n communicating the results to the other location lab. hence u cease to exist in the first place and pop up at the other (philosophical question that if it is the same you or not, but biologically u will be same). of course doing that practically is a huge challenge n often considered impossible for a large system.

in the recent Science journal paper, they say that they have been able to teleport a single qubit (a quantum register that stores quantum information just like a bit (0/1) in classical information) across 3m distance. in other words they've taken an unknown quantum state of a qubit and recreated it at another location.

4. May 31, 2014

### vishnumanu

I recently watched a documentary on youtube which explains quantum teleportation in a very simple way. Here's the link.

Last edited by a moderator: Sep 25, 2014
5. May 31, 2014

6. Jun 1, 2014

### thephystudent

I mean, imagine one of the two particles moving. Then what is instantly? At the same time in the lorentz-frame of the sender or from the receiver? It seems to me you can break causality this way? Or is this space travel and an argument for multiversum interpretations?

Or, does nobody know and are all physicists desperately looking for the solution? :p

7. Jun 1, 2014

Using the Many Worlds interpretation of quantum theory, is 'action at a distance' still a strange feature?
In my mind, if there is no wave function collapse then there is nothing unusual about the fact that finding out information on one particle will imply the same information on a 'entangled' particle light years away.
Or am I wrong and there is still some action happening at a distance?

8. Jun 1, 2014

### Strilanc

Entanglement enables more than just finding out information about the other particle. You can use it to coordinate without communicating in ways that aren't possible classically, like winning the Mermin-Peres game 100% of the time instead of 88% of the time.

9. Jun 1, 2014

### kurros

Well it is still a little freaky. The fact that the particles are entangled means that the different worlds, in which the particles are in each of their possible states, are still entangled, i.e. they have not split yet. When you do one of the measurements and find out what state one particle is in, then the world-splitting propagates away from you at the speed of light. By the time you can talk to the other guy about what their measurement was, the world-splitting already reached them, so you will only find that their measurement was the one determined by your measurement.

Maybe that is more understandable than the usual story, but to me it is still a pretty wild concept.

10. Jun 1, 2014

### Staff: Mentor

The experiment you mention is consistent with modern QFT and does not violate causality in any way.

Please keep the discussion consistent with the professional scientific literature and do not engage in personal speculation.

11. Jun 2, 2014

### thephystudent

I'm sorry for my speculation, but please let me clearify.
With pole-barn I meant the version where a vase stands on a table. There is a machanism that lets the vase fall on the ground driven by an AND-gate between signals from the two gates, so the vase should fall if both are closed the same time and if not it should stand on the table, so the result will be different in the frame of the barn than in that of the pole. The solution of this paradox in special relativity is that the signals from the gate to the vase can't go faster than light and so if it falls or not is the same in every frame. But if sending qbits goes instantly, in the frame of the barn, the vase falls and in the frame of the pole it doesn't.

But ok, if you say that instantly is by definition in the frame of the sender, then I have another issue. Imagine a particle A moving at high velocity past a particle B and sending a qbit of information to it instantly. For particle B it seems that it receives the information before it was sent. Now, B can be coupled to another particle B' in the same frame via a mechanism so that if B has received signal B' sends another signal to A' in the same frame as A, instantly in the B-frame. And one can think of a mechanism that lets A' prevent A to send the information to B in the first place.

If this does not violate causality in modern QFT, what are its solutions to this paradoxes? I tried to look for it on the internet because I thought many people would have asked the same question(I'm taught so far that entanglement can only cause a collapse but not send information, precisely because of these paradoxes with SR), but I didn't find the solution.

Last edited: Jun 2, 2014
12. Jun 2, 2014

### Staff: Mentor

However, sending qubits does not go instantly, at least not the way you're thinking.

Look at the wikipedia article at http://en.wikipedia.org/wiki/Quantum_teleportation, then consider how you might use the two "teleported" qubits as inputs to an AND gate. It won't violate causality.

13. Jun 2, 2014

### Thinkor

They have not successfully teleported true information. They have successfully created the same information in two places simultaneously.

The utility lies in the secure random generation of encryption keys. I have some qubits in an indeterminate state. I query them, thereby determining their state, and their entangled counterparts necessarily will have the same state.

You cannot send the message "Special relativity has just been proved wrong" faster than light in this way. You can however ensure that two people separated by any spatial distance who have previously entangled two qubit registers using lightspeed communication, can instantaneously share a newly generated random sequence of bits, which could, for example, be used as an encryption key or as a guide to performing other coordinated activities at the two locations.

The scientific article by Hanson et. al. is here.

14. Jun 2, 2014

### naima

simultaneously? the article cites the non cloning theorem.

15. Jun 2, 2014

### Thinkor

I am talking about the final result.

The no-cloning theorem "forbids the creation of identical copies of an arbitrary unknown quantum state" according to the wiki article on the theorem. Entanglement is not cloning. "Cloning is a process whose result is a separable state with identical factors", according to the same article.

The experiment creates an entangled state and then at one end (it doesn't matter which), the state is queried, determining both states, i.e., rendering them known simultaneously. And, the states are both identical in this experiment. That's my understanding of it.

Last edited: Jun 2, 2014
16. Jun 2, 2014

### Strilanc

This is incorrect. Quantum teleportation is a way of moving quantum information, not a way of creating entangled pairs.

17. Jun 2, 2014

### Thinkor

What I wrote was "They have not successfully teleported true information. They have successfully created the same information in two places simultaneously."

To "teleport quantum information" (not "true information") you need to create an entangled state. Nothing is really "moved" between the two locations. The state is resolved instantaneously in both when the entangled state is resolved by querying it at either location. This is called "quantum teleportation" of "quantum information". Essentially it creates the same information in two places simultaneously.

18. Jun 2, 2014

### thephystudent

So in fact, there is not much difference with the classical example of the singlet where you measure one particle and it appears to collapse in spin up and you know the other one collapses in spin down instantly?

19. Jun 2, 2014

### Strilanc

No, that is not what quantum teleportation is. That's just entanglement. Quantum teleportation doesn't create information, it moves it. It uses entanglement, and a classical communication channel, to move quantum information. E.g. see the wikipedia article:

Quantum teleportation starts with one qubit in some unknown state $\alpha \left| 0 \right> + \beta \left| 1 \right>$ and finishes with a different qubit in that state instead (it also works for entangled states). It's interesting because it works even if there's no longer a quantum channel between the sender and receiver (they only need bell pairs and a classical channel).

You may find the video about quantum teleportation in the "quantum computing for the determined" series useful.

Last edited: Jun 2, 2014
20. Jun 2, 2014

### Thinkor

That's what I was assuming, wrongly. When I understand it, I'll post again.